1. HIGHWAY CAPACITY MANUAL
  2. HCM.,000
  3. HIGHWAY CAPACITY MANUAL
  4. THE NATIONAL ACADEMIES
      1. Advisers to the Nation on Science, Engineering, and Medicine
      2. www.national - academies.org
      3. CONTENTS
      4. PREFACE
      5. CONTRIBUTORS AND ACKNOWLEDGMENTS
      6. INTRODUCTION
      7. CAPACITY AND LEVEL-OF-SERVICE CONCEPTS
      8. APPLICATIONS
      9. DECISION MAKING
      10. GLOSSARY
      11. SYMBOLS
      12. TRAFFIC FLOW PARAMETERS
      13. TRAFFIC CHARACTERISTICS
      14. ANALYTICAL PROCEDURES OVERVIEW
      15. URBAN STREET CONCEPTS
  5. ............. ............. IMMENNEMMEN 40
      1. PEDESTRIAN AND BICYCLE CONCEPTS
  6. • / ‘
  7. • / /
      1. HIGHWAY CONCEPTS
      2. FREEWAY CONCEPTS
      3. TRANSIT CONCEPTS
      4. URBAN STREETS
      5. SIGNALIZED INTERSECTIONS
      6. North t
  8. -- ,-, ?
  9. V .4 ?
      1. MENNEMENI 'EL-LINEMEN aEEMEMEM MENNEMENI •••••••Mi
      2. MEMENNEN rjE••••••
  10. 1 I•••••••• •••••••• ••••••••
    1. North t
      1. UNSIGNALIZED INTERSECTIONS
  11. -)10.
  12. g ? A
      1. .••.••• ?
  13. „. ?? Mr
  14. .••.•••
      1. • ,,,
  15. , ? V/
  16. 99999999999999999
  17. . -,-, w ? •/
      1. PEDESTRIANS
      2. BICYCLES
      3. TWO-LANE HIGHWAYS
      4. MULTILANE HIGHWAYS
      5. Millilli
      6. memmoinammilm
      7. FREEWAY FACILITIES
      8. BASIC FREEWAY SEGMENTS
  18. 00 EllE .1111 ? .
  19. MEM ? .
      1. FREEWAY WEAVING
      2. RAMPS AND RAMP JUNCTIONS
      3. INTERCHANGE RAMP TERMINALS
  20. 1,.... =
  21. N -4
  22. ...,T
      1. TRANSIT
  23. r tC ± ?
  24. The Situation ?
      1. ASSESSMENT OF MULTIPLE FACILITIES
      2. CORRIDOR ANALYSIS
      3. AREAWIDE ANALYSIS
      4. SIMULATION AND OTHER MODELS
      5. INDEX

HIGHWAY
CAPACITY
MANUAL
TRANSPORTATION RESEARCH BOARD
OF THE NATIONAL ACADEMIES

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HCM.,000
AR00042733

TRANSPORTATION RESEARCH BOARD
2005 EXECUTIVE COMMITTEE*
Chair: John R. Njord,
Executive Director, Utah Department of
Transportation, Salt Lake City
Vice Chair: Michael D. Meyer,
Professor, School of Civil and
Environmental Engineering, Georgia Institute of Technology,
Atlanta
Executive Director: Robert E. Skinner, Jr.,
Transportation
Research Board
Michael W. Behrens,
Executive Director, Texas Department of
Transportation, Austin
Allen D. Biehler,
Secretary, Pennsylvania Department of
Transportation, Harrisburg
Larry L. Brown, Sr.,
Executive Director, Mississippi Department
of Transportation, Jackson
Deborah H. Butler,
Vice President, Customer Service, Norfolk
Southern Corporation and Subsidiaries, Atlanta, Georgia
Anne P. Canby,
President, Surface Transportation Policy Project,
Washington, D.C.
John L. Craig,
Director, Nebraska Department of Roads, Lincoln
Douglas G. Duncan,
President and CEO, FedEx Freight, Memphis,
Tennessee
Nicholas J. Garber,
Professor of Civil Engineering, University of
Virginia, Charlottesville
Angela Gittens,
Vice President, Airport Business Services, HNTB
Corporation, Miami, Florida
Genevieve Giuliano,
Director, Metrans Transportation Center,
and Professor, School of Policy, Planning, and Development,
University of Southern California, Los Angeles (Past Chair,
2003)
Bernard S. Groseclose, Jr.,
President and CEO, South Carolina
State Ports Authority, Charleston
Susan Hanson,
Landry University Professor of Geography,
Graduate School of Geography, Clark University, Worcester,
Massachusetts
James R. Hertwig,
President, CSX Intermodal, Jacksonville,
Florida
Gloria J. Jeff,
Director, Michigan Department of Transportation,
Lansing
Adib K. Kanafani,
Cahill Professor of Civil Engineering,
University of California, Berkeley
Herbert S. Levinson,
Principal, Herbert S. Levinson Transportation
Consultant, New Haven, Connecticut
Sue McNeil,
Director and Professor, Urban Transportation Center,
University of Illinois, Chicago
Michael Morris,
Director of Transportation, North Central Texas
Council of Governments, Arlington
Carol A. Murray,
Commissioner, New Hampshire Department of
Transportation, Concord
Michael S. Townes,
President and CEO, Hampton Roads Transit,
Virginia (Past Chair, 2004)
C. Michael Walton,
Ernest H. Cockrell Centennial Chair in
Engineering, University of Texas, Austin
Linda S. Watson,
Executive Director, LYNX–Central Florida
Regional Transportation Authority, Orlando
Marion C. Blakey,
Administrator, Federal Aviation Administration,
U.S. Department of Transportation (ex officio)
Joseph H. Boardman,
Administrator, Federal Railroad
Administration, U.S. Department of Transportation (ex officio)
Rebecca M. Brewster,
President and COO, American
Transportation Research Institute, Smyrna, Georgia (ex officio)
George Bugliarello,
Chancellor, Polytechnic University, Brooklyn,
New York; Foreign Secretary, National Academy of
Engineering, Washington, D.C. (ex officio)
* Membership as of July 2005.
Thomas H. Collins
(Adm., U.S. Coast Guard), Commandant,
U.S. Coast Guard, Washington, D.C. (ex officio)
Jennifer L. Dorn,
Administrator, Federal Transit Administration,
U.S. Department of Transportation (ex officio)
James J. Eberhardt,
Chief Scientist, Office of FreedomCAR and
Vehicle Technologies, U.S. Department of Energy (ex officio)
Edward R. Hamberger,
President and CEO, Association of
American Railroads, Washington, D.C. (ex officio)
Jolm C. Horsley,
Executive Director, American Association of
State Highway and Transportation Officials, Washington, D.C.
(ex officio)
Jolm E. Jamian,
Acting Administrator, Maritime Administration,
U.S. Department of Transportation (ex officio)
Edward Johnson,
Director, Applied Science Directorate, National
Aeronautics and Space Administration, John C. Stennis Space
Center, Mississippi (ex officio)
Ashok G. Kaveeshwar,
Administrator, Research and Innovative
Technology Administration, U.S. Department of Transportation
(ex officio)
Rick Kowalewski,
Deputy Director, Bureau of Transportation
Statistics, U.S. Department of Transportation (ex officio)
Brigham McCown,
Deputy Administrator, Pipeline and Hazardous
Materials Safety Administration, U.S. Department of
Transportation (ex officio)
William W. Millar,
President, American Public Transportation
Association, Washington, D.C. (ex officio) (Past Chair, 1992)
Mary E. Peters,
Administrator, Federal Highway Administration,
U.S. Department of Transportation (ex officio)
Suzanne Rudzinski,
Director, Transportation and Regional
Programs, U.S. Environmental Protection Agency (ex officio)
Jeffrey W. Runge,
Administrator, National Highway Traffic Safety
Administration, U.S. Department of Transportation (ex officio)
Almette M. Sandberg,
Administrator, Federal Motor Carrier Safety
Administration, U.S. Department of Transportation (ex officio)
Jeffrey N. Shane,
Under Secretary for Policy, U.S. Department of
Transportation (ex officio)
Carl A. Strock
(Maj. Gen., U.S. Army), Chief of Engineers and
Commanding General, U.S. Army Corps of Engineers,
Washington, D.C. (ex officio)
Transportation Research Board publications
may be ordered
directly from the TRB Business Office, through the internet at
www.TRB.org/bookstore/, or by annual subscription through
organization or individual affiliation with TRB. Affiliates and
library subscribers are eligible for substantial discounts. For further
information, contact the Transportation Research Board Business
Office, 500 Fifth Street, NW, Washington, DC 20001 (telephone
202-334-3213; fax 202-334-2519; or e-mail TRBSales@nas.edu ).
© 2000 by the National Academy of Sciences.
All rights reserved.
Printed in the United States of America.
Second printing, July 2005, incorporating corrections.
For updates on HCM 2000 errata, go to http://www.ahb40.org/.
Library of Congress Cataloging in Publication Data
Highway capacity manual.
p. cm.
"HCM 2000."
Includes bibliographic references.
ISBN 0-309-06681-6 [metric]
ISBN 0-309-06746-4 [standard]
1. Highway capacity—Handbooks, manuals, etc.
HE336.H48 H54 2000
388.3'14—dc2100
?
061507
AR00042734

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HIGHWAY
CAPACITY
MANUAL
TRANSPORTATION RESEARCH BOARD
OF THE NATIONAL ACADEMIES
Washington,
TE;133
D.C.
HCMCO®
www.TRB.org
AR00042735

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THE NATIONAL ACADEMIES
Advisers to the Nation on Science, Engineering, and Medicine
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars
engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their
use for the general welfare. On the authority of the charter granted to it by the Congress in 1863, the Academy has
a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone
is president of the National Academy of Sciences.
The National Academy of Engineering was established in 1964, under the charter of the National Academy of
Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the
selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal
government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national
needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A.
Wulf is president of the National Academy of Engineering.
The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of
eminent members of appropriate professions in the examination of policy matters pertaining to the health of the
public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional
charter to be an adviser to the federal government and, on its own initiative, to identify issues of medical care,
research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine.
The National Research Council was organized by the National Academy of Sciences in 1916 to associate the
broad community of science and technology with the Academy's purposes of furthering knowledge and advising
the federal government. Functioning in accordance with general policies determined by the Academy, the Council
has become the principal operating agency of both the National Academy of Sciences and the National Academy of
Engineering in providing services to the government, the public, and the scientific and engineering communities.
The Council is administered jointly by both the Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and
Dr. William A. Wulf are chair and vice chair, respectively, of the National Research Council.
The Transportation Research Board is a division of the National Research Council, which serves the National
Academy of Sciences and the National Academy of Engineering. The Board's mission is to promote innovation and
progress in transportation through research. In an objective and interdisciplinary setting, the Board facilitates the
sharing of information on transportation practice and policy by researchers and practitioners; stimulates research
and offers research management services that promote technical excellence; provides expert advice on transportation
policy and programs; and disseminates research results broadly and encourages their implementation The Board's
varied activities annually engage more than 5,000 engineers, scientists, and other transportation researchers and
practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public
interest. The program is supported by state transportation departments, federal agencies including the component
administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the
development of transportation. www.TRB.org
www.national - academies.org
AR00042736

CONTENTS
PREFACE ?
vii
CONTRIBUTORS AND ACKNOWLEDGMENTS ?
ix
PART I: OVERVIEW
1. INTRODUCTION ?
1-1
2. CAPACITY AND LEVEL-OF-SERVICE CONCEPTS ?
2-1
3. APPLICATIONS ?
3-1
4. DECISION MAKING ?
4-1
5. GLOSSARY ?
5-1
6. SYMBOLS ?
6-1
PART II: CONCEPTS
7. TRAFFIC FLOW PARAMETERS ?
7-1
8. TRAFFIC CHARACTERISTICS ?
8-1
9. ANALYTICAL PROCEDURES OVERVIEW ?
9-1
10. URBAN STREET CONCEPTS ?
10-1
11. PEDESTRIAN AND BICYCLE CONCEPTS ?
11-1
12. HIGHWAY CONCEPTS ?
12-1
13. FREEWAY CONCEPTS ?
13-1
14. TRANSIT CONCEPTS ?
14-1
PART III: METHODOLOGIES
15. URBAN STREETS ?
15-1
16. SIGNALIZED INTERSECTIONS ?
16-1
17. UNSIGNALIZED INTERSECTIONS ?
17-1
18. PEDESTRIANS ?
18-1
19. BICYCLES ?
19-1
20. TWO-LANE HIGHWAYS ?
20-1
21. MULTILANE HIGHWAYS ?
21-1
22. FREEWAY FACILITIES ?
22-1
23. BASIC FREEWAY SEGMENTS ?
23-1
24. FREEWAY WEAVING ?
24-1
25. RAMPS AND RAMP JUNCTIONS ?
25-1
26. INTERCHANGE RAMP TERMINALS ?
26-1
27. TRANSIT ?
27-1
PART IV: CORRIDOR AND AREAWIDE ANALYSES
28. ASSESSMENT OF MULTIPLE FACILITIES ?
28-1
29. CORRIDOR ANALYSIS ?
29-1
30. AREAWIDE ANALYSIS ?
30-1
PART V: SIMULATION AND OTHER MODELS
31. SIMULATION AND OTHER MODELS ?
31-1
AR00042737

Highway Capacity Manual 2000
PREFACE
The Transportation Research Board's (TRB's)
Highway Capacity Manual
(HCM)
provides a collection of state-of-the-art techniques for estimating the capacity and
determining the level of service for transportation facilities, including intersections and
roadways as well as facilities for transit, bicycles, and pedestrians. For more than 50
years, the HCM has fulfilled this goal, earning a unique place in the esteem of the
transportation community.
Developed and revised under the direction of the TRB Committee on Highway
Capacity and Quality of Service, this newest edition, HCM 2000, presents the best
available techniques for determining capacity and level of service for transportation
facilities at the start of the new millennium. However, this comprehensive manual does
not establish a legal standard for highway design or construction.
HISTORICAL PERSPECTIVE
Originally published in 1950, the HCM was the first document to quantify the
concept of capacity for transportation facilities. The 1965 edition in turn was the first to
define the concept of level of service, which has become the foundation for determining
the adequacy of transportation facilities from the perspectives of planning, design, and
operations. The 1985 edition, along with its 1994 and 1997 updates, is TRB's most
widely used document. Translated into several languages, it has become the standard
reference on capacity and level-of-service procedures, relied on by transportation analysts
around the world.
DEVELOPMENT OF HCM 2000
To produce HCM 2000, TRB's Committee on Highway Capacity and Quality of
Service developed a comprehensive program of research. The research was implemented
through the funding efforts of the National Cooperative Highway Research Program
(NCHRP) and the Transit Cooperative Research Program. In addition, the Federal
Highway Administration supported TRB with a variety of research endeavors. These
combined efforts produced the basic research reviewed by the committee and
incorporated into HCM 2000.
All of the research results contributing to HCM 2000 underwent an iterative and
interactive review. When a funded research project was completed, the group that guided
its development—for example, an NCHRP panel—reviewed the findings first. If
accepted by the group, the research was then presented for consideration by one of the 12
working subcommittees of the Highway Capacity and Quality of Service Committee.
The subcommittee, including several committee members as well as other active
professionals, then provided its recommendations to the full committee. The final
approval for each chapter of HCM 2000 rested with the Highway Capacity and Quality of
Service Committee, composed of 30 members representing the research community,
government agencies, and private industry.
CONTENTS OF HCM 2000
The
Highway Capacity Manual
2000 represents a significant revision and expansion
of the material provided in previous editions. The manual has grown from 14 to 31
chapters. These chapters are divided into five parts:
I. Overview,
II.
Concepts,
III.
Methodologies,
IV. Corridor and Areawide Analyses, and
V. Simulation and Other Models.
Parts I and III contain information that corresponds to the contents of previous
editions. Part II provides concepts and estimated default values for use in planning-level
vii ?
Preface
AR00042738

Highway Capacity Manual 2000
analytical work. Part IV presents computational techniques and general analysis
guidelines for corridor and areawide analyses. Part V offers background and information
on alternative models that may be appropriate for systemwide or more complex analyses.
A companion version of the manual is available in CD-ROM, including tutorials and
video clips to enhance the communication of the concepts. In addition, there are links
between the text and the glossary to facilitate understanding of the manual by less-
experienced users.
SPECIAL ACKNOWLEDGMENTS
HCM 2000 incorporates significant advances in the state of knowledge in
determining capacity and quality-of-service values for all modes of surface
transportation.
Hundreds of professionals have volunteered their time and energy to the work of the
Committee on Highway Capacity and Quality of Service. Twice every year, the
committee meets to perform a major review of relevant research and to identify new
research needs in response to changes in roadway design standards, driver behavior, and
vehicle operating characteristics.
Members of the committee and its subcommittees are listed on pages ii—vii. Special
recognition is extended to those who have chaired the committee: O.K. Normann, Carl C.
Saal, Robert C. Blumenthal, James H. Kell, Carlton C. Robinson, and Adolf D. May. In
acknowledgment of their sustained contributions to the committee and to the
development of HCM 2000, Robinson and May have been designated members emeritus
of the committee.
Complementing the volunteer efforts vital to the work of the committee, TRB staff
has provided outstanding support. Special thanks are given to Richard Cunard, Engineer
of Traffic and Operations, and to B. Ray Den, NCHRP Senior Program Officer, for their
contributions.
The Committee on Highway Capacity and Quality of Service invites comments and
suggestions on HCM 2000 while continuing its mission of enhancing and improving the
design, operation, and planning of transportation facilities.
John D. Zegeer
Chairman, TRB Committee on Highway Capacity and Quality of Service
Preface ?
viii
AR00042739

Highway Capacity Manual 2000
CONTRIBUTORS AND ACKNOWLEDGMENTS
HCM 2000 is the result of the coordinated efforts of many individuals, groups,
research organizations, and government agencies. The TRB Committee on Highway
Capacity and Quality of Service is responsible for the content of the
Highway Capacity
Manual;
preparation of the volume was accomplished through the efforts of the following
groups and individuals:
TRB COMMITTEE ON HIGHWAY CAPACITY AND QUALITY OF SERVICE
(Members as of January 31, 2000)
John Zegeer,
Kittelson & Associates, Inc.—Chairman
Richard Dowling,
Dowling Associates, Inc.—Secretary
James Bonneson,
Texas A & M University
Werner Brilon,
Ruhr University, Bochum, Germany
Robert Bryson,
City of Milwaukee
Kenneth Courage,
University of Florida
Alan Danaher,
Kittelson & Associates, Inc.
Rafael DeArazoza,
Florida Department of Transportation
Lily Elefteriadou,
Pennsylvania State University
Dan Fambro,
Texas A & M University (deceased)
Ronald Giguere,
Federal Highway Administration
Albert Grover,
Albert Grover & Associates
Mariano GullOn Low,
Centro de Estudios de Carreteras (deceased)
Fred L. Hall,
McMaster University, Canada
Douglas Harwood,
Midwest Research Institute
Chris Hoban,
The World Bank
Wayne Kittelson,
Kittelson & Associates, Inc.
Michael Kyte,
University of Idaho
Adolf D. May,
University of California at Berkeley
Douglas McLeod,
Florida Department of Transportation
Barbara Ostrom,
LAW PCS
James Powell,
Parsons Transportation Group
Nagui Rouphail,
North Carolina State University
Erik Ruehr,
Valley Research and Planning Associates
Rikke Rysgaard,
Danish Road Directorate
James Schoen,
Catalina Engineering, Inc.
Alex Sorton,
Northwestern University
Dennis Strong,
Strong Concepts
Stan Teply,
University of Alberta, Canada
Rod Troutbeck,
Queensland University of Technology, Australia
Richard Cunard,
Transportation Research Board Staff Representative
Emeritus Members
(As of February 1, 2000)
Adolf D. May,
University of California at Berkeley
Carlton C. Robinson,
Consultant
Subcommittee on Arterials
James Bonneson,
Texas A & M University—Chair
Janice Daniel,
New Jersey Institute of Technology
Ronald Giguere,
Federal Highway Administration
Joel Marcuson,
Sverdrup Civil, Inc.
Doug McLeod,
Florida Department of Transportation
(continued)
ix ?
Contributors and Acknowledgments
AR00042740

Highway Capacity Manual 2000
Subcommittee on Arterials
(continued)
Dennis Strong,
Strong Concepts
Andrzej Tarko,
Purdue University
Mark Vandehey,
Kittelson & Associates, Inc.
Subcommittee on Concepts and Definitions
Barbara Ostrom,
LAW PCS—C hair
Fred L. Hall,
McMaster University, Canada
Doug McLeod,
Florida Department of Transportation
Stan Teply,
University of Alberta, Canada
Subcommittee on Freeways and Multilane Highways
Adolf D. May,
University of California at Berkeley—Group Leader, Uninterrupted
Flow
Nagui Rouphail,
North Carolina State University—Group Leader, Uninterrupted
Flow
Lily Elefteriadou,
Pennsylvania State University—Leader, Ramp and Weaving
Junctions
James Schoen,
Catalina Engineering, Inc.—Leader, Basic Freeway Segments and
Multilane Highways
Michael Cassidy,
University of California at Berkeley
Michael Church,
California Department of Transportation
Brian Eads,
Crawford, Murphy & Tilly, Inc.
Lily Elefteriadou,
Pennsylvania State University
Joseph Fazio,
Illinois Institute of Technology
Fred L. Hall,
McMaster University, Canada
Abdul-Rahman Hamad,
H. W. Lochner, Inc.
Lee Han,
University of Tennessee
Joel Leisch,
Consultant
John Leonard,
Georgia Institute of Technology
Barbara Ostrom,
LAW PCS
Thomas Parlante,
Arizona Department of Transportation
Ronald Pfefer,
Northwestern University
William Prosser,
Federal Highway Administration
William Reilly,
Catalina Engineering, Inc.
Bruce Robinson,
Kittelson & Associates, Inc.
Roger Roess,
Polytechnic University
Fred Rooney,
California Department of Transportation
Rikke Rysgaard,
Danish Road Directorate
James Schoen,
Catalina Engineering, Inc.
Ronald Sonntag,
Marquette University
Andrzej Tarko,
Purdue University
Michelle Thomas,
Federal Highway Administration
Jose Ulerio,
Polytechnic University
Tom Urbanik,
Texas A & M University
Subcommittee on Interchange Ramp Terminals
James Powell,
Parsons Transportation Group—Chair
James Bonneson,
Texas A & M University
Robert Bryson,
City of Milwaukee
Michael Church,
California Department of Transportation
Thomas Creasey,
Jordan, Jones & Goulding, Inc.
Janice Daniel,
New Jersey Institute of Technology
(continued)
Contributors and Acknowledgments
AR00042741

Highway Capacity Manual 2000
Subcommittee on Interchange Ramp Terminals
(continued)
Michael Holling,
Transcore
B. Kent La11,
Portland State University
Joel Leisch,
Consultant
Joel Marcuson,
Sverdrup Civil, Inc.
Scott Parker,
Edwards & Kelcey, Inc.
Fred Rooney,
California Department of Transportation
Subcommittee on Pedestrians and Bicycles
Alex Sorton,
Northwestern University—Chair
Patrick Allen,
California Department of Transportation
Hein Botma,
Delft University, The Netherlands
Jeff Davis,
The Citadel
Joseph Fazio,
Illinois Institute of Technology
Chris Hoban,
The World Bank
Bruce Landis,
Sprinkler Associates
John LaPlante,
TYLin-Bascor
Joe Milazzo,
North Carolina State University
John Monall,
University of Calgaiy, Canada
Virginia Sisiopiku,
Michigan State University
Mark Virkler,
University of Missouri at Columbia
Thomas Walsh,
Madison Department of Transportation
Subcommittee on Planning Applications
Douglas McLeod,
Florida Department of Transportation—Chair
Jim Altenstadter,
Arizona Department of Transportation
Robert Bryson,
City of Milwaukee
Thomas Creasey,
Jordan, Jones & Goulding, Inc.
Richard Dowling,
Dowling Associates, Inc.
Kurt Eichin,
Florida Department of Transportation
Abdul-Rahman Hamad,
H. W.
Lochner, Inc.
John Karachepone,
Kittelson & Associates, Inc.
Wayne Kittelson,
Kittelson & Associates, Inc.
William McShane,
Polytechnic University
Barbara Ostrom,
LAW PCS
Elena Prassas,
Polytechnic University
Erik Ruehr,
VRPA Technologies
Paul Ryus,
Kittelson & Associates, Inc.
Terrel Shaw,
Reynolds, Smith & Hills, Inc.
Stan Teply,
University of Alberta, Canada
Subcommittee on Research
Fred L. Hall,
McMaster University, Canada—Chair
Alan Danaher,
Kittelson & Associates, Inc.
Richard Dowling,
Dowling Associates, Inc.
Lily Elefteriadou,
Pennsylvania State University
John Leonard,
Georgia Institute of Technology
George List,
Rensselaer Polytechnic University
Pawan Maini,
University of Colorado at Denver
James Powell,
DeLeuw Cather & Company
Larry Sutherland,
Ohio Department of Transportation
Rod Troutbeck,
Queensland University of Technology
Davey Warren,
Federal Highway Administration
xi ?
Contributors and Acknowledgments
AR00042742

Highway Capacity Manual 2000
Subcommittee on Signalized Intersections
Dennis Strong,
Strong Concepts—Chair
Rahmi Akcelik,
Akcelik & Associates
Rahim Benekohal,
University of Illinois
Robert Bryson,
City of Milwaukee
Kenneth Courage,
University of Florida
Glenn Grigg
Albert Grover,
Albert Grover & Associates
David Hook,
Hook Engineering
John Leonard,
Georgia Institute of Technology
Feng-Bor Lin,
Clarkson University
Pawan Maini,
University of Colorado at Denver
Carroll Messer,
Texas A & M University
Elena Prassas,
Polytechnic University
Bruce Robinson,
Kittelson & Associates, Inc.
Roger Roess,
Polytechnic University
Nagui Rouphail,
North Carolina State University
Stan Teply,
University of Alberta, Canada
Robert Wortman,
University of Arizona
Subcommittee on Transit Systems
Alan Danaher,
Kittelson & Associates, Inc.—Chair
Tara Bartee,
Florida Department of Transportation
Howard Benn,
Montgomery County, MD Transit
Joseph Goodman, Federal Transit Administration
William Hoey,
Consultant
Michael Kyte,
University of Idaho
Herbert Levinson,
Consultant
David Miller,
Parsons Brinckerhoff
Rikke Rysgaard,
Danish Road Directorate
Paul Ryus,
Kittelson & Associates, Inc.
Kevin St. Jacques,
Wilbur Smith and Associates
Joel Volinski,
Center for Urban Transportation Research—University of South
Florida
Subcommittee on Two
-
Lane Roads
Douglas Harwood,
Midwest Research Institute—Chair
Jan Botha,
San Jose State University
Hein Botma,
Delft University, The Netherlands
Albert Grover,
Albert Grover & Associates
Mariano GullOn Low,
Centro de Estudios de Carreteras (deceased)
Christopher Hoban,
The World Bank
Greg Laragan,
Idaho Department of Transportation
David Lovell,
University of Maryland
Adolf D. May,
University of California at Berkeley
Carroll Messer,
Texas A & M University
John Monall,
University of Calgary, Canada
William Prosser,
Federal Highway Administration
Guido Radelat
Alex Sorton,
Northwestern University Traffic Institute
Davey Warren,
Federal Highway Administration
Alexander Werner,
Reid Crowther Consultants, Ltd.
Contributors and Acknowledgments ?
xii
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Highway Capacity Manual 2000
Subcommittee on Unsignalized Intersections
Rod Troutbeck,
Queensland University of Technology, Australia—Chair
Werner Brilon,
Ruhr University, Bochum, Germany
Robert Bryson,
City of Milwaukee
Joon Byun,
Federal Highway Administration
Mitzi
M. Dobersek,
Wisconsin Department of Transportation
Aimee Flannery,
Mitrekek Systems
Glenn Grigg
Mariano GullOn Low,
Centro de Estudios de Carreteras (deceased)
Wayne Haussler,
Goodkind & O'Dea, Inc.
Dane Ismart,
Federal Highway Administration
R. Ian Kingham,
GMK Transportation, Ltd., Canada
Wayne Kittelson,
Kittelson & Associates, Inc.
Michael Kyte,
University of Idaho
B. Kent Lall,
Portland State University
George List,
Rensselaer Polytechnic Institute
Charles Manning,
Creighton Manning, Inc.
Joseph Marek,
Clackamas County
Michael O'Rourke,
Eng-Wong-Taub & Associates
Bruce Robinson,
Kittelson & Associates, Inc.
Lee Rodegerdts,
Kittelson & Associates, Inc.
Erik Ruehr,
VRPA Technologies
John Sampson,
Jeffares & Green, Inc.
Zong Tian,
Kittelson & Associates, Inc.
Marian Tracz,
Cracow Technical University, Poland
Kenneth Voigt,
HNTB Corporation
Andrew Wolfe,
Union College
Subcommittee on User Liaison
Wayne Kittelson,
Kittelson & Associates, Inc.—Chair
Robert Foyle,
ITRE
Ronald Giguere,
Federal Highway Administration
Joel Leisch,
Consultant
John Leonard,
Georgia Institute of Technology
William Prosser,
Federal Highway Administration
Dennis Strong,
Strong Concepts
Charles Wallace,
University of Florida
NCHRP 3
-
55 PANEL
Carlton C. Robinson,
Consultant—Chair
Rafael DeArazoza,
Florida Department of Transportation
Richard Dowling,
Dowling Associates, Inc.
Ronald Giguere,
Federal Highway Administration
Wayne Kittelson,
Kittelson & Associates, Inc.
Barbara Ostrom,
LAW PCS
William Prosser,
Federal Highway Administration
Nagui Rouphail,
North Carolina State University
Ronald Sonntag,
Marquette University
Stan Teply,
University of Alberta, Canada
Edward Thomas,
Federal Transit Administration
John Zegeer,
Kittelson & Associates, Inc.
B. Ray Den,
Transportation Research Board Staff Representative
xiii ?
Contributors and Acknowledgments
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Highway Capacity Manual 2000
RESEARCH TEAM
William Reilly, Principal Investigator,
Catalina Engineering, Inc.
Susan Donahue,
Catalina Engineering, Inc.
Michael Ereti,
Catalina Engineering, Inc.
Wei Lien Liang,
Catalina Engineering, Inc.
Khang Nguyen,
Catalina Engineering, Inc.
Andrea Reilly,
Catalina Engineering, Inc.
James Schoen,
Catalina Engineering, Inc.
Roger Roess,
Polytechnic University
Elena Prassas,
Polytechnic University
Jose Ulerio,
Polytechnic University
Rahmi Akcelik,
Akcelik & Associates
Ronald Pfefer,
Maron Engineering, Ltd.
HCM 2000 was edited and produced under the supervision of Nancy A. Ackerman,
director of the TRB Office of Reports and Editorial Services; Javy Awan and Norman
Solomon edited the manual.
Contributors and Acknowledgments ?
xiv
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Highway Capacity Manual 2000
CHAPTER 1
INTRODUCTION
CONTENTS
I. INTRODUCTION ?
1-1
Purpose of the Manual ?
1-1
Scope of the Manual ?
1-1
Use of the Manual ?
1-1
Results from the Metric and U.S. Customary Versions ?
1-1
North American and International Applications ?
1-2
Online Manual ?
1-2
Calculation Software ?
1-2
II. HISTORY OF THE MANUAL ?
1-2
III. WHAT'S NEW IN HCM 2000 ?
1-3
Part I: Overview ?
1-3
Part II: Concepts ?
1-4
Part III: Methodologies ?
1-5
Urban Streets ?
1-5
Signalized Intersections ?
1-5
Unsignalized Intersections ?
1-5
Pedestrians ?
1-5
Bicycles ?
1-5
Two-Lane Highways ?
1-5
Multilane Highways ?
1-5
Freeway Facilities ?
1-5
Basic Freeway Segments ?
1-5
Freeway Weaving ?
1-5
Ramps and Ramp Junctions ?
1-5
Interchange Ramp Terminals ?
1-5
Transit ?
1-5
Part IV: Corridor and Areawide Analyses ?
1-6
Part V: Simulation and Other Models ?
1-6
IV. RESEARCH BASIS FOR HCM 2000 ?
1-6
V. REFERENCE ?
1-6
EXHIBITS
Exhibit 1-1. ?
HCM 1985 Edition: Organization and Updates ?
1-3
Exhibit 1-2. ?
HCM 2000 Organization ?
1-4
Exhibit 1-3. ?
Related Research Projects ?
1-7
1-i ?
Chapter 1 - Introduction
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Highway Capacity Manual 2000
I. INTRODUCTION
PURPOSE OF THE MANUAL
The
Highway Capacity Manual
(HCM) provides transportation practitioners and
researchers with a consistent system of techniques for the evaluation of the quality of
service on highway and street facilities. The HCM does not set policies regarding a
desirable or appropriate quality of service for various facilities, systems, regions, or
circumstances. Its objectives include providing a logical set of methods for assessing
transportation facilities, assuring that practitioners have access to the latest research
results, and presenting sample problems. This fourth edition of the HCM is intended to
provide a systematic and consistent basis for assessing the capacity and level of service
for elements of the surface transportation system and also for systems that involve a
series or a combination of individual facilities. The manual is the primary source
document embodying research findings on capacity and quality of service and presenting
methods for analyzing the operations of streets and highways and pedestrian and bicycle
facilities. A complementary volume,
Transit Capacity and Quality of Service Manual-
now in development by the Transportation Research Board (TRB)-presents methods for
analyzing transit services from the perspectives of both the user and the operator.
SCOPE OF THE MANUAL
This manual is divided into five parts. Part I provides an overview of the traffic flow
concepts inherent in capacity and level-of-service analyses, a discussion of their
applications, and a description of policy decision making based on this fourth edition. It
also includes a glossary of terms and a list of symbols. Part II describes the concepts and
provides the estimated default values for use in the analytical work presented in Part III.
Part III offers specific methods for assessing roadway, bicycle, pedestrian, and transit
facilities in relation to their performance, capacity, and level of service.
For the analyst who must assess more than an individual facility, Part IV of this
manual provides a framework for the analysis of corridors, areas, and multimodal
operations. In some cases, it provides specific computational techniques, while in others
it provides a more general analysis of the facility or facilities. Part V offers background
and information on the type of models appropriate for systemwide or more complex
capacity and level-of-service analyses.
Additional information beyond this manual is available on the World Wide Web at
http://national-academies.org/trb/hcm.
USE OF THE MANUAL
In addition to the service measures necessary to determine quality of service, this
manual identifies analytical procedures for other performance measures. These allow the
analyst to assess different aspects of an existing or planned facility. Moreover, this
document makes it possible to evaluate broader systems of facilities and to establish a
link between operational and planning models.
This manual is intended for use by a range of practitioners, including traffic
engineers, traffic operations personnel, design engineers, planners, management
personnel, teachers, and university students. To use the manual effectively and to apply
its methodologies, some technical background is desirable-typically university-level
training or technical work in a public agency or consulting firm.
RESULTS FROM THE METRIC AND U.S. CUSTOMARY VERSIONS
This fourth edition of the manual is published in two versions, one in metric units
and one in U.S. customary units. Although the methodologies in the metric and U.S.
customary versions of the manual are identical, parameters, level-of-service thresholds,
and other values will be hard-converted. This means that analysis results calculated using
Part!: Overview
1. Introduction
2.
Capacity and Level-of-
Service Concepts
3. Applications
4.
Decision Making
5.
Glossary
6.
Symbols
Part II: Concepts
7. Traffic Flow Parameters
8.
Traffic Characteristics
9. Analytical Procedures
Overview
10. Urban Street Concepts
11. Pedestrian and Bicycle
Concepts
12. Highway Concepts
13. Freeway Concepts
14. Transit Concepts
Part III: Methodologies
15. Urban Streets
16. Signalized Intersections
17. Unsignalized
Intersections
18. Pedestrians
19. Bicycles
20. Two-Lane Highways
21. Multilane Highways
22. Freeway Facilities
23. Basic Freeway Segments
24. Freeway Weaving
25. Ramps and Ramp
Junctions
26. Interchange Ramp
Terminals
27. Transit
Part IV: Corridor and
Areawide Analyses
28. Assessment of Multiple
Facilities
29. Corridor Analysis
30. Areawide Analysis
Part V: Simulation and
Other Models
31. Simulation and Other
Models
1-1
?
Chapter 1 - Introduction
Introduction
AR00042747

Highway Capacity Manual 2000
the metric version may differ slightly from those calculated using the U.S. customary
version. Transportation agencies may want to specify which system of units they and
their consultants will use and discourage conversions between systems of units.
NORTH AMERICAN AND INTERNATIONAL APPLICATIONS
During the 1990s, capacity and level-of-service analysis generated interest on an
international scale. Therefore, increased attention and effort has focused on incorporating
into the HCM research results and proposed procedures from countries outside of North
America. Also, by producing its first HCM with metric units, TRB has taken a step
toward making these methods and procedures more applicable to international work.
However, the user of the manual is cautioned that the majority of the research base, the
default values, and the typical applications are from North America, particularly from the
United States. Although there is considerable value in the general methods presented,
their use outside of North America requires additional emphasis on calibrating the
equations and the procedures to local conditions as well as recognizing major differences
in the composition of traffic; in driver, pedestrian, and bicycle characteristics; and in
typical geometries and control measures.-
ONLINE MANUAL
HCM 2000 is available in electronic format on CD-ROM. The online edition offers
several multimedia, user-interactive components that allow for viewing of simulated and
real-world traffic conditions, explanations of capacity and level of service concepts, and a
step-by-step graphic presentation of the solutions to sample problems. The online manual
faithfully presents the material and procedures described in this book.
CALCULATION SOFTWARE
As a companion tool to this manual, commercial software is available to perform the
numerical calculations for the chapters in Part III. The CD-ROM online manual has a
feature to incorporate the user's preferred software as required. Although there are
several calculation software packages available, TRB does not produce, review, or
endorse any.
CD-ROM version
Software for
implementing HCM
methodologies
II. HISTORY OF THE MANUAL
The first edition of the HCM was published in 1950 by the U.S. Bureau of Public
Roads as a guide to the design and operational analysis of highway facilities. In 1965,
TRB—then known as the Highway Research Board—published the second edition under
the guidance of its Highway Capacity Committee. The third edition, published by TRB
in 1985, reflected more than two decades of comprehensive research conducted by a
variety of agencies under the sponsorship of several organizations, primarily the National
Cooperative Highway Research Program and the Federal Highway Administration. Its
development was guided by the TRB Committee on Highway Capacity and Quality of
Service. As a result of continuing research in capacity, the third edition of the HCM was
updated in 1994 and 1997. Exhibit 1-1 lists the 1985 HCM chapters along with their
most recent updates.
The 1997 update included extensive revisions to Chapters 3, 9, 10, and 11. In
addition, Chapters 1, 4, 5, 6, and 7 were modified to make them consistent with other
revised chapters.
The basic freeway sections chapter (Chapter 3) revised the procedure for determining
capacity based on density. It also proposed that capacity values under ideal flow
conditions varied by free-flow speed.
Chapter 1 - Introduction ?
1-2
Introduction
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Highway Capacity Manual 2000
EXHIBIT 1-1. HCM 1985 EDITION: ORGANIZATION AND UPDATES
Chapter
?
Description/Facility Type
?
Final Update
1
Introduction, Concepts, and Applications
1997
2
Traffic Characteristics
1994
Uninterrupted-Flow Facilities
3
Basic Freeway Sections
1997
4
Weaving Areas
1997
5
Ramps and Ramp Junctions
1997
6
Freeway Systems
1997
7
Multilane Rural and Suburban Highways
1997
8
Two-Lane Highways
1985
Interrupted-Flow Facilities
9
Signalized Intersections
1997
10
Unsignalized Intersections
1997
11
Arterial Streets
1997
12
Transit Capacity
1985
13
Pedestrians
1985
14
Bicycles
1985
The signalized intersections chapter included findings from research on actuated
traffic signals. The delay equation was modified to account for signal coordination,
oversaturation, variable length analysis periods, and the presence of initial queues at the
beginning of an analysis period. The level-of-service measure was changed from stopped
delay to control delay. Adjustments were made to the permitted left-turn movement
model and to the left-turn equivalency table.
The chapter on unsignalized intersections was completely revised to incorporate the
results of a nationwide research project in the United States examining two-way and four-
way stop-controlled intersections. In addition, it addressed the impact of an upstream
traffic signal on capacity at a two-way stop-controlled intersection. Procedures were
provided to account for flared approaches, upstream signals, pedestrian crossings, and
two-stage gap acceptance (when vehicles seek refuge in a median before crossing a
second stream of traffic).
The arterial streets chapter in the 1997 HCM incorporated the relevant changes from
the signalized intersections chapter. It also established a new arterial classification for
high-speed facilities. The delay equation was modified to account for the effect of
platoons from upstream signalized intersections.
III. WHAT'S NEW IN HCM 2000
This fourth edition of the HCM is published in two versions: metric and U.S.
customary units. The chapter organization also has changed—HCM 2000 consists of five
parts with a total of 31 chapters. Exhibit 1-2 lists the parts and chapters. The changes to
these are summarized in the next sections.
PART I: OVERVIEW
Part I presents the basic concept of level of service and capacity as applied
throughout the manual. In addition, specific discussions cover different types of
applications, decision making, and guidelines for using results from the methodologies in
this manual. A glossary of terms and a list of symbols—previously at the end of the
manual—now appear in the first part and are significantly expanded.
Part I: Chapters 1-6
1-3 ?
Chapter 1 - Introduction
History of Manual
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Highway Capacity Manual 2000
EXHIBIT 1-2. HCM 2000 ORGANIZATION
Chapter
?
Description/Facility Type
Part I: Overview
1
?
Introduction
2
?
Capacity and Level-of-Service Concepts
3
?
Applications
4
?
Decision Making
5
?
Glossary
6
?
Symbols
Part II: Concepts
7
?
Traffic Flow Parameters
8
?
Traffic Characteristics
9
?
Analytical Procedures Overview
10
?
Urban Street Concepts
11
?
Pedestrian and Bicycle Concepts
12
?
Highway Concepts
13
?
Freeway Concepts
14
?
Transit Concepts
Part III: Methodologies
15 ? Urban Streets
16 ?
Signalized Intersections
17 ?
Unsignalized Intersections
18 ? Pedestrians
19 ? Bicycles
20 ?
Two-Lane Highways
21 ?
Multilane Highways
22 ?
Freeway Facilities
23 ?
Basic Freeway Segments
24 ?
Freeway Weaving
25 ?
Ramps and Ramp Junctions
26 ?
Interchange Ramp Terminals
27 ? Transit
Part IV: Corridor and Areawide Analyses
28 ?
Assessment of Multiple Facilities
29 ?
Corridor Analysis
30 ?
Areawide Analysis
Part V: Simulation and Other Models
31 ?
Simulation and Other Models
PART II: CONCEPTS
Part II presents the concepts of the facility types with methodologies described in the
manual and includes discussions of typical capacity parameters. In the past, these
materials were presented together with the methodology for each facility. New discussion
reviews the precision and accuracy of variables in the HCM. Default values are offered
to aid the analyst in obtaining input values for the methodologies that are presented in
Part III. In addition, the second part includes several sample service volume tables and,
in Chapter 10, a modified quick-estimation method for evaluating signalized
intersections.
Part II: Chapters 7-14
Chapter 1 - Introduction ?
1-4
What's New in HCM 2000
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Highway Capacity Manual 2000
PART III: METHODOLOGIES
Part III contains the analytical methodologies, which generally correspond to the 12
facility chapters in the 1997 version of the HCM.
Urban Streets
Titled "Arterial Streets" in the 1997 HCM, this chapter does not change the
methodology significantly, but includes new worksheets.
Signalized Intersections
A methodology for the estimation of back of queue is added, along with new
saturation flow rate adjustment factors for pedestrian and bicycle effects. New
consolidated worksheets are provided.
Unsignalized Intersections
Additions to this chapter include a new 95th percentile queue estimation equation
and newly designed worksheets.
Pedestrians
This chapter expands the 1985 HCM methodology, enabling the evaluation of
several pedestrian facility types previously not addressed.
Bicycles
A new methodology for evaluating bicycle facilities, based on the concept of events
and hindrance, has replaced the previous version in its entirety.
Two
-
Lane Highways
A new methodology for evaluating two-lane highways by direction of travel or by
both directions combined has replaced the previous version in its entirety.
Multilane Highways
New truck equivalency values are introduced.
Freeway Facilities
A new methodology is presented.
Basic Freeway Segments
Again, new truck equivalency values are introduced.
Freeway Weaving
The 1997 HCM methodology has been slightly revised.
Ramps and Ramp Junctions
A new speed prediction model is presented.
Interchange Ramp Terminals
Although this new chapter does not describe a methodology, it presents concepts for
analyzing interchange areas.
Transit
A new methodology is presented, based on research conducted for TRB's
Transit
Capacity and Quality of Service Manual (1).
Part III: Chapters 15-27
1-5 ?
Chapter 1 - Introduction
What's New in HCM 2000
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Highway Capacity Manual 2000
PART IV: CORRIDOR AND AREAWIDE ANALYSES
The methodologies for corridor and areawide analyses are new additions to the
HCM. The chapters show how to aggregate results from the Part III chapters to analyze
the combined effects of different facility types.
PART V: SIMULATION AND OTHER MODELS
Part V is a new addition, presenting concepts and numerical exercises using traffic
simulation models. In addition, it demonstrates typical applications of simulation models
to complement HCM methodologies. An extensive reference list points to more
information on simulation and other models.
IV. RESEARCH BASIS FOR HCM 2000
Exhibit 1-3 lists the major research projects performed since 1990 that have
contributed significantly to the contents of HCM 2000.
V. REFERENCE
1.
Transit Capacity and Quality of Service Manual.
Transit Cooperative Research
Program Web Document No. 6. TRB, National Research Council, Washington,
D.C., 1999. Online. Available:
http://www4.nationalacademies.org/trb/crp.nsf/all+projects/tcrp+a15.
Part IV: Chapters 28-30
Part V: Chapter 31
Chapter 1 - Introduction ?
1-6
What's New in HCM 2000
AR00042752

Highway Capacity Manual 2000
EXHIBIT 1-3. RELATED RESEARCH PROJECTS
Research
NCHRP 3-33
NCHRP 3-37
NCHRP 3-37(2)
NCHRP 3-45
NCHRP 3-46
NCHRP 3-47
NCHRP 3-48
NCHRP 3-49
NCHRP 3-55
NCHRP 3-55(2)
Research Title
Objective
Capacity and Level-of-Service Procedures
for Multilane Rural and Suburban
Highways
Capacity and Level of Service at Ramp-
Freeway Junctions
Capacity and Level of Service at Ramp-
Freeway Junctions (Phase II)
Speed-Flow Relationships for Basic
Freeway Segments
Capacity and Level of Service at
Unsignalized Intersections
Capacity Analysis of Interchange Ramp
Terminals
Capacity Analysis for Actuated Intersections
Capacity and Operational Effects of
Midblock Left-Turn Lanes
Highway Capacity Manual
for the Year
2000
Techniques to Estimate Speeds and Service
Volumes for Planning Applications
Develop procedures to determine capacity
and level of service of multilane highways
Develop methodology to determine
capacity and level of service at ramp-
freeway junctions
Validate methodology produced by
NCHRP 3-37
Revise material on speed-flow
relationships to update HCM 1994
analysis of Basic Freeway Sections
Develop capacity analysis procedure for
stop-controlled intersections and correlate
with the warrants for installation of traffic
signals in the
Manual on Uniform
Traffic Control Devices
Develop methodology to determine
capacity and level of service at signalized
ramp terminals
Develop capacity and level of service
analysis at intersections with actuated
control
Develop qualitative methodology for
evaluating alternative midblock left-turn
treatments on urban streets
Recommend user-preferred format and
delivery system for HCM 2000
Develop extended planning techniques for
estimating measures of effectiveness
(MOEs)
Develop draft chapters related to planning
for HCM 2000
Improve methods to determine capacity
and quality of service of two-lane
highways
Recommend MOEs and additional
performance measures
Improved methods for capacity and
quality of service analyses of weaving
areas
Complete HCM 2000 document
Develop procedures to determine capacity
and level of service of bus flow on
arterials
Expand field testing and validation of
procedures developed in TCRP A-07
Provide transit input to HCM 2000
Update method for analyzing effects of
pedestrians and bicycles at signalized
intersections; recommend improvements
Develop procedure to determine capacity
and level of service of a freeway facility
NCHRP 3-55(2)A
Planning Applications for the Year 2000
Highway Capacity Manual
Capacity and Quality of Service for Two-
Lane Highways
Performance Measures and Levels of
Service in the Year 2000
Highway
Capacity Manual
Capacity and Quality of Service of Weaving
Areas
Production of the Year 2000
Highway
Capacity Manual
Operational Analysis of Bus Lanes on
Arterials
Operational Analysis of Bus Lanes on
Arterials: Extended Field Investigations
Development of Transit Capacity and
Quality of Service Principles, Practices and
Procedures
Capacity Analysis of Pedestrian and Bicycle
Facilities Project (DTFH61-92-R-00138)
Capacity and Level of Service Analysis for
Freeway Systems Project
(DTFH61-95-Y-00086)
NCHRP 3-55(3)
NCHRP 3-55(4)
NCHRP 3-55(5)
NCHRP 3-55(6)
TCRP A-07
TCRP A-07A
TCRP A-15
FHWA
FHWA
1-7
?
Chapter 1 - Introduction
Reference
AR00042753

Highway Capacity Manual 2000
CHAPTER 2
CAPACITY AND LEVEL-OF-SERVICE CONCEPTS
CONTENTS
I. INTRODUCTION ?
2-1
II. CAPACITY ?
2-2
III. DEMAND ?
2-2
IV. QUALITY AND LEVELS OF SERVICE ?
2-2
Service Flow Rates ?
2-3
Performance Measures ?
2-3
Service Measures ?
2-3
V. FACTORS AFFECTING CAPACITY AND LOS ?
2-3
Base Conditions ?
2-3
Roadway Conditions ?
2-4
Traffic Conditions ?
2-4
Vehicle Type ?
2-4
Directional and Lane Distribution ?
2-5
Control Conditions ?
2-5
Technology ?
2-6
2-i ?
Chapter 2 - Capacity and Level-of-Service Concepts
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Highway Capacity Manual 2000
I. INTRODUCTION
This manual presents methods for analyzing capacity and level of service for a broad
range of transportation facilities. It provides procedures for analyzing streets and
highways, bus and on-street light rail transit, and pedestrian and bicycle paths.
Facilities are classified into two categories of flow: uninterrupted and interrupted.
Uninterrupted-flow facilities have no fixed elements, such as traffic signals, that are
external to the traffic stream and might interrupt the traffic flow. Traffic flow conditions
result from the interactions among vehicles in the traffic stream and between vehicles and
the geometric and environmental characteristics of the roadway.
Interrupted-flow facilities have controlled and uncontrolled access points that can
interrupt the traffic flow. These access points include traffic signals, stop signs, yield
signs, and other types of control that stop traffic periodically (or slow it significantly),
irrespective of the amount of traffic.
Uninterrupted and interrupted flows describe the type of facility, not the quality of
the traffic flow at any given time. A freeway experiencing extreme congestion, for
example, is still an uninterrupted-flow facility because the causes of congestion are
internal.
Freeways and their components operate under the purest form of uninterrupted flow.
Not only are there no fixed interruptions to traffic flow, but access is controlled and
limited to ramp locations. Multilane highways and two-lane highways also can operate
under uninterrupted flow in long segments between points of fixed interruption. On
multilane and two-lane highways, it is often necessary to examine points of fixed
interruption as well as uninterrupted-flow segments.
The analysis of interrupted-flow facilities must account for the impact of fixed
interruptions. A traffic signal, for example, limits the time available to various
movements in an intersection. Capacity is limited not only by the physical space but by
the time available for movements.
Transit, pedestrian, and bicycle flows generally are considered to be interrupted.
Uninterrupted flow might be possible under certain circumstances, such as in a long
busway without stops or along a pedestrian corridor. However, in most situations,
capacity is limited by stops along the facility.
Capacity analysis, therefore, is a set of procedures for estimating the traffic-carrying
ability of facilities over a range of defined operational conditions. It provides tools to
assess facilities and to plan and design improved facilities.
A principal objective of capacity analysis is to estimate the maximum number of
persons or vehicles that a facility can accommodate with reasonable safety during a
specified time period. However, facilities generally operate poorly at or near capacity;
they are rarely planned to operate in this range. Accordingly, capacity analysis also
estimates the maximum amount of traffic that a facility can accommodate while
maintaining its prescribed level of operation.
Operational criteria are defined by introducing the concept of level of service.
Ranges of operating conditions are defined for each type of facility and are related to the
amount of traffic that can be accommodated at each service level.
The two principal concepts of this manual—capacity and level of service—are
defined in the following sections.
Uninterrupted-flow facility
defined
Interrupted-flow facility
defined
Capacity analysis defined
2-1 ?
Chapter 2 - Capacity and Level-of-Service Concepts
Introduction
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Highway Capacity Manual 2000
II. CAPACITY
The capacity of a facility is the maximum hourly rate at which persons or vehicles
reasonably can be expected to traverse a point or a uniform section of a lane or roadway
during a given time period under prevailing roadway, traffic, and control conditions.
Vehicle capacity is the maximum number of vehicles that can pass a given point
during a specified period under prevailing roadway, traffic, and control conditions. This
assumes that there is no influence from downstream traffic operation, such as the backing
up of traffic into the analysis point.
Person capacity is the maximum number of persons that can pass a given point
during a specified period under prevailing conditions. Person capacity is commonly used
to evaluate public transit services, high-occupancy vehicle lanes, and pedestrian facilities.
Prevailing roadway, traffic, and control conditions define capacity; these conditions
should be reasonably uniform for any section of facility analyzed. Any change in the
prevailing conditions changes the capacity of the facility.
Capacity analysis examines segments or points (such as signalized intersections) of a
facility under uniform traffic, roadway, and control conditions. These conditions
determine capacity; therefore, segments with different prevailing conditions will have
different capacities.
Reasonable expectancy is the basis for defining capacity. That is, the stated capacity
for a given facility is a flow rate that can be achieved repeatedly for peak periods of
sufficient demand. Stated capacity values can be achieved on facilities with similar
characteristics throughout North America. Capacity is not the absolute maximum flow
rate observed on such a facility. Driver characteristics vary from region to region, and
the absolute maximum flow rate can vary from day to day and from location to location.
Persons per hour, passenger cars per hour, and vehicles per hour are measures that
can define capacity, depending on the type of facility and type of analysis. The concept
of person flow is important in making strategic decisions about transportation modes in
heavily traveled corridors and in defining the role of transit and high-occupancy vehicle
priority treatments. Person capacity and person flow weigh each type of vehicle in the
traffic stream by the number of occupants it carries.
III. DEMAND
In this manual, demand is the principal measure of the amount of traffic using a
given facility. Demand relates to vehicles arriving; volume relates to vehicles
discharging. If there is no queue, demand is equivalent to the traffic volume at a given
point on the roadway. Throughout this manual, the term volume generally is used for
operating conditions below the threshold of capacity.
IV. QUALITY AND LEVELS OF SERVICE
Quality of service requires quantitative measures to characterize operational
conditions within a traffic stream. Level of service (LOS) is a quality measure describing
operational conditions within a traffic stream, generally in terms of such service measures
as speed and travel time, freedom to maneuver, traffic interruptions, and comfort and
convenience.
Capacity defined
Capacity is defined on
the basis of reasonable
expectancy
Concepts of demand and
volume
Quality and level of
service defined
Chapter 2 - Capacity and Level-of-Service Concepts ?
2-2
Capacity
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Highway Capacity Manual 2000
Six LOS are defined for each type of facility that has analysis procedures available.
Letters designate each level, from A to F, with LOS A representing the best operating
conditions and LOS F the worst. Each level of service represents a range of operating
conditions and the driver's perception of those conditions. Safety is not included in the
measures that establish service levels.
SERVICE FLOW RATES
The analytical methods in this manual attempt to establish or predict the maximum
flow rate for various facilities at each level of service—except for LOS F, for which the
flows are unstable or the vehicle delay is high. Thus, each facility has five service flow
rates, one for each level of service (A through E). For LOS F, it is difficult to predict
flow due to stop-and-start conditions.
The service flow rate is the maximum hourly rate at which persons or vehicles
reasonably can be expected to traverse a point or uniform segment of a lane or roadway
during a given period under prevailing roadway, traffic, and control conditions while
maintaining a designated level of service. The service flow rates generally are based on a
15-min period. Typically, the hourly service flow rate is defined as four times the peak
15-min volume.
Note that service flow rates are discrete values, whereas levels of service represent a
range of conditions. Because the service flow rates are the maximums for each level of
service, they effectively define the flow boundaries between levels of service.
Most design or planning efforts typically use service flow rates at LOS C or D, to
ensure an acceptable operating service for facility users.
PERFORMANCE MEASURES
Each facility type that has a defined method for assessing capacity and level of
service (see Part III of this manual) also has performance measures that can be calculated.
These measures reflect the operating conditions of a facility, given a set of roadway,
traffic, and control conditions. Travel speed and density on freeways, delay at signalized
intersections, and walking speed for pedestrians are examples of performance measures
that characterize flow conditions on a facility.
SERVICE MEASURES
For each facility type, one or more of the stated performance measures serves as the
primary determinant of level of service. This LOS-determining parameter is called the
service measure or sometimes the measure of effectiveness (MOE) for each facility type.
V. FACTORS AFFECTING CAPACITY AND LOS
BASE CONDITIONS
Many of the procedures in this manual provide a formula or simple tabular or graphic
presentations for a set of specified standard conditions, which must be adjusted to account
for prevailing conditions that do not match. The standard conditions so defined are
termed base conditions.
Base conditions assume good weather, good pavement conditions, users familiar
with the facility, and no impediments to traffic flow. Other, more specific base
conditions are identified in each chapter of Part III. Examples of base conditions for
uninterrupted-flow facilities and for intersection approaches are given below.
Base conditions for uninterrupted-flow facilities include the following:
• Lane widths of 12 ft,
Service flow rate defined
Service measure defined
Base conditions defined
2-3 ?
Chapter 2 - Capacity and Level of Service Concepts
Quality and Levels of Service
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Highway Capacity Manual 2000
• Clearance of 6 ft between the edge of the travel lanes and the nearest obstructions
or objects at the roadside and in the median,
• Free-flow speed of 60 mi/h for multilane highways,
• Only passenger cars in the traffic stream (no heavy vehicles),
• Level terrain,
• No no-passing zones on two-lane highways, and
• No impediments to through traffic due to traffic control or turning vehicles.
Base conditions for intersection approaches include the following:
• Lane widths of 12 ft,
Level grade,
• No curb parking on the approaches,
• Only passenger cars in the traffic stream,
• No local transit buses stopping in the travel lanes,
Intersection located in a noncentral business district area, and
No pedestrians.
In most capacity analyses, prevailing conditions differ from the base conditions, and
computations of capacity, service flow rate, and level of service must include
adjustments. Prevailing conditions are generally categorized as roadway, traffic, or
control.
ROADWAY CONDITIONS
Roadway conditions include geometric and other elements. In some cases, these
influence the capacity of a road; in others, they can affect a performance measure such as
speed, but not the capacity or maximum flow rate of the facility.
Roadway factors include the following:
• Number of lanes,
• The type of facility and its development environment,
Lane widths,
• Shoulder widths and lateral clearances,
Design speed,
• Horizontal and vertical alignments, and
Availability of exclusive turn lanes at intersections.
The horizontal and vertical alignment of a highway depend on the design speed and
the topography of the land on which it is constructed.
In general, the severity of the terrain reduces capacity and service flow rates. This is
significant for two-lane rural highways, where the severity of terrain not only can affect
the operating capabilities of individual vehicles in the traffic stream, but also can restrict
opportunities for passing slow-moving vehicles.
TRAFFIC CONDITIONS
Traffic conditions that influence capacities and service levels include vehicle type
and lane or directional distribution.
Vehicle Type
The entry of heavy vehicles—that is, vehicles other than passenger cars (a category
that includes small trucks and vans)—into the traffic stream affects the number of
vehicles that can be served. Heavy vehicles are vehicles that have more than four tires
touching the pavement.
Trucks, buses, and recreational vehicles (RVs) are the three groups of heavy vehicles
addressed by the methods in this manual. Heavy vehicles adversely affect traffic in two
ways:
• They are larger than passenger cars and occupy more roadway space; and
• They have poorer operating capabilities than passenger cars, particularly with
respect to acceleration, deceleration, and the ability to maintain speed on upgrades.
Impact of roadway
conditions
Impact of traffic
conditions
Chapter 2 - Capacity and Level-of-Service Concepts ?
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Factors Affecting Capacity and LOS
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Highway Capacity Manual 2000
The second impact is more critical. The inability of heavy vehicles to keep pace with
passenger cars in many situations creates large gaps in the traffic stream, which are
difficult to fill by passing maneuvers. The resulting inefficiencies in the use of roadway
space cannot be completely overcome. This effect is particularly harmful on sustained,
steep upgrades, where the difference in operating capabilities is most pronounced, and on
two-lane highways, where passing requires use of the opposing travel lane.
Heavy vehicles also can affect downgrade operations, particularly when downgrades
are steep enough to require operation in a low gear. In these cases, heavy vehicles must
operate at speeds slower than passenger cars, forming gaps in the traffic stream.
Trucks cover a wide range of vehicles, from lightly loaded vans and panel trucks to
the most heavily loaded coal, timber, and gravel haulers. An individual truck's
operational characteristics vary based on the weight of its load and its engine
performance.
RVs also include a broad range: campers, both self-propelled and towed; motor
homes; and passenger cars or small trucks towing a variety of recreational equipment,
such as boats, snowmobiles, and motorcycle trailers. Although these vehicles might
operate considerably better than trucks, the drivers are not professionals, accentuating the
negative impact of RVs on the traffic stream.
Intercity buses are relatively uniform in performance. Urban transit buses generally
are not as powerful as intercity buses; their most severe impact on traffic results from the
discharge and pickup of passengers on the roadway. For the methods in this manual, the
performance characteristics of buses are considered to be similar to those of trucks.
Directional and Lane Distribution
In addition to the distribution of vehicle types, two other traffic characteristics affect
capacity, service flow rates, and level of service: directional distribution and lane
distribution. Directional distribution has a dramatic impact on two-lane rural highway
operation, which achieves optimal conditions when the amount of traffic is about the
same in each direction. Capacity analysis for multilane highways focuses on a single
direction of flow. Nevertheless, each direction of the facility usually is designed to
accommodate the peak flow rate in the peak direction. Typically, morning peak traffic
occurs in one direction and evening peak traffic occurs in the opposite direction. Lane
distribution also is a factor on multilane facilities. Typically, the shoulder lane carries
less traffic than other lanes.
CONTROL CONDITIONS
For interrupted-flow facilities, the control of the time for movement of specific
traffic flows is critical to capacity, service flow rates, and level of service. The most
critical type of control is the traffic signal. The type of control in use, signal phasing,
allocation of green time, cycle length, and the relationship with adjacent control measures
affect operations. All of these are discussed in detail in Chapters 10 and 16.
Stop signs and yield signs also affect capacity, but in a less deterministic way. A
traffic signal designates times when each movement is permitted; however, a stop sign at
a two-way stop-controlled intersection only designates the right-of-way to the major
street. Motorists traveling on the minor street must stop and then find gaps in the major
traffic flow to maneuver. The capacity of minor approaches, therefore, depends on traffic
conditions on the major street. An all-way stop control forces drivers to stop and enter
the intersection in rotation. Capacity and operational characteristics can vary widely,
depending on the traffic demands on the various approaches.
Other types of controls and regulations can affect capacity, service flow rates, and
LOS significantly. Restriction of curb parking can increase the number of lanes
available on a street or highway. Turn restrictions can eliminate conflicts at intersections,
increasing capacity. Lane use controls can allocate roadway space to component
Impact of control conditions
2-5 ?
Chapter 2 - Capacity and Level-of-Service Concepts
Factors Affecting Capacity and LOS
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Highway Capacity Manual 2000
movements and can create reversible lanes. One-way street routings can eliminate
conflicts between left turns and opposing traffic.
TECHNOLOGY
Emerging transportation technologies, also known as intelligent transportation
systems (ITS), will enhance the safety and efficiency of vehicles and roadway systems.
ITS strategies aim to increase the safety and performance of roadway facilities. For this
discussion, ITS includes any technology that allows drivers and traffic control system
operators to gather and use real-time information to improve vehicle navigation, roadway
system control, or both.
To date, there has been little research to determine the impact of ITS on capacity and
level of service. The procedures in this manual relate to roadway facilities without ITS
enhancements.
Current ITS programs might have the following impacts on specific capacity
analyses:
• For freeway and other uninterrupted-flow highways, ITS might achieve some
decrease in headways, which would increase the capacity of these facilities. In addition,
even with no decrease in headways, level of service might improve if vehicle guidance
systems offered drivers a greater level of comfort than they currently experience in
conditions with close spacing between vehicles.
• For signal and arterial operations, the major benefits of ITS would be a more
efficient allocation of green time and an increase in capacity. ITS features likely will
have a less pronounced impact on interrupted flow than on uninterrupted-flow facilities.
• At unsignalized intersections, capacity improvements might result if ITS assisted
drivers in judging gaps in opposing traffic streams or if it somehow controlled gaps in
flow on the major street.
Many of these ITS improvements—such as incident response and driver information
systems—are occurring at the system level. Although ITS features will benefit the
overall roadway system, they will not have an impact on the methods to calculate
capacity and level of service for individual roadways and intersections.
Intelligent transportation
systems
Chapter 2 - Capacity and Level-of-Service Concepts ?
2-6
Factors Affecting Capacity and LOS
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Highway Capacity Manual 2000
CHAPTER 3
APPLICATIONS
CONTENTS
I. INTRODUCTION ?
3-1
II. FRAMEWORK FOR APPLICATION OF THE HCM ?
3-1
Analysis of Individual Elements ?
3-1
System Analysis ?
3-1
Range of Operational Conditions Covered ?
3-4
III. ANALYSIS OBJECTIVES ?
3-5
Levels of Analysis ?
3-5
HCM Analyses as Part of a Broader Process ?
3-6
IV. REFERENCES ?
3-7
EXHIBITS
Exhibit 3-1. ?
Facilities and Road User Types Included in HCM Analyses ?
3-2
Exhibit 3-2. ?
Example of HCM Application to Analysis of Urban Systems ?
3-3
Exhibit 3-3. ?
Components of HCM Analysis of Urban Systems ?
3-4
Exhibit 3-4. ?
Levels and Objectives of Typical HCM Analyses ?
3-6
Exhibit 3-5. ?
HCM Performance Measures for Environmental and
Economic Analyses ?
3-8
3-i ?
Chapter 3 - Applications
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Highway Capacity Manual 2000
I. INTRODUCTION
This chapter provides an overview of the
Highway Capacity Manual
(HCM)
analyses and describes how to apply them to a range of facilities. The scope of the
manual and the framework for its application is followed by a description of the levels at
which an analyst can apply the methods. The chapter concludes with an outline of how to
use HCM analyses as input to other models.
II. FRAMEWORK FOR APPLICATION OF THE HCM
ANALYSIS OF INDIVIDUAL ELEMENTS
The purpose of the HCM is to produce estimates of performance measures for
individual elements or facilities of a transport system, as well as to combine those
elements to expand the view of the system. Exhibit 3-1 tabulates the various system
elements for which the HCM provides analysis methodologies. The chapters shown
appear in Part III of the HCM, which deals with methodologies. Other chapters provide
background on related concepts.
SYSTEM ANALYSIS
Measures of effectiveness (MOEs)—performance measures that can be estimated
quantitatively—are produced for individual system elements (and in some cases,
subelements) by the methods in each chapter of Part III. These measures allow
combination of the elements to produce an expanded view of a facility. For example, an
analysis of a signalized intersection might consider individual movements, or groups of
movements, on each approach. The results then can be successively combined to
determine MOEs for each approach, each street, and the intersection as a whole.
Similarly, the outputs from models for analyzing each element of a freeway facility can
be combined to provide a result for a section of the freeway, including ramp junctions,
weaving segments, and basic segments.
It is also possible to extend this procedure by combining the results of analyses of
individual facilities to represent successively larger portions of a whole system, as
addressed in Part IV of this manual. A system includes the corridors, with one or more
types of facility or mode, as well as the areas representing all or part of the transportation
network under study.
Exhibit 3-2 depicts a system analysis—combining the analyses of individual
elements to produce an aggregate view of a facility, a corridor, or an area. The diagram
provides an example that applies only to urban systems. Each box represents a method of
analysis covered in this manual, indicating the element, or combination of elements,
included. The box also indicates the chapter in which the applicable methodology is
presented (Parts III and IV); however, there are also materials in other parts of the manual
that might apply, especially in Part II. Finally, each box indicates the appropriate
performance measures that can be derived from the chapter and that are applicable to a
system analysis.
In general, speed and delay are the variables that derive from an analysis of
individual elements and that can be used to calculate measures for system analysis.
Usually this is done by converting the estimates of speed and delay into travel times and
then aggregating the travel times across individual elements. In some cases, however,
speed and delay can be averaged and used as performance measures even at aggregate
levels.
Concept of system analysis
3-1 ?
Chapter 3 - Applications
Introduction
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Highway Capacity Manual 2000
EXHIBIT 3-1. FACILITIES AND ROAD USER TYPES INCLUDED IN HCM ANALYSES
Service
Reference
Performance Measure Used
Element
Chapter'
Measureb
Points on
to Calculate Travel Time
Exhibit 3-3
Systems Analysis
Vehicular
Interrupted Flow
Urban street
15
speed
L, P
speed
Signalized intersection
16
delay
H, 0
delay
Two-way stop intersection
17
delay
I, J, M, N
delay
All-way stop intersection
17
delay
I, J, M, N
delay
Roundabout
17
c
K
delay
Interchange ramp terminal
26
delay
Q, R, S
delay
Uninterrupted Flow
Two-lane highway
20
speed, percent
time-spent-
following
speed
Multilane highway
21
density
speed
Freeway
Basic segment
23
density
B, X, Z
speed
Ramp merge
25
density
A, E, V, Y
speed
Ramp diverge
25
density
C, D, G, U, W
speed
Weaving
24
speed
F
speed
Other Road Users
Transit
27
d
e
speed
Pedestrian
18
space, delay
f
speed, delay
Bicycle
19
event, delay
g
speed, delay
Notes:
a. Only Part Ill chapters are listed. When performing planning level analyses, the analyst should refer to Part II, for further
guidelines and for selection of default values.
b. The service measure for a given facility type is the primary performance measure and determines the level of service.
c. HCM does not include a method for estimating performance measures for roundabouts. Non-HCM models that produce a
delay estimate must be employed.
d. Several measures capture the multidimensional nature of transit performance when defining LOS; see Chapter 27.
e. Transit facilities, such as buses in mixed traffic, buses on exclusive lanes, buses in high-occupancy vehicle (HOV) lanes, and
rail vehicles, can be analyzed separately as a transit system, or combined for a multimodal analysis.
f. Pedestrian facilities, such as sidewalks and walkways, form a system and can be analyzed separately. Pedestrian delay at
signalized intersections can be predicted or measured, and a multimodal analysis can include estimates of person delay, person
travel time, and speed.
g. Bicycle facilities—such as bicycles in traffic, bicycle lanes, and separate bicycle paths—form a system and can be analyzed
separately. Speed of bicycles in traffic and on bicycle lanes can be predicted or measured, and a multimodal analysis can
include estimates of person delay, person travel time, and speed.
The boxes referring to the basic analysis of individual elements are placed on the
periphery of the diagram. The results of these analyses are aggregated at successively
higher levels, until the objective is achieved. For example, Chapter 15 shows the analyst
how to combine the results of delay estimates for unsignalized and signalized
intersections with speed and travel time on the links between these points, to determine
an average speed for an urban street segment. The analysis of a street segment can
include pedestrian, bicycle, and transit modes. These can be combined with parallel
segments to arrive at a result for a corridor analysis. A corridor analysis (Chapter 29) can
involve combining results from analyses of uninterrupted-flow facilities, as well as
transit, pedestrian, and bicycle facilities. Areawide analysis is the highest level of study
possible (Chapter 30). The systems analyses that can be performed using this manual are
shown in the central box of Exhibit 3-2.
Chapter 3 - Applications ?
3- 2
Framework for Application of the HCM
AR00042763

Legend
(-
Urban street segment<
(15)
-
4 ?
Analysis element
Applicable chapter
Applicable performance measure
(used in computing travel time)
Buses operating in
mixed traffic
(27)
_Speed
Corridor analysis'
(auto only or
multi modal)
(29)
Areawide analysis'
(auto only or
multi modal)
(30)
Exclusive arterial ?
(
-
Freeway HOV
street bus facility ?
facility
(27) ? (27)
\Speed
?
I ?
Speed
Freeway facilities (auto
only)
(22)
Speed/delay
Systems Analyses
I
rban street corrido- r ?
r
Urban
?
street network'
--
(auto only or ?
(auto only or multimodal)
multimodal) ?
(30)
429)
Freeway Elements
Weaving seg ment
(24)
Speed
r
Entrance ramp
(25)
Speed
rExit ramp
(25)
Speed
rBasic freeway
seg ment
(23)
_Speed
Urban street segment
(15)
S
peed
Freeway network'
(auto only or
multimodal)
(30)
Public Transit Elementsb
Interrupted Flow
(lwo-way or
stop control
(17)
\Delay
l'Z
--
oundabouta
(17)
Capacity
Signal control
(16)
Qi)elay
dnterchange
ramp
terminal
(26)
qi)elay
Pedestrian and Bicycle Facilitiesd
(0n-street bicycle
facility
(19)
Speed/dela
y
Off-street bicycle facility
(19)
peed/delay
Pedestrian facility
(18)
Speed/delay
Highway Capacity Manual 2000
EXHIBIT 3-2. EXAMPLE OF HCM APPLICATION TO ANALYSIS OF URBAN SYSTEMS
Notes:
a. Current HCM methods do not provide models for estimating delay at roundabouts. The user may employ other models to
complete the analysis.
b. Public transit elements can be analyzed as a separate system, using a variety of performance measures provided in Chapter
27, or as part of a larger system using travel speed as the common performance measure.
c. The chapters on corridor and areawide analysis do not specify a specific MOE for defining LOS. Instead, performance measures
are defined for five dimensions: quantity of service produced by the system; intensity of congestion; extent of congestion; variability
of the measures; and accessibility.
d. Pedestrian and bicycle elements can be analyzed as a separate system, using the performance measures provided in Chapters
18 and 19, or as part of a larger system using travel speed as the common performance measure.
Exhibit 3-3 is a schematic of a typical urban network. The interrupted-flow elements
along an arterial are included when determining LOS for urban street segments; for
example, analysis of urban street Segment L will include the results from analysis of
Intersections H, I, J, and K. These may be further combined for an arterial corridor
analysis (designated as 2 in the exhibit). Similarly, the freeway facility (designated by 1)
is a combination of the individual elements within it. A freeway corridor analysis
combines the freeway with one or more parallel arterials. An area analysis (designated
by 3) further accumulates the values for the appropriate performance measures from
preceding stages. System analyses can consider only one mode or user type or combine
several modes or user types.
An example of how to
aggregate individual elements
of urban systems to perform a
system analysis
3-3 ?
Chapter 3 - Applications
Framework for Application of the HCM
AR00042764

0
4
6 0
R
8 ?
0 ?
0
0
■■
D'10
CDICD-t
o
b
c
.
.,
0
CD
E
E ?
))
0
E
g ?
A
Legend
O
Signal
E Two-way or all-way stop
0
Roundabout
Interchange
°Individual analysis elements
°System analysis
Highway Capacity Manual 2000
EXHIBIT 3-3. COMPONENTS OF HCM ANALYSIS OF URBAN SYSTEMS
ED
Looking at Exhibit 3-1, the right portion identifies the performance measures used to
compute travel time and to analyze the constituent elements of the system in Exhibit 3-3.
Exhibit 3-2 lists the chapters in HCM Parts III and IV that include guidelines and
methods for combining performance measures.
RANGE OF OPERATIONAL CONDITIONS COVERED
The HCM can be used to analyze a wide range of operational conditions. The
methodologies can determine the performance and LOS for undersaturated conditions
and, in some cases, for oversaturated conditions. There are two primary ways of dealing
with oversaturation: one is to conduct analyses over successive 15-min periods of
congestion; the other is to account for queue interference when downstream conditions
cause queue buildup to affect upstream elements.
The analyst can work with individual 15-min periods, or hourly periods for which
peak-hour factors are established. This flexibility expedites analyses over several hours
of the day, allowing the analyst to consider both peak and off-peak conditions, as well as
24-h totals.
Chapter 3 - Applications ?
3-4
Framework for Application of the HCM
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Highway Capacity Manual 2000
III. ANALYSIS OBJECTIVES
HCM analyses produce information for decision making. Users of the manual
generally are trying to achieve one of three objectives: identify problems, select
countermeasures (a priori evaluation), or evaluate previous actions (post hoc).
Problems usually are identified when performance measures for a network or a
facility—or a portion of one—do not meet established standards. For example, when the
service on a facility falls below LOS D, the resultant queuing might interfere with
operation upstream. Although the HCM is well suited for predicting performance
measures, an analyst studying current conditions should make direct field measurements
of the performance attributes. These direct measurements then can be applied in the same
manner as predicted values to determine LOS. The HCM, however, is particularly useful
when a current situation is being studied in the context of future conditions, or when an
entirely new element of the system is being considered for implementation.
Once a problem is identified in measurable terms, the analyst can establish the likely
underlying causes and countermeasures, with the goal of making operational
improvements. For example, an analyst might identify a problem with pedestrian
queuing at an intersection. Review of the physical conditions leads to several alternative
countermeasures, including removal of sidewalk furniture or expanding the sidewalk
area. These countermeasures can be tested for any attribute of the facility that is reflected
in the HCM models. For example, an analyst can compare alternatives for intersection
control, certain geometric design improvements, or improvements in traffic signal timing.
Historically, there is little evidence that the HCM has been used to evaluate the
effectiveness of actions once they have been implemented, but it can be useful for this.
However, it is imperative to make direct field measurements of the appropriate
performance measures while working within the general framework of the HCM process.
LEVELS OF ANALYSIS
The levels of analyses commonly performed by users of the HCM can be grouped
into three categories: operational, design, and planning.
Operational analyses are applications of the HCM generally oriented toward current
or anticipated conditions. They aim at providing information for decisions on whether
there is a need for minor, typically low-cost, improvements that can be implemented
quickly. Occasionally, an analysis is made to determine if a more extensive planning
study is needed. Sometimes the focus is on a network, or a part of one, that is
approaching oversaturation or an undesirable LOS: When, in the near term, is the facility
likely to fail? Answering this question requires an estimate of the service flow rate
allowable under a specified LOS.
HCM analyses also help in making decisions about operating conditions. Typical
alternatives often involve the following: lane-use configurations, application of traffic
control devices, signal timing and phasing, spacing and location of bus stops, frequency
of bus service, and addition of an HOV lane or a bicycle lane. The analysis produces
operational measures for a comparison of the alternatives.
Because of the immediate, short-term focus of operational analyses, it is possible to
provide detailed inputs to the models. Many of the inputs may be based on field
measurements of traffic, physical features, and control devices. Generally, the use of
default values is inappropriate at this level of analysis.
Design analyses apply the HCM primarily to establish the detailed physical features
that will allow a new or modified facility to operate at a desired LOS. Design projects
usually are targeted for mid- to long-term implementation. Not all the physical features
that a designer must determine are reflected in the HCM models. Typically, analysts
using the HCM are seeking to determine such elements as the basic number of lanes
required and the need for auxiliary or turning lanes. However, an analyst also can use the
HCM to establish values for elements such as lane width, steepness of grade, the length
Why an analyst might want to
use the HCM
Operational, design, and
planning analyses
3-5
?
Chapter 3 - Applications
Analysis Objectives
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Highway Capacity Manual 2000
Environmental impact
analysis
of added lanes, the size of pedestrian queuing areas, sidewalk and walkway widths, and
the dimensions of bus turnouts.
The data required for design analyses are fairly detailed and are based substantially
on proposed design attributes. However, the intermediate- to long-term focus of the work
will require use of some default values. This simplification is justified in part by the
limits on the accuracy and precision of the traffic predictions with which the analyst will
be working.
Planning analyses are applications of the HCM generally directed toward strategic
issues; the time frame usually is long-term. Typical studies address the possible
configuration of a highway system (or portion of one); a set of bus routes; the expected
effectiveness of a new rail service; or the likely impact of a proposed development. An
analyst often must estimate the future times at which the operation of the current and
committed systems will fall below the desired LOS. Planning studies also can assess
proposed systemic policies, such as lane-use control for heavy vehicles, application of
systemwide freeway ramp metering, and the use of demand-management techniques,
such as congestion pricing.
Exhibit 3-4 demonstrates the general relationship between the levels of analysis and
their objectives. Each of the methodological chapters (Part III of the HCM) has one basic
method adapted to facilitate each of the levels of analysis. Planning analyses generally
are simplified by using more default values than analyses of design and operations.
EXHIBIT 3-4. LEVELS AND OBJECTIVES OF TYPICAL HCM ANALYSES
Level of Analysis
Analysis Objective
Problem Identification
Countermeasure
Selection (A Priori)
Evaluation
(Post Hoc)
Operational
Design
Planning
Primary
Not applicable
Secondary
Primary
Primary
Primary
Primary
Secondary
Not applicable
HCM ANALYSES AS PART OF A BROADER PROCESS
Since its first edition in 1950, the HCM has provided transportation analysts with the
analytical tools to estimate traffic operational measures such as speed, density, and delay.
It also has provided insights and specific tools for estimating the effects of various traffic,
roadway, and other conditions on the capacity of facilities. In the past 10 to 15 years, the
calculated values from the HCM increasingly have been used in other transportation
work, such as project analysis both in terms of the environment and in terms of user costs
and benefits. This practice of using estimated or calculated values from HCM work as
the foundation for estimating user costs and benefits in terms of economic value,
environmental changes (especially air and noise), and even implications on safety, is
particularly pronounced in transportation priority programs and in the justification of
projects. A good description of what non-HCM users do with HCM-produced material is
found in a handbook,
Environmental and Energy Considerations (1,
p. 447):
The environmental analyst is required carefully and objectively to
examine project data provided by transportation planners and
designers, review existing environment laws and regulations which
may affect the project, make appropriate calculations of impact,
compare impact values against acceptable criteria, and
recommend mitigation where needed.
In a similar manner, the economic analysis of transportation improvements depends
heavily on information generated directly through use of the HCM. From an authoritative
source of traditional road user benefit and cost analysis, the following excerpt indicates
the degree to which such analyses depend on the HCM:
Chapter 3 - Applications ?
3-6
Analysis Objectives
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Many of the highway user cost factors in this manual are shown as
a function of either traffic speed or of the ratio of traffic volume to
highway capacity (v/c ratio). The key highway design and traffic
characteristics that define capacity and traffic speed can be
translated into these parameters through the use of such
documents as the
Highway Capacity Manual
(2,
p. 1).
This indicates the strong link between economic analysis and HCM results.
A paper in
Transportation Quarterly
identifies the need for measures of performance
that take into account person movement through a system or area
(3).
The paper suggests
that by taking both accessibility and mobility into account, an areawide measure of
service level can be developed. Also, many environmental analyses (e.g., of ozone
formation) and economic analyses (e.g., of vehicle miles of travel or system hours of
travel) can be conducted only from a systemwide or areawide perspective.
The three performance measures that play key roles in programs related to the Clean
Air Act Amendments of 1990 and in related air quality monitoring are vehicle miles of
travel, vehicle trips, and average travel speeds. These measures also are applicable to
assessments of air quality
(1) .
This manual provides a measure of average travel speeds
for many facility types, but in some cases uses another measure (such as density) to
describe LOS. The Intermodal Surface Transportation Efficiency Act regulations of 1991
specify that the movement of people and not just vehicles should be measured in the
ongoing monitoring programs. Part IV of this manual addresses person movement in the
context of corridor and areawide analyses.
The economic analysis of highway improvements is an important decision-making
tool. A recent analysis of a highway widening project
(4)
referred to the HCM (1985
edition), using average running speed along the highway in question as the important
variable in the model. In addition to running speed and delay, the model's major
component was the change in number of accidents from before to after the highway
improvement. It is noteworthy that some 95 percent of the benefits ascribed to the project
came from delay savings and from reductions in vehicle operating costs—both measures
calculated with the foundation of HCM speed data.
In summary, almost all economic analyses and all air and noise environmental
analyses have relied directly on one or more measures estimated or produced with HCM
calculations. Exhibit 3-5 lists the performance measures from this manual that are
applicable to environmental or economic analyses.
IV. REFERENCES
1. Environmental and Energy Considerations.
Transportation Engineering
Handbook, Institute of Transportation Engineers, Washington, D.C., 1991.
2. A
Manual of User Benefit Analysis of Highway and Bus Transit Improvements.
American Association of State Highway and Transportation Officials, Washington,
D.C., 1977.
3.
Ewing, R. Measuring Transportation Performance.
Transportation Quarterly,
Winter, 1995, pp. 91-104.
4. Wildenthal, M. T., J. L. Buffington, and J. L. Memmott Application of a User
Cost Model To Measure During and After Construction Costs and Benefits:
Highway Widening Projects. In
Transportation Research Record 1450,
TRB,
National Research Council, Washington, D.C., 1995, pp. 38-43.
Economic analysis
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EXHIBIT 3-5. HCM PERFORMANCE MEASURES FOR ENVIRONMENTAL AND ECONOMIC ANALYSES
Chapter
Performance Measure
("Service Measure)
Appropriate for Use
Air
Noise
Economic
15 Urban Streets
Travel speed"
Running time
Intersection control delay
Ai
Ai
Ai
Ai
Ai
Ai
Ai
16 Signalized Intersections
Control delay"
v/c ratio
Ai
Ai
Ai
Ai
17 Unsignalized Intersections
Control delay"
Queue length
v/c ratio
Ai
Ai
Ai
Ai
Ai
Ai
Ai
18 Pedestrians
Space"
Pedestrian delay"
Speed
v/c ratio
Ai
Ai
19 Bicycles
Hindrance"
Events
Control delay"
Travel speed"
Ai
Ai
20 Two-Lane Highways
Percent time-spent-following"
Speed"
Ai
Ai
Ai
21
Multilane Highways
Density"
Speed
v/c ratio
Ai
Ai
Ai
Ai
Ai
22 Freeway Facilities
Density
Veh-h delay
Speed
Travel time
Ai
Ai
Ai
Ai
Ai
23 Basic Freeway Segments
Density"
Speed
v/c ratio
Ai
Ai
Ai
Ai
Ai
24 Freeway Weaving
Density"
Weaving speed
Nonweaving speed
Ai
Ai
Ai
Ai
Ai
Ai
25 Ramps and Ramp Junctions
Density"
Speed
Ai
Ai
Ai
26 Interchange Ramp Terminals
Control delay"
Ai
Ai
27 Transit
Service frequency"
Hours of service"
Passenger loading"
Reliability"
Ai
Ai
Ai
Ai
Ai
Ai
Ai
Ai
Ai
Ai
Ai
Ai
Chapter 3 - Applications ?
3-8
References
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CHAPTER 4
DECISION MAKING
CONTENTS
I. INTRODUCTION ?
4-1
II. DECISION MAKING ?
4-1
Types of Decisions to Which the HCM Applies ?
4-1
Operational ?
4-1
Design ?
4-2
Planning ?
4-2
Roles of Performance, Effectiveness, and Service Measures and LOS ?
4-2
III. PRESENTING RESULTS TO FACILITATE INTERPRETATION ? 4-3
Selecting Appropriate Measures ?
4-3
Understanding Sensitivity of Measures ?
4-4
Graphic Representation of Results ?
4-4
IV. REFERENCES ?
4-6
EXHIBITS
Exhibit 4-1. ?
Example of a Graphic Display of LOS ?
4-4
Exhibit 4-2. ?
Example of a Thematic Graphic Display of LOS ?
4-5
Exhibit 4-3. ?
Example of a Cost-Effectiveness Graph ?
4-5
4-i ?
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I. INTRODUCTION
This chapter explains how to use the results of the
Highway Capacity Manual
(HCM) analyses in making decisions for planning, designing, and operating
transportation facilities. It begins with the types of decisions to which the HCM usually
is applied; discusses the role of measures of effectiveness (MOEs), level of service
(LOS), and other performance measures; and concludes with some guidelines and
examples on the presentation of results to facilitate interpretation.
II. DECISION MAKING
TYPES OF DECISIONS TO WHICH THE HCM APPLIES
Chapter 3 has described the analysis levels of operational, design, and planning.
This section now turns to the types of decisions frequently associated with each of these
levels. Combining service measures with performance measures allows the user to match
the evaluation process to the problem at hand. However, decisions related to safety
cannot be made effectively using the methodologies and performance measures in the
HCM.
Operational
Operational analyses generally identify the existence and nature of a problem.
Therefore, in making any decision, an analyst first considers whether a given element,
facility, area, or system has a potential problem requiring study. In this case, the analyst
simply decides if there is or will be a problem. This is what highway needs studies do.
The prediction models of the HCM can be used even if the performance cannot be
directly measured in the field. The analyst often uses the HCM as a framework to
document a problem about which the agency has been alerted by the public or by other
agencies.
However, operational analyses often do not end with the confirmation of a problem.
They usually also entail a decision on how the problem might be remedied (i.e., through
countermeasures). Typically, several alternatives for improvement are proposed, leading
to the next decision. One alternative must be selected as the recommended plan. The
HCM can be used to predict the change in performance measures for each alternative, to
help in selecting and recommending a plan.
Decisions that use results from the HCM include choosing among alternatives for
intersection controls, for signal phasing and timing arrangements, and for minor changes
to control and marking (e.g., location of parking and bus stops, reconfiguring the number
and the use of lanes, frequency of bus service, and relocating or eliminating street
furniture for pedestrians), as well as choosing among a combination of actions.
There also may be a need to decide on the feasibility of a proposed operational
improvement. The addition of exclusive turning lanes or the extension of existing turning
lanes can be considered at intersections. Another example is that a bicycle lane or a high-
occupancy vehicle lane might be recommended for placement within the current right-of-
way of an urban street. HCM analyses can determine if the space lost to other modes of
travel (i.e., pedestrians and other vehicles) will result in an unacceptably low LOS,
making the alternative unfeasible.
HCM methods are used to estimate performance measures for assessing alternative
actions. Combined with other factors as desired, these then can assist decision makers in
comparing alternatives and choosing the most appropriate course.
Examples of decisions for
which the HCM can be used
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Design
Design determinations for which the HCM is used most commonly involve decisions
on the number of lanes, or the amount of space, needed to operate a facility at a desired
LOS. For example, if a basic freeway segment is to be designed for an LOS with a
service flow rate of 2,000 passenger cars per hour per lane (pc/h/ln) and the demand flow
rate is 4,500 pc/h, the number of lanes required is calculated as 2.25 (from 4,500/2,000).
Based on this information only, the analyst might choose to design the segment with three
lanes. However, the segment may be one of several alternative designs under
consideration. Others might have better geometrics, closer to base conditions, and might
result in a higher service flow rate, indicating a need for only two lanes.
This is the simplest form of design determination found in the HCM. The
relationship between service flow rate and geometrics and controls is much more
complex for other facility types covered—computing the number of lanes required is not
a simple matter. The HCM can be used to select among alternative designs either by
comparing the LOS at which each alternative would operate or by finding the attributes of
the design that result in a targeted LOS.
Planning
HCM analyses are useful for such planning decisions as determining the need to
improve a system (e.g., a highway network). This kind of analysis is similar to an
operational analysis, except that it requires less detail for the inputs and uses a greater
number of default values. The decision not only involves whether improvements are
needed, but if so, what type and where. This is determined by testing a series of
alternatives and comparing their performance measures. The measures produced by the
HCM methodologies either will play a role as criteria for decision making, or they will
act as interim inputs to a planning model that will generate its own performance
measures. Ultimately, the HCM methods produce results that support decision making.
Planning decisions involving the HCM often relate to the feasibility of a new
commercial or residential development. For example, if a shopping center is proposed
for a location, the HCM analyses can be used to decide if the traffic generated by the
development would result in an undesirable quality of service. This decision involves the
determination of service measures, LOS, and other appropriate performance measures
(e.g., v/c ratio and queue lengths). If the development is found unfeasible as proposed,
due to an unacceptable impact on street or intersection operation, the HCM also can be
used to assess alternative improvements to make it feasible. In this way, the HCM can be
used in deciding what should be required of a new commercial or residential development
as well as cost-sharing for any public improvements in conjunction with the development.
For example, the developer might be required to change the location, number, or
geometrics of access points based on tests made using the HCM.
Planning analyses also can be performed to decide on the feasibility of a proposed
policy. For example, if a city is considering a policy to provide special lanes for bicycles
or high-occupancy vehicles, scenarios can be tested to allow decision makers to arrive at
the most appropriate requirements for the policy.
ROLES OF PERFORMANCE, EFFECTIVENESS, AND SERVICE MEASURES
AND LOS
As described in Chapter 2, operations on each facility type or element of the overall
transportation system can be characterized by a set of performance measures, both
qualitative and quantitative. Quantitative measures estimated using the analytical
methods of this manual are termed measures of effectiveness (MOEs). For each facility
type, a single MOE has been identified as the service measure that defines the operating
LOS for the specific facility. (More than one MOE is used in the LOS determination for
transit facilities and for two-lane highways).
Problem identification
Alternative analyses and
design determination
Planning decisions
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Analysis and decision making using the HCM methods almost always involves
estimating or determining a service measure and the related LOS. Parts III and IV
provide methods for generating performance measures in addition to the specific service
measure; these can be useful inputs in decision making. In some cases, performance
measures can be more important to the decision than the LOS rating. An example is the
length of queue caused by oversaturation. If the analysis predicts a problem due to a
queue backup into an upstream intersection, the next steps are to generate and select
alternatives to resolve the problem. Another example is the volume/capacity (v/c) ratio
for signalized intersections. Although delay is used to establish the LOS, the v/c ratio
sometimes can indicate potential problems, even when the LOS is acceptable.
Each of the methodological chapters provides a different set of performance
measures, summarized in Chapter 9. Users of this manual should become familiar with
the performance measures that can be estimated using the HCM, and with how the
performance measures can enhance decision making.
III. PRESENTING RESULTS TO FACILITATE INTERPRETATION
SELECTING APPROPRIATE MEASURES
Several performance measures can result from HCM analyses. Determining the most
appropriate measures to use for a decision depends on the particular case. However,
decision-making situations generally can be divided into those involving the public (e.g.,
city councils or community groups) and those involving technicians (e.g., state or local
engineering staff or transit planners).
The HCM is highly technical and complex. The results of the analyses can be
difficult for people to interpret for decision making, unless the data are carefully
organized and presented. In general, the results should be presented as simply as
possible. This might include using a small set of performance measures and providing
the data in an aggregate form, without losing the ability to relate to the underlying
variations and factors that have generated the results.
The LOS concept was created, in part, to make the presentation of results easier to
understand than if the numerical values of the MOEs and service measures were reported
directly. It is easier to understand a grading scale similar to that of the traditional school
report card than to deal with measures such as density and v/c ratio. Although there are
limitations to their usefulness, LOS ratings remain a part of the HCM because of their
acceptance by the public and elected officials. Decision makers who are not analytically
oriented often prefer to have a single number or letter represent a condition. It is
generally not effective to provide representatives of the public with a large set of differing
measures or with a frequency distribution for a specific performance measure. If the
analyst has several measures available, it is preferable to select the one that best fits the
situation and keep the others in reserve until needed.
Decision makers who represent the public usually prefer measures that their
constituents can understand; the public can relate to LOS grades. Unit delay (e.g.,
seconds per vehicle) and travel speed also are readily understood. However, v/c, density,
percent time spent following, and vehicle hours of travel are not measures to which the
public easily relates. When selecting the measures to present, therefore, it is important
for the analyst to recognize the orientation of the decision maker and the context in which
the decision will be made. In general, these measures can be differentiated as system-
user or system-manager oriented. When making a presentation to technical members of a
public agency, such as highway engineers and planners, it might be necessary to use more
LOS is only one of several
ways to evaluate operational
conditions
Performance measures
selected should be related to
the problem being addressed
4-3 ?
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than one performance measure, especially when providing both the system-user and
system-manager perspectives.
UNDERSTANDING SENSITIVITY OF MEASURES
Once one or more performance measures have been selected for reporting analysis
results, decision making can be improved by demonstrating how the numerical values (or
the LOS letter grade) change when one or more of the assumed input values change. It
can be important for the decision maker to know how an assumed increase of 15 percent
in future traffic volume (compared with the standard forecast volume) will affect delay
and LOS at a signalized intersection. By providing a central value along with values
based on upward and downward assumptions on key input variables (especially volume),
the analyst ensures that decision making is based on a full understanding of sensitivities.
The
Traffic Engineering Handbook (1)
provides examples of tabular presentations of
sensitivity results for signalized intersections.
GRAPHIC REPRESENTATION OF RESULTS
Historically, data and analysis results have been presented primarily in tables.
However, results sometimes are best presented as pictures and only supplemented as
necessary with the underlying numbers. Graphs and charts should not be used to decorate
data or to make dull data entertaining; they should be conceived and fashioned to aid in
the interpretation of the meaning behind the numbers
(2).
Most of the performance measures in the HCM are quantitative, continuous,
variables. LOS grades, however, are qualitative measures of performance; they do not
lend themselves to graphing. When placed on a scale, LOS grades must be given an
equivalent numeric value, as shown in Exhibit 4-1, which presents the LOS for a group of
intersections. The letter grade is indicated, and shaded areas are defined as unacceptable
LOS that do not meet the objective of LOS D. The size of the indicator at each
intersection is intended to show the relative delay values for the indicated LOS.
Evaluate how results
change with input
assumptions
Present results to make
them very plain (obvious)
to the audience
EXHIBIT 4-1. EXAMPLE OF A GRAPHIC DISPLAY OF
LOS
Chapter 4 - Decision Making
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Difference
in
de
lay
Desired LOS
Difference in cost
Highway Capacity Manual 2000
The issue is whether the change in value between successive grades of LOS (i.e., the
interval) should all be shown as equal. For instance, is it appropriate for the LOS Grades
A through F to be converted to a scale of 0 through 5? Should the numerical equivalent
assigned to the difference of the thresholds between LOS A and B be the same as the
difference between LOS E and F? These questions have not been addressed in the
research. Furthermore, LOS F is not given an upper bound. Therefore, a graph of LOS
should be considered ordinal, not interval, because the numeric differences between
levels of service would not appear significant.
However, it is difficult to refrain from comparing the differences. A scale
representing the relative values of the LOS grades would have to incorporate the
judgment of the analyst and the opinions of the public or of decision makers—a difficult
task. A thematic style of graphic presentation, however, avoids this issue. In Exhibit 4-2,
for example, shading is used to highlight time periods and basic freeway segments that do
not meet the objective LOS (in this case, D).
EXHIBIT 4-2. EXAMPLE OF A THEMATIC GRAPHIC DISPLAY OF
LOS
Start Time
Segment I
Segment II
Segment III
Segment IV
5:00 p.m.
A
B
B
A
5:15 p.m.
B
B
D
A
5:30 p.m.
B
B
F
A
5:45 p.m.
B
D
F
A
6:00 p.m.
B
F
F
A
6:15 p.m.
D
F
E
A
6:30 p.m.
D
E
C
A
6:45 p.m.
B
B
B
A
Simple graphics often can facilitate decision making among available alternatives.
For example, in the cost-effectiveness graph shown in Exhibit 4-3, the estimated delays
resulting from alternative treatments have been plotted against their associated cost. The
graph shows more clearly than a tabulation of the numbers that Alternative III both is
more costly and creates higher delay than Alternative II. This eliminates Alternative III.
EXHIBIT 4-3. EXAMPLE OF A COST-EFFECTIVENESS GRAPH
Cost
4 -5 ?
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Whether Alternative I or II should be chosen, however, is a matter for the decision
maker's judgment. Alternative II is more expensive than Alternative I, but is predicted to
deliver a significantly lower delay. A useful measure for decision makers is provided by
the slope of the line between the alternatives, which shows the seconds of delay saved per
dollar of cost.
For this example, assume that Alternative IV provides the minimum acceptable LOS
at significantly less cost than Alternative III. The dashed lines in Exhibit 4-3 indicate the
relative cost-effectiveness of moving from Ito IV or IV to II. The steepest slope, Ito IV,
signifies a high level of cost-effectiveness. The two alternatives that meet or exceed the
LOS objective are II and IV. The most appropriate alternative for selection, therefore, is
Alternative IV.
The HCM provides valuable assistance in making transport management decisions in
a wide range of situations. It offers the user a selection of performance measures to meet
a variety of needs. The analyst should recognize that using the HCM involves a bit of art
along with the science. Sound judgment is needed not only for interpreting the values
produced, but also in summarizing and presenting the results.
IV. REFERENCES
1. Traffic Engineering Handbook.
Institute of Transportation Engineering,
Washington, D.C., 1992.
2. Tufte, E. R.
The Visual Display of Quantitative Information.
Graphics Press,
Cheshire, Connecticut, 1983.
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CHAPTER 5
GLOSSARY
This chapter defines the terms used in the manual.
Acceleration lane - A paved auxiliary lane, including tapered areas, allowing vehicles to
accelerate when entering the through-traffic lane of the roadway.
Access point - An intersection, driveway, or opening on the right-hand side of a roadway.
An entry on the opposite side of a roadway or a median opening also can be
considered as an access point if it is expected to influence traffic flow significantly in
the direction of interest.
Access-point density - The total number of access points on a roadway divided by the
length of the roadway and then averaged over a minimum length of 3 mi.
Accuracy - The degree of a measure's conformity to a standard or true value.
Adjustment - An additive or subtractive quantity that adjusts a parameter for a base
condition to represent a prevailing condition.
Adjustment factor - A multiplicative factor that adjusts a parameter for a base condition
to represent a prevailing condition.
Aggregate delay - The summation of delays for multiple lane groups, usually aggregated
for an approach, an intersection, or an arterial route.
Alighting time - The time required for a passenger to leave a transit vehicle, expressed as
time per passenger or total time for all passengers.
All-way stop-controlled - An intersection with stop signs at all approaches. The driver's
decision to proceed is based on the rules of the road (e.g., the driver on the right has
the right-of-way) and also on the traffic conditions of the other approaches.
Analysis period - A single time period during which a capacity analysis is performed on
a transportation facility. If the demand exceeds capacity during an analysis period,
consecutive analysis periods can be selected to account for initial queue from the
previous analysis period. Also referred to as time interval.
Analytical model - A model that relates system components using theoretical
considerations tempered, validated, and calibrated by field data.
Angle loading area - A bus bay design, similar to an angled parking space, requiring
buses to back up to exit and allowing more buses to stop in the given linear space.
Typically used when buses must occupy berths for a long period of time (e.g., at an
intercity bus terminal).
Annual average daily traffic - The total volume of traffic passing a point or segment of
a highway facility in both directions for one year divided by the number of days in
the year.
Approach - A set of lanes at an intersection that accommodates all left-turn, through, and
right-turn movements from a given direction.
Approach grade - The grade of an intersection approach, expressed as a percentage, with
positive values for upgrade and negative for downgrade.
Area type - A geographic parameter reflecting the variation of saturation flows in
different areas.
Arrival rate - The mean of the statistical distribution of vehicles arriving at a point or
uniform segment of a lane or roadway.
Arrival type - Six assigned categories for determining the quality of progression at a
signalized intersection.
Arterial - A signalized street that primarily serves through-traffic and that secondarily
provides access to abutting properties, with signal spacings of 2.0 mi or less.
Articulated bus or articulated trolleybus - An extralong, high-capacity bus or
trolleybus with a rear body section or sections flexibly but permanently connected to
the forward section. The vehicle can bend for curves but does not require an interior
barrier between its sections.
Acceleration lane—Articulated
bus or articulated trolleybus
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Auxiliary lane—Capacity
Auxiliary lane - An additional lane on a freeway to connect an on-ramp and an off-ramp.
Average travel speed - The length of the highway segment divided by the average travel
time of all vehicles traversing the segment, including all stopped delay times.
Back of queue - The distance between the stop line of a signalized intersection and the
farthest reach of an upstream queue, expressed as a number of vehicles. The vehicles
previously stopped at the front of the queue are counted even if they begin moving.
Base condition - The best possible characteristic in terms of capacity for a given type of
transportation facility; that is, further improvements would not increase capacity; a
condition without hindrances or delays.
Base saturation flow rate - The maximum steady flow rate—expressed in passenger cars
per hour per lane—at which previously stopped passenger cars can cross the stop line
of a signalized intersection under base conditions, assuming that the green signal is
available and no lost times are experienced.
Basic freeway segment - A length of freeway facility whose operations are unaffected by
weaving, diverging, or merging.
Berth - A position for a bus to pick up and discharge passengers, including curb bus
stops and other types of boarding and discharge facilities.
Bicycle - A vehicle with two wheels tandem, propelled by human power, and usually
ridden by one person.
Bicycle facility - A road, path, or way specifically designated for bicycle travel, whether
exclusively or with other vehicles or pedestrians.
Bicycle lane - A portion of a roadway designated by striping, signing, and pavement
markings for the preferential or exclusive use of bicycles.
Bicycle path - A bikeway physically separated from motorized traffic by an open space
or barrier, either within the highway right-of-way or within an independent right-of-
way.
Bicycle speed - The riding speed of bicycles, in miles per hour or feet per second.
Boarding time - The time for a passenger to board a transit vehicle, expressed as time
per passenger or total time for all passengers.
Body ellipse - The space provided per pedestrian on a pedestrian facility, expressed as
square feet per pedestrian.
Bottleneck - A road element on which demand exceeds capacity.
Breakdown - The onset of a queue development on a freeway facility.
Breakdown flow - Also called forced flow, occurs either when vehicles arrive at a rate
greater than the rate at which they are discharged or when the forecast demand
exceeds the computed capacity of a planned facility.
Bus - A self-propelled, rubber-tired road vehicle designed to carry a substantial number
of passengers (at least 16) and commonly operated on streets and highways.
Bus lane - A highway or street lane reserved primarily for buses during specified periods.
It may be used by other traffic under certain circumstances, such as making a right or
left turn, or by taxis, motorcycles, or carpools that meet the requirements of the
jurisdiction's traffic laws.
Bus platoon - A convoy of several buses, with each bus following the operating
characteristics of the one in front.
Bus stop - An area in which one or more buses load and unload passengers. It consists of
one or more loading areas and may be on line or off line.
Busway - A right-of-way restricted to buses by a physical separation from other traffic
lanes.
Calibration - The process of comparing model parameters with real-world data to ensure
that the model realistically represents the traffic environment. The objective is to
minimize the discrepancy between model results and measurements or observations.
Capacity - The maximum sustainable flow rate at which vehicles or persons reasonably
can be expected to traverse a point or uniform segment of a lane or roadway during a
specified time period under given roadway, geometric, traffic, environmental, and
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control conditions; usually expressed as vehicles per hour, passenger cars per hour,
or persons per hour.
Captive riders - Transit riders, such as people with disabilities, the elderly, young
adolescents, and adults without driver's licenses, who do not have alternative means
of travel.
Change interval - The yellow plus all-red interval that occurs between phases of a traffic
signal to provide for clearance of the intersection before conflicting movements are
released.
Circulating flow - The volume of traffic on the principal roadway of a roundabout at a
given time.
Circulating roadway - The continuous-flow section of a roundabout that requires other
vehicles entering the roadway to yield.
Circulation area - The portion of a sidewalk street corner used by moving pedestrians
passing through the area; in square feet.
Clearance lost time - The time, in seconds, between signal phases during which an
intersection is not used by any traffic.
Clearance time - The time loss at a transit stop, not including passenger dwell times.
This parameter can be the minimum time between one transit vehicle leaving a stop
and the following vehicle entering and can include any delay waiting for a sufficient
gap in traffic to allow the transit vehicle to reenter the travel lane.
Climbing lane - A passing lane added on an upgrade to allow traffic to pass heavy
vehicles whose speeds are reduced.
Collector street - A surface street providing land access and traffic circulation within
residential, commercial, and industrial areas.
Commuter rail - The portion of passenger railroad operations that carries passengers
within urban areas, or between urban areas and their suburbs; unlike rapid rail transit,
the passenger cars generally are heavier, the average trip lengths are usually longer,
and the operations are carried out over tracks that are part of the area's railroad
system.
Composite grade - A series of adjacent grades along a highway that cumulatively has a
more severe effect on operations than each grade separately.
Compound left-turn protection - A signal phasing scheme that provides both a
protected and permitted phase in each cycle for a left turn. See also
protected plus
permitted
and
permitted plus protected.
Conflicting approach - The approach opposite the subject approach at an all-way stop-
controlled intersection.
Conflicting flow rate - The flow rate of traffic that conflicts with a specific movement at
an unsignalized intersection.
Conflicting movements - The traffic streams in conflict at an unsignalized intersection.
Congested flow - A traffic flow condition caused by a downstream bottleneck.
Constrained operation - An operating condition in a weaving segment, involving
geometric and traffic constraints, that prevents weaving vehicles from occupying a
large portion of the lanes available to achieve balanced operation.
Control condition - The traffic controls and regulations in effect for a segment of street
or highway, including the type, phasing, and timing of traffic signals; stop signs; lane
use and turn controls; and similar measures.
Control delay - The component of delay that results when a control signal causes a lane
group to reduce speed or to stop; it is measured by comparison with the uncontrolled
condition.
Corridor - A set of essentially parallel transportation facilities designed for travel
between two points. A corridor contains several subsystems, such as freeways, rural
(or two-lane) highways, arterials, transit, and pedestrian and bicycle facilities.
Coverage - The geographical area that a transit system serves, normally based on
acceptable walking distances from loading points.
Captive riders—Coverage
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Crawl speed—Design
speed
Crawl speed - The maximum sustained speed that can be maintained by a specified type
of vehicle on a constant upgrade of a given percent; in miles per hour.
Critical density - The density at which capacity occurs for a given facility, usually
expressed as vehicles per mile per lane.
Critical gap - The minimum time, in seconds, between successive major-stream vehicles,
in which a minor-street vehicle can make a maneuver. Also see
Pedestrian critical
gap.
Critical lane group - The lane groups that have the highest flow ratio for a given signal
phase.
Critical speed - The speed at which capacity occurs for a facility, usually expressed as
miles per hour.
Critical volume-to-capacity ratio - The proportion of available intersection capacity
used by vehicles in critical lane groups.
Cross flow - A pedestrian flow that is approximately perpendicular to and crosses
another pedestrian stream. The smaller of the two flows is the cross-flow condition.
Crosswalk - A marked area for pedestrians crossing the street at an intersection or
designated midblock location.
Crown line - A lane marking that connects from the entrance gore area directly to the
exit gore area.
Crush load - The maximum number of passengers that can be accommodated on a transit
vehicle.
Cycle - A complete sequence of signal indications.
Cycle length - The total time for a signal to complete one cycle.
Deceleration lane - A paved auxiliary lane, including tapered areas, allowing vehicles
leaving the through-traffic lane of the roadway to decelerate.
Default value - A representative value that may be appropriate in the absence of local
data.
Delay - The additional travel time experienced by a driver, passenger, or pedestrian.
Demand - The number of users desiring service on the highway system, usually
expressed as vehicles per hour or passenger cars per hour.
Demand-responsive service - Passenger cars, vans, or buses with fewer than 25 seats,
dispatched by a transit operator in response to calls from passengers or their agents.
Demand starvation - A condition when portions of the green time at a downstream
intersection cannot be used because conditions at an upstream intersection prevent
vehicles from reaching the stop line downstream at an interchange ramp terminal.
Demand to capacity ratio - The ratio of demand flow rate to capacity for a traffic
facility.
Density - The number of vehicles on a roadway segment averaged over space, usually
expressed as vehicles per mile or vehicles per mile per lane. Also see
Pedestrian
density.
Departure headway - The average headway in seconds between two consecutive
vehicles departing from a lane at an all-way stop-controlled intersection.
Descriptive model - A mathematical model that applies concepts or theoretical principles
to represent the behavior of a system.
Design application - Using capacity analysis procedures to determine the size (number
of lanes) required for a specified level of service.
Design category - A type of urban street defined by geometric features and roadside
environment.
Design hour - An hour with a traffic volume that represents a reasonable value for
designing the geometric and control elements of a facility.
Design-hour factor (K-factor) - The proportion of the 24-h volume that occurs during
the design hour.
Design speed - A speed used to design the horizontal and vertical alignments of a
highway.
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Deterministic model - A mathematical model that is not subject to randomness. The
result of one analysis can be repeated with certainty.
Diamond interchange - An interchange that results in two or more closely spaced
surface intersections, so that one connection is made to each freeway entry and exit,
with one connection per quadrant.
Directional design-hour volume - The traffic volume for the design hour in the peak
direction of flow, in vehicles per hour.
Directional distribution - A characteristic of traffic, that volume may be greater in one
direction than in the other during any particular hour on a highway.
Directional flow rate - The flow rate of a highway in one direction.
Directional segment - A length of two-lane highway in one travel direction, with
homogeneous cross sections and relatively constant demand volume and vehicle mix.
Directional split - The directional distribution of hourly volume on a highway, expressed
in percentages.
Diverge - A movement in which a single lane of traffic separates into two lanes without
the aid of traffic control devices.
Double-stream door - A transit vehicle door, generally 3.74 to 4.50 ft wide, that permits
two passengers to board, alight, or board and alight simultaneously.
Downstream - The direction of traffic flow.
Downtown street - A surface facility providing access to abutting property in an urban
area.
Drive-through (pull-through) loading area - A bus bay design for compact areas,
providing several adjacent loading islands, between which buses stop, drive through,
and then exit.
Driver population - A parameter that accounts for driver characteristics and their effects
on traffic.
Duration of congestion - A measure of the maximum amount of time that congestion
occurs anywhere in the transportation system.
Dwell time - The time a transit unit (vehicle or train) spends at a station or a stop,
measured from stopping to starting.
Effective green time - The time during which a given traffic movement or set of
movements may proceed; it is equal to the cycle length minus the effective red time.
Effective red time - The time during which a given traffic movement or set of
movements is directed to stop; it is equal to the cycle length minus the effective
green time.
Effective walkway width - The width, in feet, of a walkway usable by pedestrians, or the
total walkway width minus the width of unusable buffer zones along the curb and
building line.
85th-percentile speed - A speed value that is less than 15 percent of a set of field
measured speeds.
Empirical model - A model that describes system performance based on the statistical
analysis of field data.
Entrance ramp - A ramp that allows traffic to enter a freeway.
Equilibrium distance - The distance between the next upstream ramp and the subject
ramp, or between the next downstream ramp and the subject ramp, that produces a
P FNI or P
FD value indicating that the subject ramp is isolated.
Event - A meeting or a passing on a bicycle facility.
Event-based model - A simulation model that advances from one event to the next,
skipping over intervening points in time when no event occurs.
Exclusive bus lane - A highway or street lane reserved for buses.
Exclusive turn lane - A designated left- or right-turn lane or lanes used only by vehicles
making those turns.
Exit ramp - A ramp for traffic to depart from a freeway.
Deterministic model—Exit
ramp
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Extension of effective
green time—Gore area
Extension of effective green time - The amount of the change and clearance interval at
the end of the phase for a lane group, usable for movement of its vehicles.
Extent of congestion - The maximum geographic extent of congestion on the
transportation system at any one time.
Facility - A length of highway composed of connected sections, segments, and points.
Failure rate - The probability that a bus will find all available loading areas occupied by
other buses at a bus stop.
Far-side stop - A transit stop that requires transit units to cross an intersection before
stopping to serve passengers.
Fixed obstruction - Obstructions along a roadway, including light poles, signs, trees,
abutments, bridge rails, traffic barriers, and retaining walls.
Fixed-route service - Service provided by transit vehicles on a repetitive, fixed schedule
along a specific route, picking up and delivering passengers to specific locations;
each fixed route serves an assigned origin and destination.
Flared approach - A shared right-turn lane that allows right-turning vehicles to complete
their movement while other vehicles are occupying the lane.
Flow rate - The equivalent hourly rate at which vehicles, bicycles, or persons pass a
point on a lane, roadway, or other trafficway; computed as the number of vehicles,
bicycles, or persons passing the point, divided by the time interval (usually less than
1 h) in which they pass; expressed as vehicles, bicycles, or persons per hour.
Flow ratio - The ratio of the actual flow rate to the saturation flow rate for a lane group at
an intersection.
Follow-up time - The time between the departure of one vehicle from the minor street
and the departure of the next vehicle using the same gap under a condition of
continuous queuing, in seconds.
Free flow - A flow of traffic unaffected by upstream or downstream conditions.
Free-flow speed - (1) The theoretical speed of traffic, in miles per hour, when density is
zero, that is, when no vehicles are present; (2) the average speed of vehicles over an
urban street segment without signalized intersections, under conditions of low
volume; (3) the average speed of passenger cars over a basic freeway or multilane
highway segment under conditions of low volume.
Freeway - A multilane, divided highway with a minimum of two lanes for the exclusive
use of traffic in each direction and full control of access without traffic interruption.
Freeway facility - An aggregation of sections comprising basic freeway segments, ramp
segments, and weaving segments.
Fully actuated control - A signal operation in which vehicle detectors at each approach
to the intersection control the occurrence and length of every phase.
Functional category - An urban street defined by the traffic service it provides.
Functional class - A transportation facility defined by the traffic service it provides.
Gap - The time, in seconds, for the front bumper of the second of two successive vehicles
to reach the starting point of the front bumper of the first.
Gap acceptance - The process by which a minor-street vehicle accepts an available gap
to maneuver.
Gate - A point at which a major facility crosses the boundary of a corridor.
Gate tree - A list of segments connected to the entry gate of a corridor.
General terrain - A classification used for analysis in lieu of a specific grade.
Geometric condition - The spatial characteristics of a facility, including approach grade,
the number and width of lanes, lane use, and parking lanes.
Geometric delay - The component of delay that results when geometric features cause
vehicles to reduce their speed in negotiating a facility.
Gore area - The area located immediately between the left edge of a ramp pavement and
the right edge of the roadway pavement at a merge or diverge area.
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Green time - The duration, in seconds, of the green indication for a given movement at a
signalized intersection.
Green time ratio - The ratio of the effective green time of a phase to the cycle length.
Group critical gap - The minimum time during which a platoon of pedestrians will not
attempt to cross a stop-controlled intersection, expressed in seconds.
Growth factor - A percentage increase applied to current traffic demands to estimate
future demands.
Headway - (1) The time, in seconds, between two successive vehicles as they pass a
point on the roadway, measured from the same common feature of both vehicles (for
example, the front axle or the front bumper); (2) the time, usually expressed in
minutes, between the passing of the front ends of successive transit units (vehicles or
trains) moving along the same lane or track (or other guideway) in the same
direction.
Heavy rail - A transit system using trains of high-performance, electrically powered rail
cars operating in exclusive right-of-way.
Heavy vehicle - A vehicle with more than four wheels touching the pavement during
normal operation.
High-occupancy vehicle (HOV) - A vehicle with a defined minimum number of
occupants (>1); HOVs often include buses, taxis, and carpools, when a lane is
reserved for their use.
Hindrance - A concept related to the comfort and convenience of bicyclists, used to
derive level of service for a bicycle facility. Often, the number of events is used as a
surrogate for hindrance.
Impedance - The reduction in the capacity of lower-priority movements, caused by the
congestion of higher-priority movements at a stop-controlled approach.
Incident - Any occurrence on a roadway that impedes the normal flow of traffic.
Incident delay - The component of delay that results from an incident, compared with
the no-incident condition.
Incremental delay - The second term of lane group control delay, it accounts for
nonuniform arrivals and temporary random delays as well as delays caused by
sustained periods of oversaturation.
Influence area - (1) An area that incurs operational impacts of merging vehicles in Lanes
1 and 2 of the freeway and the acceleration lane for 1,500 ft from the merge point
downstream; (2) an area that incurs operational impacts of diverging vehicles in
Lanes 1 and 2 of the freeway and the deceleration lane for 1,500 ft from the diverge
point upstream.
Initial queue - The unmet demand at the beginning of an analysis period, either observed
in the field or carried over from the computations of a previous analysis period.
Initial queue delay - The third term of lane group control delay refers to the delay due to
a residual queue identified in a previous analysis period and persisting at the start of
the current analysis period. This delay results from the additional time required to
clear the initial queue.
Intelligent transportation system (ITS) - A transportation technology that enhances the
safety and efficiency of vehicles and roadway systems.
Intensity of congestion - A measure of the total number of person-hours of delay and
mean trip speed or mean delay per person-trip.
Interchange density - The average number of interchanges per mile, computed for 6 mi
of freeway including the basic freeway segment.
Interchange ramp terminal - A junction with a surface street to serve vehicles entering
or exiting a freeway.
Internal link - The segment between two signalized intersections at an interchange ramp
terminal.
Green time—Internal link
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Internal zone—Load
factor
Internal zone - A diamond-shaped area identified in a corridor analysis for each arterial
street segment that lies between intersections. An internal zone represents the
geographic area likely to generate trips to each segment.
Interrupted flow - A category of traffic facilities characterized by traffic signals, stop
signs, or other fixed causes of periodic delay or interruption to the traffic stream.
Intersection delay - The total additional travel time experienced by drivers, passengers,
or pedestrians as a result of control measures and interaction with other users of the
facility, divided by the volume departing from the corresponding cross section of the
facility.
Interval - A period of time in which all traffic signal indications remain constant.
Isolated intersection - An intersection at least 1 mi from the nearest upstream signalized
intersection.
Jam density - The density at which congestion stops all movement of persons or
vehicles, usually expressed as vehicles per mile per lane or pedestrians per square
feet.
Kiss and ride - An access mode to transit allowing passengers (usually commuters) to be
driven to a transit stop to board a transit unit and then to be met after their return.
Lane 1 - The highway lane adjacent to the shoulder.
Lane 2 - The highway lane adjacent and to the left of Lane 1.
Lane balance - The number of lanes leaving a diverge point is equal to the number of
lanes approaching it, plus one.
Lane distribution - A parameter used when two or more lanes are available for traffic in
a single direction, and the volume distribution varies widely, depending on traffic
regulation, traffic composition, speed and volume, the number of and location of
access points, the origin–destination patterns of drivers, the development
environment, and local driver habits.
Lane group - A set of lanes established at an intersection approach for separate capacity
and level-of-service analysis.
Lane group delay - The control delay for a given lane group.
Lane utilization - The distribution of vehicles among lanes when two or more lanes are
available for a movement; however, as demand approaches capacity, uniform lane
utilization develops.
Lane width - The arithmetic mean of the lane widths of a roadway in one direction,
expressed in feet.
Lateral clearance - (1) The total left- and right-side clearance from the outside edge of
travel lanes to fixed obstructions on a multilane highway; (2) the right-side clearance
distance from the rightmost travel lane to fixed obstructions on a freeway.
Level of service - A qualitative measure describing operational conditions within a traffic
stream, based on service measures such as speed and travel time, freedom to
maneuver, traffic interruptions, comfort, and convenience.
Level terrain - A combination of horizontal and vertical alignments that permits heavy
vehicles to maintain approximately the same speed as passenger cars; this generally
includes short grades of no more than 1 to 2 percent.
Light rail transit (LRT) - A metropolitan electric railway system operating single cars
or short trains along exclusive rights-of-way at ground level, on aerial structures, in
subways, or occasionally in streets; an LRT also can board and discharge passengers
at track or car floor level.
Linear loading area - A bus bay design in which buses stop directly behind each other,
so that the bus in front must leave its bay before the following bus can exit; often
used when buses occupy a bay only for a short time (e.g., at an on-street bus stop).
Link - A segment of highway ending at a major intersection on an urban street or at a
ramp merge or diverge point on a freeway. Links have a node at each end.
Load factor - The number of passengers occupying a transit vehicle, divided by the
number of seats on the vehicle.
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Loading area - (1) A branch from, or widening of, a road that permits buses to stop,
without obstructing traffic, while laying over or while passengers board and alight;
(2) a specially designed or designated location at a transit stop, station, terminal, or
transfer center at which a bus stops to allow passengers to board and alight; (3) a lane
in a garage facility for parking or storing buses, often for maintenance.
Loading island - (1) A pedestrian refuge within the right-of-way and traffic lanes of a
highway or street, designated for transit stops, to protect transit passengers from
traffic while awaiting boarding or alighting; (2) a protected spot for the loading and
unloading of passengers within a rail transit or bus station; (3) a passenger loading
platform in the middle of the street, level with the street or more usually raised to
curb height, for streetcar and light rail systems.
Local bus - A bus that stops for passengers within 250 ft of the stop line of an
intersection approach.
Loop ramp - A ramp requiring vehicles to execute a left turn by turning right,
accomplishing a 90-degree left turn by making a 270-degree right turn.
Lost time - The time, in seconds, during which an intersection is not used effectively by
any movement; it is the sum of clearance lost time plus start-up lost time.
Low floor bus - A bus without steps at its entrances and exits.
Macroscopic model - A mathematical model that employs traffic flow rate variables.
Mainline - The primary through roadway as distinct from ramps, auxiliary lanes, and
collector-distributor roads.
Major diverge segment - A segment in which one freeway segment with multiple lanes
diverges, to form two primary freeway segments.
Major merge segment - A segment in which two primary freeway segments, each with
multiple lanes, merge to form a single freeway segment.
Major street - The street not controlled by stop signs at a two-way stop-controlled
intersection.
Major weaving segment - A weaving segment with at least three entry and exit legs,
each with two or more lanes.
Maximum load point - The point on a transit line or route at which the passenger
volume is the greatest. There is one maximum load point in each direction.
Measure of effectiveness - A quantitative parameter indicating the performance of a
transportation facility or service.
Meeting - An encounter of bicycles or pedestrians moving in the opposite direction of the
subject bicycle flow.
Merge - A movement in which two separate lanes of traffic combine to form a single lane
without the aid of traffic signals or other right-of-way controls.
Mesoscopic model - A mathematical model for the movement of clusters or platoons of
vehicles, incorporating equations to indicate how these clusters interact.
Microscopic model - A mathematical model that captures the movement of individual
vehicles.
Midblock stop - A transit stop located at a point away from intersections.
Minor arterial - A functional category of a street allowing trips of moderate length
within a relatively small geographical area.
Minor movement - A vehicle making a specific directional entry into an unsignalized
intersection from a minor street.
Minor street - The street controlled by stop signs at a two-way stop-controlled
intersection; also referred to as a
side street.
Mixed-traffic bus facility - Buses operating in mixed traffic with automobiles.
Mountainous terrain - A combination of horizontal and vertical alignments causing
heavy vehicles to operate at crawl speeds for significant distances or at frequent
intervals.
Loading area—Mountainous
terrain
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Movement capacity—
Paid area
Movement capacity - The capacity of a specific traffic stream at a stop-controlled
intersection approach, assuming that the traffic has exclusive use of a separate lane,
in passenger cars per hour.
Multilane highway - A highway with at least two lanes for the exclusive use of traffic in
each direction, with no control or partial control of access, but that may have
periodic interruptions to flow at signalized intersections no closer than 2 mi.
Multimodal - A transportation facility for different types of users or vehicles.
Multiple weaving segment - A segment formed when one merge is followed by two
diverge points, or two merge points are followed by one diverge point.
Near-side stop - A transit stop located on the approach side of an intersection. The
transit units stop to serve passengers before crossing the intersection.
No-passing zone - A segment of a two-lane, two-way highway along which passing is
prohibited in one or both directions.
Node - The endpoint of a link; also used interchangeably with point.
Nonweaving flow - The traffic movements in a weaving segment that are not engaged in
weaving movements.
Normative model - A mathematical model that identifies a set of parameters that provide
the best system performance.
Off-line loading - A transit unit (vehicle or train) stops outside the main track or travel
lane, so that its passengers can board and alight and other units can pass.
Off-line model - A mathematical model in which the output neither directly nor
immediately influences traffic operations.
Off-line stop - A location outside the main track or travel lane at which a transit unit
(vehicle or train) stops, so that passengers can board and alight and other units can
pass.
Off-ramp - See
Exit ramp.
Off-street path - A path physically separated from highway traffic for the use of
pedestrians, bicycles, and nonmotorized traffic.
Offset - The difference, in seconds, between the start of green time at the two signalized
intersections of a diamond interchange for through traffic on the internal link or the
time between the start of individual green times and a specified time datum in a
system of signalized intersections.
On-line loading - A station stop for transit units on the main track or travel lane.
On-line model - A model that influences the control system operation in real time.
On-line stop - A transit unit stop in the main track or travel lane.
On-ramp - See
Entrance ramp.
Open fare collection system - A system for collecting transit fares that does not have
turnstiles or fare gates.
Operating margin - The amount of time that a train can run behind schedule without
interfering with following trains.
Operational application - A use of capacity analysis to determine the level of service on
an existing or projected facility, with known or projected traffic, roadway, and
control conditions.
Opposing approach - The approach approximately 180 degrees opposite the subject
approach at an all-way stop-controlled intersection.
Opposing flow rate - The flow rate for the direction of travel opposite to the direction
under analysis.
Overflow queue - Queued vehicles left over from a green phase at a signalized
intersection.
Oversaturation - A traffic condition in which the arrival flow rate exceeds capacity.
Paid area - (1) An area that a passenger may enter only after paying a fare or showing
credentials; (2) a station area set off by barriers or gates to restrict access to transit
only to those who have paid fares or secured passes.
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Paratransit - Transportation services that are more flexible and personalized than
conventional fixed-route, fixed-schedule services; however, such exclusive services
as charter bus trips are not considered paratransit. The vehicles usually are low- or
medium-capacity highway vehicles, and the service often is adjustable to individual
users' requirements.
Parclo - See
Partial cloverleaf interchange.
Park and ride - An access mode to transit in which patrons drive private automobiles or
ride bicycles to a transit station, transit stop, or carpool or vanpool waiting area,
parking in the areas provided.
Partial cloverleaf interchange - Also called a parclo, an interchange with one or two
loop ramps.
Partial diamond interchange - A diamond interchange with fewer than four ramps, so
that not all of the freeway-street or street-freeway movements are served.
Passenger-car equivalent - The number of passenger cars displaced by a single heavy
vehicle of a particular type under specified roadway, traffic, and control conditions.
Passenger service time - The time required for a passenger to board or alight from a
transit vehicle, in seconds per passenger.
Passing - An encounter with a bicycle or pedestrian moving in the same direction as the
subject bicycle flow on a bicycle facility.
Passing lane - A lane added to improve passing opportunities in one direction of travel
on a conventional two-lane highway.
Passing sight distance - The visibility distance required for drivers to execute safe
passing maneuvers in the opposing traffic lane of a two-lane, two-way highway.
Peak-hour factor - The hourly volume during the maximum-volume hour of the day
divided by the peak 15-min flow rate within the peak hour; a measure of traffic
demand fluctuation within the peak hour.
Pedestrian - An individual traveling on foot.
Pedestrian critical gap - The minimum time during which a single pedestrian will not
attempt to cross an intersection, expressed in seconds.
Pedestrian density - The number of pedestrians per unit of area within a walkway or
queuing area, expressed as pedestrians per square feet.
Pedestrian effective green time - The minimum effective green time required to serve a
given pedestrian demand, expressed in seconds.
Pedestrian flow rate - The number of pedestrians passing a point per unit of time,
usually expressed as pedestrians per 15 min or pedestrians per minute.
Pedestrian queuing area - Places such as elevators, transit platforms, and street
crossings, in which pedestrians stand temporarily, while waiting to be served.
Pedestrian space - The average area provided for pedestrians in a moving pedestrian
stream or pedestrian queue, in square feet per pedestrian.
Pedestrian start-up time - The time for a platoon of pedestrians to get under way
following the beginning of the Walk interval, expressed in seconds.
Pedestrian walking speed - The average walking speed of pedestrians, in feet per
second.
Percent time-spent-following - The average percent of total travel time that vehicles
must travel in platoons behind slower vehicles due to inability to pass on a two-lane
highway.
Performance-based planning - A way of relating agency planning and project
implementation to public benefits.
Performance measure - A quantitative or qualitative characteristic describing the quality
of service provided by a transportation facility or service.
Period of unmet demand - The length of time within an analysis period during which
the unmet demand is greater than zero.
Permitted plus protected - Compound left-turn protection that displays the permitted
phase before the protected phase.
Paratransit—Permitted plus
protected
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Permitted turn—Queue
discharge flow
Permitted turn - Left or right turn at a signalized intersection that is made against an
opposing or conflicting vehicular or pedestrian flow.
Person capacity - The maximum number of persons, in persons per hour, that reasonably
can be expected to be carried past a given point on a highway or transit right-of-way
during a given time period, under specified operating conditions, without
unreasonable delay, hazard, or restriction.
Phase - The part of the signal cycle allocated to any combination of traffic movements
receiving the right-of-way simultaneously during one or more intervals.
Planning application - A use of capacity analysis to estimate the level of service, the
volume that can be accommodated, or the number of lanes required, using estimates,
HCM default values, and local default values as inputs.
Platoon - A group of vehicles or pedestrians traveling together as a group, either
voluntarily or involuntarily because of signal control, geometrics, or other factors.
Platoon ratio - A parameter useful in quantifying arrival type. Platoon ratio is calculated
by dividing the proportion of all vehicles arriving during green by the g/C ratio of the
subject movement.
Point - A boundary between segments, usually places at which traffic enters, leaves, or
crosses a facility.
Point-deviation service - Public transportation service in which the transit vehicle
arrives at designated stops on a prearranged schedule but does not follow a specific
route.
Potential capacity - The capacity of a specific movement at a stop-controlled
intersection approach, assuming that it is unimpeded by other movements and has
exclusive use of a separate lane, in vehicles per hour.
Precision - The range of accurate and acceptable numerical answers.
Prepositioning - When one or more turning movements are necessary to occupy a lane of
the lane group.
Pretimed control - A signal control in which the cycle length, phase plan, and phase
times are preset to repeat continuously.
Prevailing condition - The geometric, traffic, and control conditions during the analysis
period.
Principal arterial - A major surface street with relatively long trips between major
points, and with through-trips entering, leaving, and passing through the urban area.
Progression adjustment factor - A factor used to account for the effect of signal
progression on traffic flow; applied only to uniform delay.
Protected plus permitted - Compound left-turn protection at a signalized intersection
that displays the protected phase before the permitted phase.
Protected turn - The left or right turns at a signalized intersection that are made with no
opposing or conflicting vehicular or pedestrian flow allowed.
Quality of service - A performance indicator of a traveler's perceived satisfaction with
the trip.
Quantity of service - A measure of the utilization of the transportation system.
Queue - A line of vehicles, bicycles, or persons waiting to be served by the system in
which the flow rate from the front of the queue determines the average speed within
the queue. Slowly moving vehicles or people joining the rear of the queue are
usually considered part of the queue. The internal queue dynamics can involve starts
and stops. A faster-moving line of vehicles is often referred to as a moving queue or
a platoon.
Queue carryover - The queued vehicles left over from the analysis period due to demand
exceeding capacity.
Queue discharge - A flow with high density and low speed, in which queued vehicles
start to disperse. Usually denoted as Level of Service F.
Queue discharge flow - A traffic flow that has passed through a bottleneck and is
accelerating to the free-flow speed of the freeway.
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Queue storage ratio - The parameter that uses three parameters (back of queue, queued
vehicle spacing, and available storage space) to determine if blockage will occur.
Ramp - A short segment of roadway connecting two traffic facilities.
Ramp junction - A short segment of highway along which vehicles transfer from an
entrance ramp to the main roadway or from the main roadway to an exit ramp.
Ramp meter - A traffic signal that controls the entry of vehicles from a ramp onto a
limited access facility; the signal allows one or two vehicles to enter on each green or
green flash.
Ramp roadway - See
Ramp.
Ramp segment - See
Ramp.
Ramp-freeway terminal - The roadway segment over which an entrance or an exit ramp
joins the mainline of a freeway.
Ramp-street terminal - The roadway segment over which an entrance or an exit ramp
joins with a surface street.
Ramp-weave segment - A weaving segment formed by a one-lane entrance ramp
followed by a one-lane exit ramp joined by a continuous auxiliary lane.
Random positioning - Through vehicles can use any lane of the subject lane group.
Rank - The hierarchy of right-of-way among conflicting traffic streams at a two-way
stop-controlled intersection.
Rapid bus - A bus that operates on an exclusive or reserved right-of-way permitting
higher speeds. On limited access roads it can include reverse lane operations.
Rapid transit - Rail systems operating on exclusive right-of-way, i.e., heavy rail or
metro.
Real-time model - A model that keeps pace with actual time.
Recreational vehicle - A heavy vehicle, generally operated by a private motorist, for
transporting recreational equipment or facilities; examples include campers, boat
trailers, and motorcycle or jet-ski trailers.
Red time - The period, expressed in seconds, in the signal cycle during which, for a
given phase or lane group, the signal is red.
Residual queue - The unmet demand at the end of an analysis period, resulting from
operation while demand exceeded capacity.
Roadside obstruction - An object or barrier along a roadside or median that affects
traffic flow, whether continuous (e.g., a retaining wall) or not continuous (e.g., light
supports or bridge abutments).
Roadway characteristic - A geometric characteristic of a street or highway, including
the type of facility, number and width of lanes (by direction), shoulder widths and
lateral clearances, design speed, and horizontal and vertical alignments.
Roadway occupancy - The proportion of roadway length covered by vehicles, used to
identify the proportion of time a roadway cross section is occupied by vehicles.
Because it is easier to measure in the field, roadway occupancy is used as a surrogate
for density in control systems.
Rolling terrain - A combination of horizontal and vertical alignments causing heavy
vehicles to reduce their speed substantially below that of passenger cars but not to
operate at crawl speeds for a significant amount of time.
Roundabout - An unsignalized intersection with a circulatory roadway around a central
island with all entering vehicles yielding to the circulating traffic.
Route-deviation service - A public transportation service that operates along a public
way on a fixed route but not on a fixed schedule. It is a form of paratransit.
Running speed - The distance a vehicle travels divided by running time, in miles per
hour.
Running time - The portion of the travel time during which a vehicle is in motion.
Rural - An area with widely scattered development and a low density of housing and
employment.
Queue storage ratio—Rural
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Saturation flow rate—
Simple left turn
protection
Saturation flow rate - The equivalent hourly rate at which previously queued vehicles
can traverse an intersection approach under prevailing conditions, assuming that the
green signal is available at all times and no lost times are experienced, in vehicles per
hour or vehicles per hour per lane.
Saturation headway - The average headway between vehicles occurring after the fourth
vehicle in the queue and continuing until the last vehicle in the initial queue clears
the intersection.
Sawtooth loading area - A bus bay design with the curb indented in a sawtooth pattern,
allowing buses to enter and exit bus bays independently of other buses. Often used
at transit centers.
Segment - A portion of a facility on which a capacity analysis is performed; it is the basic
unit for the analysis, a one-directional distance. A segment is defined by two
endpoints.
Semiactuated control - A signal control in which some approaches (typically on the
minor street) have detectors, and some of the approaches (typically on the major
street) have no detectors.
Service area - (1) The jurisdiction in which a transit property operates; (2) the
geographic region in which a transit system either provides service or is required to
provide service.
Service coverage - See
Coverage.
Service flow rate - The maximum hourly rate at which vehicles, bicycles, or persons
reasonably can be expected to traverse a point or uniform segment of a lane or
roadway during a given time period (usually 15 min) under prevailing roadway,
traffic, environmental, and control conditions, while maintaining a designated level
of service; expressed as vehicles per hour or vehicles per hour per lane.
Service frequency - The number of transit units (vehicles or trains) on a given route or
line, moving in the same direction, that pass a given point within a specified interval
of time, usually 1 h; see also
Headway.
Service measure - A specific performance measure used to assign a level of service to a
set of operating conditions for a transportation facility or service.
Service time - The average time that a vehicle on the subject approach is serviced at an
all-way stop-controlled intersection, depending on arrival rates of the opposing and
conflicting approaches.
Service volume - The maximum hourly rate at which vehicles, bicycles, or persons
reasonably can be expected to traverse a point or uniform segment of a roadway
during an hour under specific assumed conditions while maintaining a designated
level of service.
Shared-lane capacity - The capacity of a lane, in vehicles per hour, at an unsignalized
intersection that is shared by two or three movements.
Shock wave - The compression wave that moves upstream through traffic as vehicles
arriving at a queue slow down abruptly, or the decompression wave of thinning
traffic that moves downstream from the point of a capacity reduction on a freeway.
Shoulder - A portion of the roadway contiguous with the traveled way for
accommodation of stopped vehicles, emergency use, and lateral support of the
subbase, base, and surface courses.
Shoulder bypass lane - A portion of the paved shoulder opposite the minor-road leg at a
three-leg intersection, marked as a lane for through traffic to bypass vehicles that are
slowing or stopped to make a left turn.
Side street - See
Minor street.
Signalization condition - A phase diagram illustrating the phase plan, cycle length, green
time, change interval, and clearance time interval of a signalized intersection.
Simple left turn protection - A signal phasing scheme that provides a single protected
phase in each cycle for a left turn.
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Simple weaving segment - A segment formed by a single merge point followed by a
single diverge point.
Simulation model - A computer program that uses mathematical models to conduct
experiments with traffic events on a transportation facility or system over extended
periods of time.
Simulation model descriptor - A fundamental descriptor (state variable, event, time step
logic, and processing logic) used in combination with others to represent a unified
and consistent simulation model.
Single-point diamond interchange - A diamond interchange that combines all the ramp
movements into a single signalized intersection.
Single-stream door - A door on a transit vehicle that allows passenger flow in only one
direction at a time.
Skip-stop service - A transit operation in which alternate units stop at alternate sets of
stations on the same route. Each set consists of some joint and some alternate
stations.
Space - See
Pedestrian space.
Space mean speed - (1) The harmonic mean of speeds over a length of roadway; (2) an
average speed based on the average travel time of vehicles to traverse a segment of
roadway; in miles per hour.
Spacing - The distance, in feet, between two successive vehicles in a traffic lane,
measured from the same common feature of the vehicles (e.g., rear axle, front axle,
or front bumper).
Specific grade - A single grade of a roadway segment or extended roadway segment
expressed in percentage.
Speed - A rate of motion expressed as distance per unit of time.
Split-diamond interchange - Diamond interchanges in which freeway entry and exit
ramps are separated at the street level, creating four intersections.
Standee - A passenger standing in a transit vehicle.
Start-up lost time - The additional time, in seconds, consumed by the first few vehicles
in a queue at a signalized intersection above and beyond the saturation headway,
because of the need to react to the initiation of the green phase and to accelerate.
Static flow model - A mathematical model in which the traffic flow rate is constant.
Stochastic model - A mathematical model that employs random variables for at least one
input parameter.
Stop time - A portion of control delay when vehicles are at a complete stop.
Street corner - The area encompassed within the intersection of two sidewalks.
Streetcar - An electrically powered rail car that is operated singly or in short trains in
mixed traffic on track in city streets.
Study period - A duration of time on which to base capacity analyses of a transportation
facility.
Subject approach - The approach under study at two-way and all-way stop-controlled
intersections.
Suburban - An area with a mixture of densities for housing and employment, where
high-density nonresidential development is intended to serve the local community.
Suburban street - A street with low-density driveway access on the periphery of an
urban area.
System level of service - The quality of service provided by the transportation system.
System performance measure - A parameter that measures the efficiency of the
transportation system.
System performance report card - A list of measures depicting the use of the
transportation system, for decision making.
System speed - A space mean speed, in miles per hour, of vehicles both in the ramp
influence area and in the outer lanes of a 1,500-ft freeway segment.
Simple weaving segment—
System speed
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Taper area—Traveler
satisfaction
Taper area - An area characterized by a reduction or increase in pavement width to
direct traffic.
Terrain type - See
General terrain.
Through vehicles - All vehicles passing directly through a street segment and not
turning.
Time interval - See
Analysis period.
Time interval scale factor - The ratio of the total freeway entrance demands to the
freeway exit counts in each time interval.
Time mean speed - The arithmetic average of individual vehicle speeds passing a point
on a roadway or lane, in miles per hour.
Time-based model - A model in which time advances from one point to the next.
Time-space domain - A graphical display of a freeway facility with a horizontal scale of
distance along the freeway, with traffic moving from left to right, and with the
freeway divided into sections.
Time-varying flow model - A simulation model in which flow changes with time.
Total delay - The sum of all components of delay for any lane group, including control
delay, traffic delay, geometric delay, and incident delay. See also
Aggregate delay.
Total lateral clearance - The total width of the left side plus the right side along one
direction of a roadway.
Total lost time - The time per signal cycle during which the intersection is effectively not
used by any movement; this occurs during the change and clearance intervals and at
the beginning of most phases.
Traffic condition - A characteristic of traffic flow, including distribution of vehicle types
in the traffic stream, directional distribution of traffic, lane use distribution of traffic,
and type of driver population on a given facility.
Traffic delay - The component of delay that results when the interaction of vehicles
causes drivers to reduce speed below the free-flow speed.
Traffic pressure - A parameter that reflects driver aggressiveness due to heavier volumes
or long delays in a confined area.
Transit accessibility - A measure of pedestrian, bicycle, automobile, and Americans with
Disabilities Act accessibility to transit.
Transit availability - A measure of a transit system's capability for use by potential
passengers, including the hours the system is in operation, route spacing, and
accessibility for the physically handicapped.
Transit quality of service - The overall measured or perceived quality of transit service
from the passenger's point of view.
Transit reliability - A measure of the time performance and the regularity of headways
between successive transit vehicles that affect the amount of time passengers must
wait at a transit stop as well as the consistency of a passenger's arrival time at a
destination.
Transit stop - An area where passengers await, board, alight, and transfer between transit
units (vehicles or trains). It is usually indicated by distinctive signs and by curb or
pavement markings and may provide service information, shelter, seating, or any
combination of these.
Transit-supportive area - An area with sufficient population or employment density to
warrant at least hourly transit service.
Travel speed - The average speed, in miles per hour, of a traffic stream computed as the
length of a highway segment divided by the average travel time of the vehicles
traversing the segment.
Travel time - The average time spent by vehicles traversing a highway segment,
including control delay, in seconds per vehicle or minutes per vehicle.
Traveler satisfaction - A measure of the quality of a trip from the perspective of the
traveler.
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Trolleybus - An electrically propelled bus that obtains power from an overhead wire
system. The power-collecting apparatus allows the bus to maneuver in mixed traffic
over several lanes.
Truck - A heavy vehicle engaged primarily in the transport of goods and materials or in
the delivery of services other than public transportation.
Turnout - A short segment of a lane—usually a widened, unobstructed shoulder area—
added to a two-lane, two-way highway, allowing slow-moving vehicles to leave the
main roadway and stop so that faster vehicles can pass.
Two-lane Class I highway - A two-lane highway that generally serves long-distance
trips or provides connecting links between facilities that serve long-distance trips.
Two-lane Class II highway - A two-lane highway that generally serves relatively short
trips, the beginning and ending portions of longer trips, or trips for which sightseeing
activities play a significant role in route choice.
Two-lane highway - A roadway with a two-lane cross section, one lane for each
direction of flow, on which passing maneuvers must be made in the opposing lane.
Two-sided weaving segment - A weaving segment in which vehicles entering the
highway approach on the right and vehicles departing the highway depart on the left,
or vice versa; weaving vehicles must cross the mainline highway flow.
Two-stage gap acceptance - A process used by drivers entering an unsignalized
intersection from the minor street and reaching the median area in a first move, then
completing the entry with a second move.
Two-way left-turn lane - A lane in the median area that extends continuously along a
street or highway and is marked to provide a deceleration and storage area, out of the
through-traffic stream, for vehicles traveling in either direction to use in making left
turns at intersections and driveways.
Two-way stop-controlled - The type of traffic control at an intersection where drivers on
the minor street or a driver turning left from the major street wait for a gap in the
major-street traffic to complete a maneuver.
Unconstrained operation - An operating condition when the geometric constraints on a
weaving segment do not limit the ability of weaving vehicles to achieve balanced
operation.
Uncontrolled ramp terminal - A ramp terminal without a traffic control device.
Undersaturation - A traffic condition in which the arrival flow rate is lower than the
capacity or the service flow rate at a point or uniform segment of a lane or roadway.
Uniform delay - The first term of the equation for lane group control delay, assuming
uniform arrivals.
Uninterrupted flow - A category of facilities that have no fixed causes of delay or
interruption external to the traffic stream; examples include freeways and
unsignalized sections of multilane and two-lane rural highways.
Unit extension - The minimum gap, in seconds, between successive vehicles moving on
a traffic-actuated approach to a signalized intersection that will cause the signal
controller to terminate the green display.
Unit width flow rate - The pedestrian flow rate expressed as pedestrians per minute per
foot of walkway or crosswalk width.
Unmet demand - The number of vehicles on a signalized lane group that have not been
served at any point in time as a result of operation in which demand exceeds
capacity, in either the current or previous analysis period. This does not include the
normal cyclical queue formation on the red and discharge on the green phase. See
also
Initial queue
and
Residual queue.
Unsignalized intersection - An intersection not controlled by traffic signals.
Upstream - The direction from which traffic is flowing.
Urban - An area typified by high densities of development or concentrations of
population, drawing people from several areas within a region.
Trolleybus—Urban
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Urban street—Zone
Urban street - A street with relatively high density of driveway access located in an
urban area and with traffic signals no farther than 2 mi apart.
Urban street class - A category of urban street based on functional and design
categories.
Urban street segment - A length of urban street (in one direction) from one signal to the
next, including the downstream signalized intersection but not the upstream
signalized intersection.
Utility - A measure of the value a traveler places on a trip choice.
Utility equation - A mathematical function for evaluating the use of highway facilities;
the numerical values depend on the attributes of the travel options and on the
characteristics of the traveler.
Validation - Determining whether the selected model is appropriate for the given
conditions and for the given task; it compares model prediction with measurements
or observations.
Variability - The probability of congestion or a confidence interval for measures of
congestion (intensity, duration, and extent).
Vehicle capacity - (1) The maximum number of passengers that a transit vehicle is
designed to accommodate comfortably, seated and standing; also known as normal
vehicle capacity or total vehicle capacity; (2) the maximum number of vehicles that
can be accommodated in a given time by a transit facility.
Volume - The number of persons or vehicles passing a point on a lane, roadway, or other
traffic-way during some time interval, often 1 h, expressed in vehicles, bicycles, or
persons per hour.
Volume to capacity ratio - The ratio of flow rate to capacity for a transportation facility.
Walkway - A facility provided for pedestrian movement and segregated from vehicular
traffic by a curb, or provided for on a separate right-of-way.
Wave speed - The speed at which a shock wave travels upstream or downstream through
traffic.
Weave type - A classification scheme that categorizes weaving configuration into one of
the three types (Types A, B, C).
Weaving - The crossing of two or more traffic streams traveling in the same direction
along a significant length of highway, without the aid of traffic control devices
(except for guide signs).
Weaving configuration - The organization and continuity of lanes in a weaving segment,
which determines lane-changing characteristics.
Weaving diagram - A schematic drawing of flows in a weaving segment, used in
analysis.
Weaving flow - The traffic movements in a weaving segment that are engaged in
weaving movements.
Weaving length - The length from a point on the merge gore at which the right edge of
the freeway shoulder lane and the left edge of the merging lane are 2 ft apart to a
point on the diverge gore at which the edges are 12 ft apart.
Weaving segment - A length of highway over which traffic streams cross paths through
lane-changing maneuvers, without the aid of traffic signals; formed between merge
and diverge points.
Weaving width - The total number of lanes between the entry and exit gore areas,
including the auxiliary lane, if present.
Work zone - A segment of highway in which maintenance and construction operations
impinge on the number of lanes available to traffic or affect the operational
characteristics of traffic flowing through the segment.
Zebra-striped crosswalk - A crosswalk painted with diagonal stripes at an unsignalized
intersection, in which pedestrians have the right-of-way.
Zone - A geographic aggregation defined by land use, which generates trips within a
corridor.
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CHAPTER 6
SYMBOLS
This chapter lists and defines the symbols and abbreviations used in the manual,
along with their units if applicable. If the same symbol has more than one meaning, the
chapter or chapters of the specific use are cited in parentheses ( ) following the definition.
A ?
1.
Passenger waiting area size, ft 2 (27). 2. Access points (21).
a ?
1. Adjustment factor for two-stage gap acceptance (17). 2. Coefficient
for estimating base percent time spent following (20). 3. Weaving
intensity factor calibration constant (24). 4. Adjacent-lane impedance
factor (27).
AADT ?
Annual average daily traffic, veh/day
AC ?
Urban street class
Al ?
Added initial time per actuation, s
a n (t) ?
Acceleration of nth vehicle at time t
a ?
Design pedestrian area occupancy, ft 2 /p
A
pbT ?
Permitted phase adjustment for pedestrian/bicycle blockage
AT ?
Arrival type
ATS ?
Average travel speed for both directions of travel combined on two-
lane highways, mi/h
ATS, ?
Average travel speed for all two-lane highway directional segments
combined, mi/h
ATS d ?
Average travel speed in the analysis direction for the entire segment
without the passing lane, mi/h
ATS
p i ?
Average travel speed for the entire segment including the passing lane,
mi/h
AVM ?
Adjusted vehicle minimum time, s
AVO ?
Average vehicle occupancy
AWDT ?
Average weekday daily traffic, veh/day
?
1.
Begin platoon event time; the time that the dispersing platoon begins
to pass through the subject two-way stop-controlled intersection (17).
2. Bus lane vehicle capacity, buses/h (27).
? 1.
Bunching factor (16). 2. Coefficient for estimating base percent
time spent following (20). 3. Weaving intensity factor calibration
constant (24).
B(a) ?
Sum of gradients for counted segments
Bbb ?
Maximum number of buses per berth per hour, buses/h
BFFS ?
Base free-flow speed, mi/h
BPTSF ?
Base percent time-spent-following, %
BPTSF d ?
Base percent time-spent-following in the analysis direction, %
B s ?
Maximum number of buses per bus stop per hour, buses/h
?
1. Signal cycle length, s. 2. Capacity, veh/h (30).
?
1. Total lane group capacity, veh/h. 2. Two-way segment capacity,
normally 3,200 pc/h for a two-way segment and 1,700 pc/h for a
directional segment (20). 3. Weaving intensity factor calibration
constant (24).
C(a) ?
Sum of square of gradients for counted segments
C a ?
Approach capacity at roundabouts, veh/h
CAF ?
Capacity adjustment factor
c
b ?
1. Capacity of bicycle lane, bicycle/h (19). 2. Bus capacity, buses/h (27).
CBD ?
Central business district
CF ?
Acceleration/deceleration correction factor
A-CF
6-1 ?
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c
/ ?
Capacity for Stage I of two-stage gap acceptance
c
// ?
Capacity for Stage II of two-stage gap acceptance
C L ?
1. Lane group capacity per lane, veh/h (16). 2. Capacity of major-
street left-turn lane, veh/h (30).
mx
, ?
Movement capacity of Minor Movement x, veh/h
C
max ?
Maximum cycle length, s
C min ?
Minimum cycle length, s
Cost ?
Out-of-pocket cost for trip, cents
Potential capacity of Minor Movement x, veh/h
Cr
p,x ?
?
Capacity of right turns at specific intersection, veh/h
CS ?
Sum of critical phase volumes, veh/h
c s ?
Capacity of stop-controlled approach, veh/h
c
SH ?
Capacity of a shared lane, veh/h
C
T ?
Available capacity in the analysis period, veh/h
T ,x ?
Total capacity for Movement x considering a two-stage gap acceptance,
veh/h
CV ?
Critical phase volume, veh/h
c v ?
Coefficient of variation of headways of transit serving a particular route
arriving at a stop
C
VS ?
Sum of critical volume to saturation flow rate ratio
?
1.
Density, veh/mi, pc/mi/h, or veh/mi/ln (7). 2. Proportion of peak-
hour traffic in peak direction (8). 3. Total initial queue delay due to an
initial queue incurred in the average cycle, s (16). 4. Density of all
vehicles in the weaving segment, pc/mi/ln (24). 5. Diverge from traffic
at an interchange (26). 6. Movement at a downstream intersection
(26). 7. Number of doors available in the peak hour (27). 8. Mean
delay for subject lane group, s/veh (30). 9. Node delay for link, s (30).
?
1.
Control delay, s/veh (11, 15, 16). 2. Demand, veh/h (22).
3. Weaving intensity factor calibration constant (24). 4. Mean trip
delay, s/person (29). 5. Direction of analysis index (29).
d 1 ?
Uniform delay, s/veh
d 2 ?
Incremental delay, s/veh
d 3 ?
Initial queue delay, s/veh
D a ?
Mean delay for subject approach, s/veh
d A ?
Approach delay, s/veh
d
a
d ?
Acceleration/deceleration correction delay, s
d b ?
Control delay, s/bicycle
D c ?
Number of doors per car
?
Combined interchange delay, s/veh
DDHV ?
Directional design-hour volume, veh/h
DF ?
Adjustment factor for progression to compute zero-flow control delay
at signalized intersection
DHV ?
Design-hour volume, bicycles/h
D
i ?
Delay incurred by vehicles in the initial queue, s/veh
?
Intersection control delay, s/veh
?
1.
Lane group delay, s/veh (16). 2. Average control delay for Lane
Group i, s/veh (26).
diff
i ?
Computed differences for the entry legs
diff ?
Computed differences for the exiting legs
d
INT ?
Average control delay per vehicle for the interchange, s/veh
?
Jam density, veh/mi or veh/mi/ln
?
1.
Pedestrian control delay at Intersection j, s (18). 2. Average bicycle
delay at Intersection j, s (19).
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DL ?
Detector length, ft
D i ?
Estimated delay for left turns, s/veh
D m ?
Delay per person-trip for modal subsystem, s
?
Distance to subject vehicle n
d n _ 1 ?
Distance from subject vehicle n to vehicle ahead n —
1
D o ?
1.
Optimum density, veh/mi or veh/mi/ln (7). 2. Zero-flow control
delay at signalized intersection, h (30).
?
Average pedestrian delay, s
?
Delay per person-trip for Point p, s
D
ped ?
Pedestrian density, p/ft 2
DQ ?
Total delay due to excess demand or queuing delay, veh-h
D R ?
Density of flow within the ramp influence area, pc/mi/ln
D r ?
Estimated delay for right turns, s/veh
drankl ?
Average control delay to Rank 1 vehicles, s/veh
DS ?
Detector setback, ft
D s ?
1.
Intermediate speed determination variable for diverge area (25).
2.
Delay per person-trip for segment, s (28).
d s ?
1.
Storage density, veh/mi/ln (7, 29, 30). 2. Saturated delay, s (16).
3. Deceleration rate, ft/s 2 , also used as a surrogate for twice the
average acceleration from zero to maximum velocity (27).
dsep ?
Average control delay for the separate lane case, s/veh
dsH ?
Average control delay for the shared lane case, s/veh
Ds0 ?
Oversaturation delay corresponding to a zero initial queue, s/veh
D t ?
Estimated delay for through vehicles, s/veh
cl 7 ?
Delay for transit passengers, h
du ?
Dwelling unit
?
Undersaturated delay, s
d vq , ?
Time-in-queue per vehicle, s
D x ?
Delay per person-trip for Segment x, s
?
1.
End platoon event time; the time that the dispersing platoon
completes passage through the subject two-way stop-controlled
intersection. 2. East intersection (26).
?
Extension of green time, s
EB ?
Eastbound approach or movement
EL
I ?
Through-car equivalents for permitted left turns
EL2 ?
Through-car equivalents for opposing movements of permitted left
turns
e o ?
Unit extension time setting, s
E R ?
Passenger-car equivalent for recreational vehicles
E T ?
1. Passenger-car equivalent for heavy vehicles in the lane group (16).
2. Passenger-car equivalent for trucks; sometimes includes buses (20,
21, 22, 23, 24, 25)
ETC ?
Passenger-car equivalent for trucks that use crawl speeds
EW ?
Sum of the volume to saturation flow rate ratio for the critical phase
pair for the east-west street
?
1.
A parameter for basic dispersion model calculated as [1 + (t a. ) -1 1
(17). 2. Total number of events on the path, events/h (18, 19).
fA ?
Adjustment to account for effect of access points on base free-flow
speed
fa ?
Adjustment factor for area type
FAR ?
Floor area ratio
fb ?
Bus-bus interference adjustment factor
fB%
?
Percentile back of queue factor
DL—f 8%
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fbb ?
Adjustment factor for the blocking effect of local buses that stop within
the intersection area
f
d/np ?
Adjustment for the combined effect of directional distribution of traffic
and the percentage of no-passing zones on percent time-spent-
following, %
fDL ?
Planning left-turn adjustment factor
FFS ?
Free-flow speed, mi/h
FFS d ?
Free-flow speed in the analysis direction, mi/h
f G ?
Grade adjustment factor for two-lane highways
f g ?
Adjustment factor for approach grade
fFiv ?
Heavy-vehicle adjustment factor
fID ?
Adjustment for interchange density, mi/h
fk ?
Impedance adjustment factor
?
Bus stop location factor for bus lane capacity
fLC ?
Adjustment for lateral clearance, mi/h
fL b ?
Pedestrian adjustment factor for left-turn movements
fLS ?
Adjustment to base free-flow speed to account for effect of lane width
and shoulder width, mi/h
L
T ?
Adjustment factor for left turns in the lane group
fLu ?
Adjustment factor for lane utilization
fLuo ?
Lane utilization factor for the opposing flow of permitted left turns
fLw ?
Adjustment for lane width, mi/h
F m ?
Number of opposing events, events/h
f
m ?
Adjustment for median type, mi/h
f m ?
1.
Left-turn adjustment factor applied only to the lane from which left
turns are made (16). 2. Mixed traffic adjustment factor (27).
fmin ?
Minimum left-turn adjustment factor applied only to the lane from
which left turns are made
fN ?
Adjustment for number of lanes, mi/h
fnp ?
Adjustment to account for the effect of percentage of no-passing zones
on free-flow speed
?
Number of passing events, events/h
?
1.
Adjustment factor for the existence of parking lane and parking
activity adjacent to the lane group (10, 16). 2. Driver population factor
(21, 23, 25, 30). 3. Bus-passing activity factor (27).
f ?
PA ?
Supplemental adjustment factor for platoon arrival during the green
f
pb
?
?
Pedestrian blockage factor, or the proportion of time that one lane on an
approach is blocked during one hour
1 ?
Factor for the effect of a passing lane on percent time-spent-following
p
and average travel time
f q ?
Queue calibration factor for randomness in arrivals
f r ?
Right-turn adjustment factor for bus lane capacity
fRpb ?
Pedestrian/bicycle adjustment factor for right-turn movements
f R
T ?
Adjustment factor for right turns in the lane group
fs ?
Skip-stop speed adjustment factor
ft ?
feet
f
w ?
Adjustment factor for lane width
fx ?
A capacity adjustment factor for Movement x that accounts for the
impeding effects of higher-ranked movements
?
1.
Approach grade, % (16). 2. Green time, s (16). 3. Percent grade
divided by 100 (17). 4. Green time for phase, if WALK + EDW is not
installed, s (18).
?
1. Effective green time for lane group or for movement, s (15).
2. Effective green time for pedestrians, s (18).
Chapter 6 - Symbols ?
6-4
f
bb
—g
AR00042798

Highway Capacity Manual 2000
g(a) ?
Gradient of a segment (29)
gdiff ?
The larger of (a) the difference between g q and g f and (b) zero, s
ge ?
Extension to the protected green time that occurs while the controller
waits for a gap in the arriving traffic long enough to terminate the
phase, s
g a ff ?
Effective green time of a signalized intersection upstream of a two-way
stop-controlled intersection, s
g/ ?
Portion of the green time in which a through vehicle in a shared lane
would not be blocked by a left-turn vehicle waiting for the opposing
movement to clear, s
Gi ?
Green time, s
gi ?
Effective green time, s
G(i,j) ?
Gradient matrix for corridor analysis (29)
Gmax ?
Maximum gradient, or the largest absolute ratio of the gradient to the
estimated number of trips (29)
?
Effective green time for the opposing flow, s
?
Minimum pedestrian green time, s
gP ?
Pedestrian green time (Walk + Don't Walk), s
gprot ?
Protected phase effective green time, s
gq ?
1. Portion of the permitted green time blocked by a queue of opposing
vehicles, s (16). 2. Total time to discharge the queue at a signalized
intersection upstream of a two-way stop-controlled intersection, s (17).
gql
?
?
The time to discharge the vehicles that arrive during red at a signalized
intersection upstream of a two-way stop-controlled intersection, s
gq2
The time to discharge the vehicles that arrive during green and join the
back of queue at a signalized intersection upstream of a two-way stop-
controlled intersection, s
GR ?
Gap reduction rate
gs ?
Portion of the protected green time required to service the queue of
vehicles that accumulated on the previous phase, s
gu ?
Portion of the permitted green time not blocked by a queue of opposing
vehicles, s
?
Number of hours in analysis period
?
1.
Saturation headway, s (7). 2. Time period index (29).
?
hour
h a d] ?
Headway adjustment to account for the proportion of left turns, right
turns, and heavy vehicles, s
hbs ?
Minimum block-signaled section train headway, s
h d ?
Departure headway, s
H i ?
Duration of congestion for Link i, h
h min ?
Minimum train headway, s
hos ?
Minimum on-street section train headway, s
hp ?
horsepower
h s ?
Scheduled headway, s
h si ?
Saturation headway for degree of conflict Case i
hst ?
Minimum single-track section train headway, s
HV ?
Percent heavy vehicles, %
?
1. Incremental delay adjustment for the filtering or metering by
upstream signals (10, 15, 16). 2. Survey count interval for field control
delay study (16). 3. Adjustment factor for type, intensity, and location
of the work activity to compute capacity on freeway facilities, pc/h/ln
(22).
in ?
inch
?
Vehicle movements subscript of Rank 1
6
-
5 ?
Chapter 6 - Symbols
g(a)—i
AR00042799

Highway Capacity Manual 2000
is ?
Interval between vehicle-in-queue counts, s
IVT ?
In-vehicle time, min
?
Calibration parameter
?
Vehicle movements subscript of Rank 2
?
1.
The proportion of annual average daily traffic occurring in the
analysis period. 2. Adjustment factor to utilize bus stops fully in a
skip-stop operation (27). 3. Parameter (30).
?
1.
Incremental delay adjustment factor for the actuated control (10, 15,
16). 2. Vehicle movements subscript of Rank 3 (17). 3. Constant to
adjust degree of conflict case probability to account for
interdependence of headways (17).
k B ?
Second term queued vehicles adjustment factor related to early arrivals
k i ?
Sum of known trips originating at i
k. ?
Sum of known trips destined to j
k min ?
Minimum factor to compute incremental delay adjustment for actuated
signal
?
1.
Length of a highway segment, mi (7). 2. Lost time per cycle or
total lost time, s (10, 16). 3. Urban street segment or section length, mi
(15). 4. Crosswalk length, ft (16, 18). 5. Length of weaving segment,
ft (24). 6. Length of the link (i-j) from the upstream stop line to the
downstream stop line, ft (26). 7. Left-side movement at an interchange
(26). 8. Train length, ft (27). 9. Analysis section length, mi (27). 10.
Length of segment, mi (29). 11. Link length, mi (30).
?
1. Vehicle movements subscript of Rank 4 (17). 2. Queue storage
length per vehicle, veh (26). 3. Segment index (29).
L
1 ?
Distance for one-block stop pattern, ft
I. ?
Start-up lost time, s
L 2 ?
Distance for multiple-block stop pattern, ft
?
Clearance lost time, s
L A ?
Total length of the acceleration lane, ft
L a ?
Available queue storage distance, ft
L Ai ?
Length of first acceleration lane on a two-lane on-ramp, ft
LA2 ?
Length of second acceleration lane on a two-lane on-ramp, ft
Laa ?
Length of added approach lane, mi
Lad ?
Length of added departure lane, mi
LAeff ?
Length of effective acceleration lane on a two-lane on-ramp, ft
lb ?
pound
LC L ?
Width of lateral clearance from the left edge of travel lanes to
obstructions in the roadway median, ft
LC
R ?
Width of lateral clearance from the right edge of travel lane to roadside
obstruction, ft
L D ?
Length of deceleration lane, ft
L d ?
1. Detector length, ft (16). 2. Length of two-lane highway
downstream of the passing lane and beyond its effective length, mi
(20).
L Di ?
Length of first deceleration lane on a two-lane off-ramp, ft
LD2 ?
Length of second deceleration lane on a two-lane off-ramp, ft
Lde ?
Downstream length of two-lane highway within the effective length of
the passing lane, mi
L'de ?
Actual distance from end of passing lane to end of analysis segment, mi
LDeff ?
Length of effective deceleration lane on a two-lane on-ramp, ft
Ldown ?
Distance from the subject ramp to the downstream adjacent ramp, ft
Chapter 6 - Symbols ?
6-6
AR00042800

Highway Capacity Manual 2000
L
EQ ?
Equilibrium distance between upstream or downstream ramp and the
subject ramp, ft
L h ?
Average queue spacing in a stationary queue, ft
L t ?
Length of Segment i, ft
?
Total start-up lost time, s
In ?
lane
?
Length of the passing lane including tapers, mi
L s ?
Length of left- or right-lane storage bay, ft
Lst ?
Length of single-track section, ft
LT ?
Left turn
L T ?
Total length of the urban street under analysis, ft
L t ?
Total length of the analysis segment, mi
LTC ?
Left-turn vehicles per cycle, veh
?
Length of two-lane highway upstream of the passing lane, mi
L up ?
Distance from the subject ramp to the upstream adjacent ramp, ft
?
Vehicle length, ft
LW ?
Lane width, ft
?
1. Circulation area per pedestrian or pedestrian space, ft 2 /p (11, 18).
2. Median type (21). 3. Merge with traffic at an interchange (26).
?
1. Number of vehicles that can be stored in median of intersection
during two-stage gap acceptance, veh (17). 2. Move-up time, s (17).
MF ?
Mainline flow rate, veh/h
ml ?
mile
min ?
minute
MnA ?
Minimum allowable gap, s
MnV ?
Minimum vehicle phase time, s
ms ?
Intermediate speed determination variable for merge area
MxG ?
Maximum green time, s
Mx! ?
Maximum initial interval, s
?
1.
Number of lanes. 2. Last vehicle in queue (7). 3. Number of
through lanes at an intersection (15). 4. Number of lanes open through
the short-term work zone (22). 5. Total number of lanes in the
weaving segment (24). 6. Number of stops or stations in the analysis
section (27).
?
1.
Number of travel times observed (7) 2 Minimum number of
observations to meet accuracy goal of mean (9). 3. Fractional added
through lane (10). 4. Maximum number of opposing vehicles that
could arrive during g diff (16). 5. Number of vehicles that can be stored
in flared right-turn approach, veh (17).
N(i) ?
Node i with the upstream segment numbered segment (i - 1) and the
downstream segment numbered (i)
N
B ?
Local buses stopping at intersection, buses/h
NB ?
Northbound approach or movement
?
1.
Size of typical pedestrian crossing platoon, p (11, 18). 2. Number
of cycles surveyed (16). 3. Number of cars per train (27).
Ncd ?
Number of channels per door for moving passengers
Neb ?
Number of effective loading areas
N
i ?
Number of analysis subperiods
NLG ?
Number of lanes in the lane group
NLT ?
Number of exclusive left-turn lanes
N m ?
Parking activity per hour, maneuvers/h
n
Max ?
The maximum value of n, the number of vehicles that can be stored in
the flared right-turn approach, above which it will operate like a
separate lane
L EQ
-n max
6-7 ?
Chapter 6 - Symbols
AR00042801

Highway Capacity Manual 2000
N„—P,
N nw ?
Number of lanes used by nonweaving vehicles
N o ?
Number of outside lanes in one direction (not including acceleration or
deceleration lanes or Lanes 1 and 2)
N o ?
Number of opposing lanes
?
1.
Spatial distribution of pedestrians, p-ft (18). 2. Number of buses
making the maneuver from the curb lane to the adjacent lane (27).
Nped ?
Number of pedestrians crossing during an interval, p
Nrec ?
Number of cross-street receiving lanes
NRT ?
Number of exclusive right-turn lanes
NS ?
Sum of the volume to saturation flow rate ratio for the critical phase for
the north-south street
?
1.
Number of alternating skip stops in sequence (27). 2. Number of
stations on single-track section (27).
NTH ?
Number of through lanes
N
tum ?
Number of turning lanes
Ntv ?
Number of vehicles during the green phase, veh
N w ?
Number of lanes used by weaving vehicles if unconstrained operation is
to be achieved
N(max) ?
Maximum number of lanes that can be used by weaving vehicles for a
given configuration
OCC bicg ?
Average bicycle occupancy
OCC
pedg ?
Average pedestrian occupancy
OCC
pedu ?
Pedestrian occupancy after the opposing queue clears
OCC r ?
Relevant average pedestrian and bicycle occupancy
OFRF ?
Off-ramp flow rate, veh/h
ONRF ?
On-ramp flow rate, veh/h
OVT ?
Out-of-vehicle travel time, min
?
1.
Proportion of all vehicles arriving during green (15, 16, 26).
2. Primary phase (16). 3. Bicycle or pedestrian directional split (19).
4. Maximum single-track capacity in passengers per peak-hour
direction (27). 5. Total number of person trips (29). 6. Number of
transit passengers onboard, p (29). 7. Length of subperiod, h (30).
?
pedestrian, passenger, or person
?
Proportion of total flow rate traveling in the subject direction
p' ?
Adjustment to the major-street left turn, minor-street through
movement impedance factor
/Dr,
?
?
Product of the probabilities of queue-free states of Rank 1 and Rank 2
vehicles
P o,x ?
A factor indicating the probability there will be no queue in the shared
lane for major-street Movements 1 and 4, where x is the particular
movement being considered
P15 ?
Passenger volume during the peak 15 min
1
3 1 ?
First parameter for percentile back of queue factor
P2 ?
Second parameter for percentile back of queue factor
P3 ?
Third parameter for percentile back of queue factor
Pa ?
1. Alighting passengers per bus through the busiest door during the
peak 15 min, p (27). 2. Proportion of travelers preferring Option a
(28).
P
a d] ?
Adjusted probability of degree of conflict
Pb ?
Boarding passengers per bus through the busiest door during the peak
15 min, p
P
131•••Pbn ?
Boarding passenger volume per transit vehicle for each route served by
the waiting area during the peak 15 min, p
Pc ?
Maximum allowed passenger load per car, p
Chapter 6 - Symbols ?
6-8
AR00042802

Highway Capacity Manual 2000
pc ?
Passenger car
P
d ?
1. Transit passenger volume through the busiest door during the peak
15 min, p (27). 2. Number of transit passengers experiencing delay, p
(29).
P[C 1 ] ?
Probability of degree of conflict case C,
PF ?
Progression adjustment factor
PF 2 ?
Adjustment factor for the effects of progression in the first term queued
vehicles
PFD ?
An adjustment factor to compute v 12 at diverge influence area
PFM ?
An adjustment factor to compute v 12 at merge influence area
PHD ?
Total person-hours of delay
PHF ?
Peak-hour factor
PHT ?
Person-hours of travel in corridor, veh-h
PHT
f ?
Person-hours of travel under free-flow condition, p-h
PH
T
T ?
Person-hours traveled on transit, h
PHV ?
Proportion of heavy vehicles
P i ?
Proportion of the analysis period for major-street flow Regime i
PL ?
Proportion of left-turning vehicles in the shared lane
PLT ?
Proportion of left-turn volume in the lane group
PLTA ?
Proportion of left protected green time to the total left green time
PLTo ?
Proportion of left turns in opposing single-lane approach
?
Loading level, p/ft
PMT ?
Person-miles of travel, person-mi
Pov ?
Minimum phase time, s
Po,x ?
Probability that conflicting movement x will operate in a queue-free
state
PR ?
Proportion of recreational vehicles in the traffic stream
PRT ?
Proportion of right-turn volume in the lane group
PRTA ?
Proportion of right protected green time to the total right green time
PT ?
Proportion of trucks in the traffic stream; also can include buses (21)
PTC ?
Proportion of all trucks in the traffic stream that use crawl speeds on a
specific downgrade
PTHo ?
Proportion of through and right-turning vehicles in opposing single-
lane approach
PTSF ?
Percent time-spent-following, %
PTSF
c ?
Percent time-spent-following for all segments combined, %
PTSF
d ?
Percent time-spent-following in the analysis direction, %
PTSF
p i ?
Percent time-spent-following for the entire segment including the
passing lane, %
PTSF
x ?
Percent time-spent-following for Segment x, %
?
1. Average number of vehicles in queue, veh (7, 16). 2. Queue left
over at end of previous time period, veh (29). 3. Capacity of basic
freeway segment, multilane highway, or two-lane highway, pc/h/ln
(30).
?
Vehicle arrival rate throughout the cycle, veh/s
Q % ?
Percentile back of queue, veh
?
Queue size at the end of the permitted green period adjusted with
sneakers, veh
?
Vehicles arriving at the subject location during Time Slice t
Q i ?
First term queued vehicles, veh
Q2 ?
Second term queued vehicles, veh
Q95 ?
95th-percentile queue at a two-way stop-controlled intersection, veh
Qa ?
Queue at beginning of a green arrow, veh
qa ?
Arrival rate, veh/s
6-9
?
Chapter 6 - Symbols
pc—q
a
AR00042803

Highway Capacity Manual 2000
Q
b
R
po
Qb ?
Initial queue at the start of the analysis period, veh
QbL ?
Lane group initial queue at the start of the analysis period per lane, veh
Q f ?
Left-turn movement free queue or queue size at the end of the interval
veh
Q g ?
Flow departing from upstream signalized intersection during green
phase
qg ?
Green arrival time, veh/s
Qga ?
Queue size at the beginning of the protected green (green arrow)
period, veh
QL ?
Queue length, mi
Q m ?
Maximum queue length, veh
Qob ?
Bicycle flow rate in the opposing direction, bicycles/h
?
Queue size at the end of the permitted green period, veh
Q Q ?
Average queue length while queue is present, veh
Q g ?
Queue size at the end of the interval g q , veh
Qr ?
Residual queue or queue at the end of the effective green time, veh
qr ?
Red arrival time, veh/s
cir 0 ?
Opposing queue ratio, the proportion of opposing flow rate originating
in opposing queues
Qsb ?
Bicycle flow rate in the same direction, bicycles/h
Qsep ?
Average queue length for the separate lane case for flared right-turn
calculations, veh
QSH ?
Average queue length for the shared-lane case for flared right-turn
calculations, veh
Qt ?
Vehicles departing from an upstream signalized intersection during
time slice t
Qtco ?
Total time spent by pedestrians waiting to cross the minor street during
one cycle, p-s
Qtdo ?
Total time spent by pedestrians waiting to cross the major street during
one cycle, p-s
Qu ?
Queue at beginning of unsaturated green, veh
Queue (0)
?
Initial queue remaining from the preceding time period
? 1.
Radius of corner curb, ft (18). 2. Adjustment for ramps to compute
capacity on freeway facilities, veh/h (22). 3. Weaving ratio; the ratio
of the smaller weaving flow to total flow in the weaving segment (24).
4. Right-side movement at an interchange (26). 5. Segment traversal
time, h (29). 6. Link traversal time, h (30). 7. Parameter (30).
?
1.
Effective red time, s (7, 10, 16). 2. Segment rank used in
algorithms to adjust for excess demand (29). 3. Ratio of off-peak
demand to peak demand rate (30).
R 0 ?
Link traversal time when demand equals capacity, h
R
d ?
Ratio of busiest door usage to average door usage
R f ?
Segment free-flow traversal time, h
R i ?
1.
Ratio of desired to actual entering volume for Entry Leg i (10).
2. Red time, s (10).
?
Effective red time, s
?
Ratio of desired to actual exiting volume for Exit Leg j
R mi ?
The minor-street red phase, or the Don't Walk phase for pedestrian
signals, s
R mj ?
The major-street red phase, or the Don't Walk phase for pedestrian
signals, s
R 0 ?
Link travel time at free-flow link speed, h
?
Platoon ratio
c) ?
Platoon ratio for the opposing flow based on opposing arrival type
p
Chapter 6 - Symbols ?
6- 10
AR00042804

Highway Capacity Manual 2000
R Q ?
Average queue storage ratio
r q ?
Queue at the end of effective red time, veh
R
Q%
?
Percentile queue storage ratio
RS ?
Reference sum flow rate, veh/h
RT ?
Right turn
RTOR ?
Right turn on red
?
1. Average travel speed, mi/h (7). 2. Pedestrian speed, ft/min (18).
3. Average passenger-car travel speed, mi/h (21, 23). 4. Space mean
speed of all vehicles in the weaving segment, mi/h (24). 5. Space
mean speed of the ramp influence area, mi/h (25). 6. Mean segment
speed, mi/h (29). 7. Link speed, mi/h (30).
?
1. Saturation flow rate, veh/h or veh/h/ln. 2. Estimated standard
deviation for the sample (9). 3. Adjusted saturation flow per through
lane, veh/h (15).
?
second
S A ?
1. Average travel speed of through vehicles in the segment or the entire
section, mi/h (15). 2. Approach speed at a signalized intersection, mi/h
(16). 3. Average pedestrian travel speed, ft/s (18).
Sets ?
Bicycle travel speed, mi/h
SB ?
Southbound approach or movement
S b ?
Mean bicycle speed on the path, ft/s
S
b ?
Saturation flow rate of the bicycle lane, bicycles/h
SC ?
Urban street class
S f ?
1. Speed for a given flow rate, mi/h (7). 2. Free-flow speed of train,
mi/h (27). 3. Segment free-flow speed, mi/h (29).
SFF ?
Free-flow speed of the freeway approaching the merge or diverge area,
mi/h
S Fm ?
Mean speed of traffic measured in the field, mi/h
SFR ?
The free-flow speed of the ramp at the point of the merge area, mi/h
SFT ?
Free-flow speed of transit on facility, mi/h
S i ?
1. Pedestrian walking speed over Segment i, ft-s (18). 2. Bicycle
running speed over Segment i, mi/h (19). 3. Mean speed of Link i,
mi/h (30).
s i ?
Saturation flow rate, veh/h
SL ?
Urban street section length, mi
SL ?
Lane group saturation flow rate, veh/h
?
Adjusted saturation flow rate per lane, veh/h
S
LT ?
Filter saturation flow rate of permitted left turns, veh/h/ln
SM ?
Speed margin (constant)
Smax ?
1. Maximum speed reached, ft/s (11). 2. Maximum speed expected in
a weaving segment, mi/h (24).
S min ?
Minimum speed expected in a weaving segment, mi/h
S
N ?
Northbound approach service time to all-way stop-controlled
intersection, s
S nw ?
Space mean speed of nonweaving vehicles in the weaving segment,
mi/h
S o ?
1. Space mean speed of vehicles traveling in outer lanes, mi/h (25).
2. Base bus speed, mi/h (27).
S o ?
1. Optimum speed, mi/h (7). 2. Link free-flow speed, mi/h (30).
s o ?
Base saturation flow rate, pc/h/ln
?
Average pedestrian speed, ft/s; mean pedestrian speed on the path, ft/s
(18)
?
Protected phase departure rate, veh/s
S
ped ?
Pedestrian speed, ft/min
R
Q
-S
ped
6
-
11 ?
Chapter 6 - Symbols
AR00042805

Highway Capacity Manual 2000
S prog -TL
S
prog ?
Platoon speed from upstream signalized intersection to two-way stop-
controlled intersection, ft/s
SR ?
1. Space mean speed, mi/h (7). 2. Space mean speed of vehicles
within the ramp influence area, mi/h (25).
s s ?
Permitted phase departure rate, veh/s
S T ?
1. Time mean speed, mi/h (7). 2. Actual speed of transit on facility
including all delays, mi/h (29).
ST ?
Urban street section (or segment) travel time, s
S t ?
1. Bus travel speed, mi/h (27). 2. Train travel speed, mi/h (27).
S
TH ?
Saturation flow rate of through traffic, veh/h/ln
s
w ?
1. Westbound approach service time to all-way stop-controlled
intersection, s (17). 2. Space mean speed of weaving vehicles in the
weaving segment, mi/h (24).
?
1. Duration of analysis period, h. 2. Total crosswalk occupancy time,
p-s (18). 3. Through movement (26). 4. Maximum number of trains
per hour (27). 5. Duration of analysis subperiod, h (29). 6. The
expected duration of the demand, h (30).
?
1. Duration of initial queue in the analysis period, h (16). 2. Total
crossing time for pedestrians, s (18). 3. Mean trip time, min/p (29).
t' ?
Length of time during which dispersing platoon passes the subject two-
way stop-controlled intersection
t 3,LT ?
Critical gap adjustment factor for intersection geometry (T-intersection)
t a ?
1. Average travel time over a length of a highway segment, h (7).
2. Travel time from the upstream signalized intersection to the subject
two-way stop-controlled intersection, s (17). 3. Passenger alighting
time, s/p (27).
t b ?
Passenger boarding time, s/p
t br ?
Operator and braking system reaction time, s
T s ?
Time to clear initial queue present at the start of analysis period, s
t s ?
1. Single pedestrian critical gap, s (11, 18). 2. Vehicular critical gap
time at a two-way stop-controlled intersection, s (17). 3. Clearance
time between successive buses or trains, s (27).
t
c,
base ?
Base critical gap, s
t
cG
, ?
Critical gap adjustment for approach grade, s
t c,HV ?
Critical gap adjustment for heavy vehicles, s
t cT
,
?
Critical gap adjustment for each part of a two-stage gap acceptance
process
t
d ?
Dwell time, s
t f ?
Follow-up time, or the time span between the departure of one vehicle
from the minor street and the departure of the next vehicle, s
t f
,
base ?
Base follow-up time, s
t f,HV ?
Follow-up time adjustment for heavy vehicles, s
t
G ?
Group critical gap, s
TH ?
Through
T t ?
Number of trips originating at Point i
t t ?
1. Travel time of the ith vehicle to traverse the section, h (7). 2. Start-
up lost time for ith vehicle, s (7).
?
Number of trips going from Origin Point i to Destination Point j
?
Estimated number of trips between Origin Zone i and Destination Zone
j (29)
?
Number of trips leaving Destination j
t jt ?
Jerk limiting time, s
TL ?
Total intersection lost time, s
Chapter 6 - Symbols
6 - 12
AR00042806

Highway Capacity Manual 2000
t L ?
Lost time per phase or movement lost time, s
TLC ?
Total lateral clearance, ft
t o ?
Time duration the detector is occupied by a passing vehicle, s
toc ?
Door opening and closing time, s
t om ?
Operating margin time, s
?
Number of person-trips using Point p
pf ?
Passenger flow time, sip
t p,1 ?
Duration of the blocked period for either the through movement or the
protected left-turn movement to a two-way stop-controlled intersection,
t c? ?
Time duration of queue, s
T R ?
Total running time on all segments, in a defined urban street section, s
f t. ?
Running time, s
t t. ,0 ?
Base bus running time, min/mi
t
n ?
Bus running time losses, min/mi
TS ?
Available time-space, ft 2 -s
Ts ?
Number of person-trips using Segment s
ts ?
1. Service time, s (17). 2. Pedestrian start-up time, s (11, 18).
TS
c ?
Total time-space available for circulating pedestrians, ft 2 -s
TS E ?
Effective time-space, ft 2 -s
tst ?
Time to cover single-track section, s
TS
tv ?
Time-space occupied by turning vehicles, ft 2 -s
TT ?
Urban street field travel time, s
T t ?
Platoon travel time from upstream signalized intersection to subject
two-way stop-controlled intersection, s
t t ?
Total travel time, s
TT 15 ?
Total travel time for all vehicles on the analysis segment during the
peak 15-min period, veh-h
TT
x ?
Total travel time for Segment x, veh-h
TWLTL ?
Two-way left-turn lane
?
Number of person-trips using Segment x
?
Movement at an upstream intersection
?
Initial queue delay parameter
U o ?
Utility function valued for Option a
?
Utility function valued for Option j
Utility ?
Measure of the traveler's perceived value of an alternative
Hourly volume, veh/h or veh/h/ln
?
1. Vehicular flow rate for peak 15-min period, veh/h or veh/h/ln.
2. Pedestrian unit flow rate, p/min/ft (18). 3. Pedestrian volume on the
subject roadway, p/15-min (18). 4. Total bicycle flow rate, both
directions, bicycles/h (19). 5. Arrival flow rate at the downstream
intersection, veh/h (26). 6. Demand rate for current time period (29).
v
5 ?
Anticipated approach flow rate in Lane 5 of the freeway, pc/h
v
12 ?
Flow rate entering Lanes 1 and 2 immediately upstream of the merge
influence area or at the beginning of the deceleration lane in the diverge
influence area, pc/h
V 15 ?
Volume during the peak 15 min of the peak hour, veh/15-min
v 15 ?
Peak 15-min pedestrian flow rate, p/15-min
v(a) ?
Counted volume for Segment a
v'(a) ?
Estimated volume for Segment a
V
A ?
Approach flow rate, veh/h
v o ?
Approach flow rate at roundabouts, veh/h
V b ?
Bicycle hourly volume, bicycles/h
Vb ?
1. Bicycle flow rate, bicycles/h (19). 2. Bus flow rate, buses/h (27).
6-13 ?
Chapter 6 - Symbols
t
L
—V
b
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Highway Capacity Manual 2000
Vbic —V nw
V bic ?
Approach bicycle volume, bicycles/h
V bicg ?
Bicycle flow rate during the green interval for 1 h, bicycles/h
v
bo ?
Flow rate of bicycles in the opposing direction, bicycles/h
v
bs ?
Flow rate of bicycles in the subject direction, bicycles/h
v/c ?
Volume to capacity ratio
v c ?
Circulating flow rate at roundabouts, veh/h
VCL ?
Critical lane flow rate, veh/h
vcc, ?
Number of pedestrians waiting to cross the minor street during one
cycle, p/cycle
conflicting flow rate for Movement x, that is, the total flow rate
conflicting with Movement x, veh/h or p/h
V
D ?
Total flow rate on the downstream adjacent ramp from the subject
ramp, pc/h
v
d ?
Passenger-car equivalent flow rate for the peak 15-min period in the
analysis direction, pc/h
v
do ?
Number of pedestrians waiting to cross the major street during one
cycle, p/cycle
veh ?
vehicle
rate in the separate lanes on minor street approach; used to
compute capacity for flared right-turn approach
Vf ?
Observed flow rate for the period when field data were obtained, veh/h
V
F ?
Maximum total flow rate approaching a merge or diverge area on the
freeway, pc/h or pc/h/ln
v
F4eff ?
Effective approaching flow rate for a four-lane freeway segment, pc/h
v
FO ?
Maximum total flow rate departing from a merge or diverge area on the
freeway, pc/h
V g ?
Unadjusted flow rate for the lane group, veh/h
v
gl ?
Unadjusted flow rate on the single lane in the lane group with the
highest volume
VHD ?
Vehicle-hours of delay, h
VHT ?
Vehicle-hours of travel, veh-h
?
1. Adjusted flow rate for Lane Group i, veh/h (10). 2. Incoming
pedestrian volume for the subject crosswalk, p/cycle (18). 3. Flow rate
for Movement i under base conditions during peak 15 min, pc/h (25).
4. Demand flow rate for Lane Group i, veh/h (26).
V iq ?
Total vehicles in queue for field control delay study
VMT ?
Vehicle-miles of travel, veh-mi
VM7 - 15 ?
Total travel on the analysis segment during the peak 15-min period,
veh-mi
VMT 60 ?
Total travel on the analysis segment during the peak hour, veh-mi
VMT x ?
Total travel for Segment x, veh-mi
V L ?
Left-turn movement volume, veh/h
VL ?
1. Lane group flow rate per lane, veh/h (16). 2. Minor left-turn flow
rate, veh/h (17).
?
Lane group flow rate including initial queue, veh/h
v
LS ?
Left-turn movement maximum sneakers, veh
VLT ?
Left volume per lane, veh/h/ln
v
LT ?
Adjusted left-turn flow rate, veh/h
?
Maximum flow rate, veh/h or veh/h/ln
?
Desired speed for Vehicle n
(t) ?
Speed of nth vehicle at time t
V n _ 1 ?
Vehicle ahead of subject Vehicle n
v nw ?
Total nonweaving flow in the weaving segment, pc/h
Chapter 6 - Symbols ?
6-14
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Highway Capacity Manual 2000
V o ?
Demand volume for the full peak hour in the opposing direction of
travel, veh/h
v o ?
1. Opposing flow rate for permitted left turns, veh/h (16). 2. Outgoing
pedestrian volume for the subject crosswalk, p/cycle (18). 3. Flow rate
of bicycles in the opposing direction, bicycles/h (19). 4. Passenger-car
equivalent flow rate for the peak 15-min period in the opposing
direction of travel, pc/h (20). 5. Sum of approach volumes on
nonsubject approaches, veh/h (30).
v 01 ?
Larger of the two outer, or nonweaving, flows in the weaving segment,
pc/h
v 02 ?
Smaller of the two outer, or nonweaving, flows in the weaving
segment, pc/h
VOA ?
Average per lane demand related to flow in the outer lanes, pc/h/ln
V oe ?
Effective opposing flow rate, veh/h
votc ?
Adjusted opposing flow rate per lane per cycle, veh
?
1. Pedestrian flow rate, p/s (11). 2. Pedestrian unit flow rate, p/min/ft
(18). 3. Peak 15-min passenger-car equivalent flow rate, pc/h/ln (20,
21, 23, 24, 25).
v
ped ?
1. Unit flow rate, p/min/ft (11). 2. Conflicting pedestrian flow rate,
p/h (16).
v
pedg ?
Pedestrian flow rate during the green interval for 1 h, p/h
v
po ?
Flow rate of pedestrians in the opposing direction, p/h
v
prog ?
Progressed flow rate from upstream signalized intersection to compute
the effect of upstream signals
Flow rate of pedestrians in the subject direction, p/h
ps ?
V
R ?
Right-turn movement volume, veh/h
VR ?
Volume ratio; the ratio of weaving to total flow in the weaving segment
v
R ?
Total ramp flow rate, pc/h
v r ?
Volume of right turns at a specific intersection, veh/h
v
R12 ?
Maximum total flow entering the ramp influence area, pc/h
VRT ?
Right-turn volume per lane, veh/h/ln
v s ?
Flow rate of bicycles in the subject direction, bicycles/h
v
SH ?
Flow rate in the shared lanes; used to compute capacity for flared
right-turn approach
V stop ?
Stopped vehicles count for field control delay study
V
T ?
Through movement volume, veh/h
V
TH ?
Through volume per lane, veh/h/ln
Vtot ?
1. Total approach volume, veh/h (10). 2. Total vehicles arriving for
field control delay study (16).
v
tot ?
Total number of circulating pedestrians in one cycle, p/cycle
v u ?
Total flow rate on the adjacent upstream ramp from the subject ramp,
pc/h
?
Total weaving flow in the weaving segment, pc/h
w
V
i ?
Larger of the two weaving flows in the weaving segment, pc/h
w
V
2 ?
Smaller of the two weaving flows in the weaving segment, pc/h
?
Flow rate for Movement x for vehicular flows and pedestrian flows,
veh/h or p/h
?
1. Average lane width, ft. 2. Effective width of sidewalk, ft (18).
2. West intersection (26).
?
Lane width, ft
WALK + FDW ?
Effective pedestrian green time at a signalized intersection, s
WB ?
Westbound approach or movement
WDW ?
Pedestrian Walk plus flashing Don't Walk, s
WE ?
Effective walkway width, ft
V o —W
E
6-15 ?
Chapter 6 - Symbols
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Highway Capacity Manual 2000
W nw -
Wnw ?
Weaving intensity factor for prediction of nonweaving speed
W a ?
Sum of widths and shy distances from obstructions on the walkway, ft
WT ?
Total walkway width, ft
W w ?
Weaving intensity factor for prediction of weaving speed
X ?
Volume to capacity ratio
?
1. Mean arrival rate, veh/h (7). 2. Degree of utilization, or vh d /3600
(17).
?
The mean value of the observation
X b ?
Volume to capacity ratio of bicycle lanes (also termed degree of
saturation)
X c ?
Critical volume to capacity ratio for the intersection
X cm ?
Critical volume to capacity ratio for planning procedure
x i ?
The ith observation of the value
x(t) ?
Position of nth vehicle at time t
Xperm ?
Permitted phase volume to capacity ratio for leading or lagging left-turn
movement with protected-plus-permitted phase
Xprot ?
Protected phase volume to capacity ratio for leading or lagging left-turn
movement with protected-plus-permitted phase
X
?
of saturation at upstream intersection
?
Yellow plus all-red change and clearance interval (intergreen), s
?
Mean service rate, veh/h
Yc ?
Sum of flow ratios for critical lane groups
Yi ?
Change and clearance interval time, s
?
Subject approach volume divided by 1000, veh/h/1000
Z a ?
The area under one tail of the normal curve beyond the acceptable
levels of probability that a queue will form at a bus stop (27), or failure
rate (29)
a ?
1. Platoon dispersion factor (17). 2. Constant to adjust degree of
conflict case probability to account for interdependence of headways
(17).
?
Dispersion factor, or (1 +
0 -
1
A ?
Minimum arrival (intrabunch) headway, s
At ?
Time interval or duration of one time step
Av ?
Amount of change in speed
/ x ?
First derivative of travel time with respect to volume to capacity ratio
?
Offset between through movements at Intersections i and j at
interchange ramp terminal, s
?
Parameter for computing green extension time, veh/s
?
Arrival rate for approach or Lane x
?
Maximum desirable error in the estimate of the mean
?
Proportion of free (unbunched) vehicles
Chapter 6 - Symbols ?
6
-
16
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Highway Capacity Manual 2000
CHAPTER 7
TRAFFIC FLOW PARAMETERS
CONTENTS
I. INTRODUCTION ?
7-1
II. UNINTERRUPTED FLOW ?
7-1
Volume and Flow Rate ?
7-1
Speed ?
7-2
Density ?
7-4
Headway and Spacing ?
7-4
Relationships Among Basic Parameters ?
7-5
III. INTERRUPTED FLOW ?
7-6
Signal Control ?
7-7
Stop- or Yield-Controlled Intersections ?
7-8
Speed ?
7-9
Delay ?
7-9
Saturation Flow Rate and Lost Time ?
7-10
Queuing ?
7-11
IV. REFERENCES ?
7-13
EXHIBITS
Exhibit 7-1. ?
Typical Relationship Between Time Mean and Space Mean Speed
7-3
Exhibit 7-2. ?
Generalized Relationships Among Speed, Density, and Flow Rate
on Uninterrupted-Flow Facilities ?
7-5
Exhibit 7-3. ?
Conditions at Traffic Interruption in an Approach Lane of a
Signalized Intersection ?
7-7
Exhibit 7-4. ?
Concept of Saturation Flow Rate and Lost Time ?
7-8
Exhibit 7-5. ?
Queuing Diagram for Signalized Intersection ?
7-12
7-i ?
Chapter 7 - Traffic Flow Parameters
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Highway Capacity Manual 2000
I. INTRODUCTION
Three basic variables—volume or flow rate, speed, and density—can be used to
describe traffic on any roadway. In this manual, volume or traffic flow is a parameter
common to both uninterrupted- and interrupted-flow facilities, but speed and density
apply primarily to uninterrupted flow. Some parameters related to flow rate, such as
spacing and headway, also are used for both types of facilities; other parameters, such as
saturation flow or gap, are specific to interrupted flow.
II. UNINTERRUPTED FLOW
VOLUME AND FLOW RATE
Volume and flow rate are two measures that quantify the amount of traffic passing a
point on a lane or roadway during a given time interval. These terms are defined as
follows:
• Volume—the total number of vehicles that pass over a given point or section of a
lane or roadway during a given time interval; volumes can be expressed in terms of
annual, daily, hourly, or subhourly periods.
• Flow rate—the equivalent hourly rate at which vehicles pass over a given point or
section of a lane or roadway during a given time interval of less than 1 h, usually 15 min
Volume and flow are variables that quantify demand, that is, the number of vehicle
occupants or drivers (usually expressed as the number of vehicles) who desire to use a
given facility during a specific time period. Congestion can influence demand, and
observed volumes sometimes reflect capacity constraints rather than true demand.
The distinction between volume and flow rate is important. Volume is the number of
vehicles observed or predicted to pass a point during a time interval. Flow rate represents
the number of vehicles passing a point during a time interval less than 1 h, but expressed
as an equivalent hourly rate. A flow rate is the number of vehicles observed in a
subhourly period, divided by the time (in hours) of the observation. For example, a
volume of 100 vehicles observed in a 15-min period implies a flow rate of 100 veh/0.25 h
or 400 veh/h.
Volume and flow rate can be illustrated by the volumes observed for four
consecutive 15-min periods. The four counts are 1,000, 1,200, 1,100, and 1,000. The
total volume for the hour is the sum of these counts, or 4,300 veh. The flow rate,
however, varies for each 15-min period. During the 15-min period of maximum flow, the
flow rate is 1,200 veh/0.25 h, or 4,800 veh/h. Note that 4,800 vehicles do not pass the
observation point during the study hour, but they do pass at that rate for 15 min
Consideration of peak flow rates is important in capacity analysis. If the capacity of
the segment of highway studied is 4,500 veh/h, capacity would be exceeded during the
peak 15-min period of flow, when vehicles arrive at a rate of 4,800 veh/h, even though
volume is less than capacity during the full hour. This is a serious problem, because
dissipating a breakdown of capacity can extend congestion for up to several hours.
Peak flow rates and hourly volumes produce the peak-hour factor (PHF), the ratio of
total hourly volume to the peak flow rate within the hour, computed by Equation 7-1:
Hourly volume
PHF - ?
Peak flow rate (within the hour)
(7-1)
If 15 -
min periods are used, the PHF may be computed by Equation 7 - 2:
PHF -
4 x
V15
Basic concepts for
uninterrupted-flow facilities:
volume, flow rate, speed,
density, headway, and
capacity
Calculating a peak-hour factor
V
(7-2)
7-1 ?
Chapter 7 - Traffic Flow Parameters
Introduction
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Highway Capacity Manual 2000
where
PHF =
peak-hour factor,
V
= hourly volume (veh/h), and
V 15 = volume during the peak 15 min of the peak hour (veh/15 min)
When the PHF is known, it can convert a peak-hour volume to a peak flow rate, as in
Equation 7-3:
where
V
v -
PHF
(7-3)
Speed parameters
V
= flow rate for a peak 15-min period (veh/h),
V
= peak-hour volume (veh/h), and
PHF =
peak-hour factor.
Equation 7-3 does not need to be used to estimate peak flow rates if traffic counts are
available; however, the chosen count interval must identify the maximum 15-min flow
period. The rate then can be computed directly as 4 times the maximum 15-min count.
When flow rates in terms of vehicles are known, a conversion to a flow rate in terms of
passenger car equivalents (pce) can be computed using the PHF and the heavy vehicle
factor.
SPEED
Although traffic volumes provide a method of quantifying capacity values, speed (or
its reciprocal of travel time) is an important measure of the quality of the traffic service
provided to the motorist. It is an important measure of effectiveness defining levels of
service for many types of facilities, such as rural two-lane highways, urban streets,
freeway weaving segments, and others.
Speed is defined as a rate of motion expressed as distance per unit of time, generally
as miles per hour (mi/h). In characterizing the speed of a traffic stream, a representative
value must be used, because a broad distribution of individual speeds is observable in the
traffic stream. In this manual, average travel speed is used as the speed measure because
it is easily computed from observation of individual vehicles within the traffic stream and
is the most statistically relevant measure in relationships with other variables. Average
travel speed is computed by dividing the length of the highway, street section, or segment
under consideration by the average travel time of the vehicles traversing it. If travel times
t1, t2, t3,..., t n (in hours) are measured for n vehicles traversing a segment of length L, the
average travel speed is computed using Equation 7-4.
nL
in
?
t
i
?
- ?
ti ?
t
a
?
1=1 ?
ni
=
l
where
s
=
(7-4)
S
= average travel speed (mi/h),
length of the highway segment (mi),
t i
?
travel time of the ith vehicle to traverse the section (h),
n
=
number of travel times observed, and
i n
t a
= —
It
1
= average travel time over L (h).
fl i=i
The travel times in this computation include stopped delays due to fixed interruptions or
traffic congestion. They are total travel times to traverse the defined roadway length.
Several different speed parameters can be applied to a traffic stream. These include
the following:
Chapter 7 - Traffic Flow Parameters ?
7-2
Uninterrupted Flow
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Highway Capacity Manual 2000
Average running speed—A traffic stream measure based on the observation of
vehicle travel times traversing a section of highway of known length. It is the length of
the segment divided by the average running time of vehicles to traverse the segment.
Running time includes only time that vehicles are in motion.
Average travel speed—A traffic stream measure based on travel time observed on a
known length of highway. It is the length of the segment divided by the average travel
time of vehicles traversing the segment, including all stopped delay times. It is also a
space mean speed.
Space mean speed—A statistical term denoting an average speed based on the
average travel time of vehicles to traverse a segment of roadway. It is called a space
mean speed because the average travel time weights the average to the time each vehicle
spends in the defined roadway segment or space.
Time mean speed—The arithmetic average of speeds of vehicles observed passing a
point on a highway; also referred to as the average spot speed. The individual speeds of
vehicles passing a point are recorded and averaged arithmetically.
Free-flow speed—The average speed of vehicles on a given facility, measured under
low-volume conditions, when drivers tend to drive at their desired speed and are not
constrained by control delay.
For most of the procedures using speed as a measure of effectiveness in this manual,
average travel speed is the defining parameter. For uninterrupted-flow facilities not
operating at level of service (LOS) F, the average travel speed is equal to the average
running speed.
Exhibit 7-1 shows a typical relationship between time mean and space mean speeds.
Space mean speed is always less than time mean speed, but the difference decreases as
the absolute value of speed increases. Based on the statistical analysis of observed data,
this relationship is useful because time mean speeds often are easier to measure in the
field than space mean speeds.
EXHIBIT 7-1. TYPICAL RELATIONSHIP BETWEEN TIME MEAN AND SPACE MEAN SPEED
60
50
?
60
It is possible to calculate both time mean and space mean speeds from a sample of
individual vehicle speeds. For example, three vehicles are recorded with speeds of 30,
40, and 50 mi/h. The time to traverse 1 mi is 2.0 min, 1 5 min, and 1.2 min, respectively.
The time mean speed is 40 mi/h, calculated as (30 + 40 + 50)13. The space mean speed is
38.3 mi/h, calculated as (60)[3 ± (2.0 + 1.5 + 1.2)].
Average running speed
Average travel speed
Space mean speed
Time mean speed
Free-flow speed
Sp
ace
Mean
Sp
ee
d,
SR
(M
i
th)
50
40
30
20
10
0
0 ?
10 ?
20 ?
30 ?
40
Time Mean Speed, S T (mi/h)
Source: Drake et al.
(1).
7- 3 ?
Chapter 7 - Traffic Flow Parameters
Uninterrupted Flow
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Highway Capacity Manual 2000
For capacity analysis, speeds are best measured by observing travel times over a
known length of highway. For uninterrupted-flow facilities operating in the range of
stable flow, the length may be as short as several hundred feet for ease of observation.
As measures of effectiveness, speed criteria must recognize driver expectations and
roadway function. For example, a driver expects a higher speed on a freeway than on an
urban street. Lower free-flow speeds are tolerable on a roadway with more severe
horizontal and vertical alignment, since drivers are not comfortable driving at high
speeds. LOS criteria reflect these expectations.
DENSITY
Density is the number of vehicles (or pedestrians) occupying a given length of a lane
or roadway at a particular instant. For the computations in this manual, density is
averaged over time and is usually expressed as vehicles per mile (veh/mi) or passenger
cars per mile (pc/mi).
Direct measurement of density in the field is difficult, requiring a vantage point for
photographing, videotaping, or observing significant lengths of highway. Density can be
computed, however, from the average travel speed and flow rate, which are measured
more easily. Equation 7-5 is used for undersaturated traffic conditions.
Computing density
v
D= —
S
where
(7-5)
v = flow rate (veh/h),
S
= average travel speed (mi/h), and
D
=
density (veh/mi).
A highway segment with a rate of flow of 1,000 veh/h and an average travel speed of
50 mi/h would have a density of
1
'
000 veh / h
D —
5 ?
— 20 veh / mi
0 ml / h
Density is a critical parameter for uninterrupted-flow facilities because it characterizes
the quality of traffic operations. It describes the proximity of vehicles to one another and
reflects the freedom to maneuver within the traffic stream.
Roadway occupancy is frequently used as a surrogate for density in control systems
because it is easier to measure. Occupancy in space is the proportion of roadway length
covered by vehicles, and occupancy in time identifies the proportion of time a roadway
cross section is occupied by vehicles.
HEADWAY AND SPACING
Spacing is the distance between successive vehicles in a traffic stream, measured
from the same point on each vehicle (e.g., front bumper, rear axle, etc.). Headway is the
time between successive vehicles as they pass a point on a lane or roadway, also
measured from the same point on each vehicle.
These characteristics are microscopic, since they relate to individual pairs of vehicles
within the traffic stream. Within any traffic stream, both the spacing and the headway of
individual vehicles are distributed over a range of values, generally related to the speed of
the traffic stream and prevailing conditions. In the aggregate, these microscopic
parameters relate to the macroscopic flow parameters of density and flow rate.
Spacing is a distance, measured in feet. It can be determined directly by measuring
the distance between common points on successive vehicles at a particular instant. This
generally requires complex aerial photographic techniques, so that spacing usually
derives from other direct measurements. Headway, in contrast, can be easily measured
with stopwatch observations as vehicles pass a point on the roadway.
Chapter 7 - Traffic Flow Parameters ?
7-4
Uninterrupted Flow
AR00042815

Legend
--- Oversaturated flow
D
1
D,
Density (veh/mi/ln)
Source: Adapted from May (2).
D, ?
D.
Density (veh/mi/ln)
0
v,,
_=
_=
u_
0
0
v,,
S t
Flow (veh/h/ln)
Highway Capacity Manual 2000
Relationships among density,
speed and flow rate, and
headway and spacing
The average vehicle spacing in a traffic stream is directly related to the density of the
traffic stream, as determined by Equation 7-6.
5,280
Density (veh/mi) =
spacing(ft I veh)
The relationship between average spacing and average headway in a traffic stream
depends on speed, as indicated in Equation 7-7.
spacing (ft/veh)
Headway (s/veh) =
speed (ft/s)
This relationship also holds for individual headways and spacings between pairs of
vehicles. The speed is that of the second vehicle in a pair of vehicles. Flow rate is
related to the average headway of the traffic stream with Equation 7-8.
3, 600
Flow rate (veh/h) =
headway (s I veh)
RELATIONSHIPS AMONG BASIC PARAMETERS
Equation 7-5 cites the basic relationship among the three parameters, describing an
uninterrupted traffic stream. Although the equation v = S * D algebraically allows for a
given flow rate to occur in an infinite number of combinations of speed and density, there
are additional relationships restricting the variety of flow conditions at a location.
Exhibit 7-2 shows a generalized representation of these relationships, which are the
basis for the capacity analysis of uninterrupted-flow facilities. The flow-density function
is placed directly below the speed-density relationship because of their common
horizontal scales, and the speed-flow function is placed next to the speed-density
relationship because of their common vertical scales. Speed is space mean speed.
EXHIBIT 7-2.
GENERALIZED RELATIONSHIPS AMONG SPEED, DENSITY, AND FLOW RATE ON
UNINTERRUPTED-FLOW FACILITIES
(7-6)
(7-7)
(7-8)
Illustration of speed-density,
flow-density, and speed-flow
relationships
The form of these functions depends on the prevailing traffic and roadway conditions
on the segment under study and on its length in determining density. Although the
diagrams in Exhibit 7-2 show continuous curves, it is unlikely that the full range of the
7-5 ?
Chapter 7 - Traffic Flow Parameters
Uninterrupted Flow
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Highway Capacity Manual 2000
functions would appear at any particular location. Survey data usually show
discontinuities, with part of these curves not present
(2).
The curves of Exhibit 7-2 illustrate several significant points. First, a zero flow rate
occurs under two different conditions. One is when there are no vehicles on the facility—
density is zero, and flow rate is zero. Speed is theoretical for this condition and would be
selected by the first driver (presumably at a high value). This speed is represented by Sf
in the graphs.
The second is when density becomes so high that all vehicles must stop—the speed
is zero, and the flow rate is zero, because there is no movement and vehicles cannot pass
a point on the roadway. The density at which all movement stops is called jam density,
denoted by Dj in the diagrams.
Between these two extreme points, the dynamics of traffic flow produce a
maximizing effect. As flow increases from zero, density also increases, since more
vehicles are on the roadway. When this happens, speed declines because of the
interaction of vehicles. This decline is negligible at low and medium densities and flow
rates. As density increases, these generalized curves suggest that speed decreases
significantly before capacity is achieved. Capacity is reached when the product of
density and speed results in the maximum flow rate. This condition is shown as optimum
speed S o (often called critical speed), optimum density D o (sometimes referred to as
critical density), and maximum flow v
m .
The slope of any ray line drawn from the origin of the speed-flow curve represents
the inverse of density, based on Equation 7-5. Similarly, a ray line in the flow-density
graph represents speed. As examples, Exhibit 7-2 shows the average free-flow speed and
speed at capacity, as well as optimum and jam densities. The three diagrams are
redundant, since if any one relationship is known, the other two are uniquely defined.
The speed-density function is used mostly for theoretical work; the other two are used in
this manual to define LOS.
As shown in Exhibit 7-2, any flow rate other than capacity can occur under two
different conditions, one with a high speed and low density and the other with high
density and low speed. The high-density, low-speed side of the curves represents
oversaturated flow. Sudden changes can occur in the state of traffic (i.e., in speed,
density, and flow rate). LOS A though E are defined on the low-density, high-speed side
of the curves, with the maximum-flow boundary of LOS E placed at capacity; by
contrast, LOS F, which describes oversaturated and queue discharge traffic, is represented
by the high-density, low-speed part of the functions.
III. INTERRUPTED FLOW
Interrupted flow is more complex than uninterrupted flow because of the time
dimension involved in allocating space to conflicting traffic streams. On an interrupted-
flow facility, flow usually is dominated by points of fixed operation, such as traffic
signals and stop signs. These controls have different impacts on overall flow.
The operational state of traffic at an interrupted traffic-flow facility is defined by the
following measures:
Volume and flow rate,
• Saturation flow and departure headways,
• Control variables (stop or signal control),
• Gaps available in the conflicting traffic streams, and
Delay.
The discussion of volume and flow rate in the first part of this chapter also is applicable
to interrupted-flow facilities. An important additional point is the screenline at which the
Basic concepts for
interrupted-flow facilities:
intersection control,
saturation flow rate, lost
time, and queuing
Chapter 7 - Traffic Flow Parameters ?
7-6
Uninterrupted Flow
AR00042817

• • • • • • • • • •
4
3
2
Vehicle in Queue
1
2
3
4
Departure in Headway
h +
h + t
2
h + t
3
h +
Legend
I
Stop line
Highway Capacity Manual 2000
traffic volume or flow rate is surveyed. Traditional intersection traffic counts yield only
the number of vehicles that have departed the intersection. The maximum flow is
therefore limited to the capacity of the facility. When demand exceeds capacity and a
queue is growing, it is advisable to survey traffic upstream, before the congestion.