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NATIONAL
NCHRP REPORT 617
COOPERATIVE
HIGHWAY
RESEARCH
PROGRAM
Accident Modification Factors for
Traffic Engineering and
ITS Improvements
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TRANSPORTATION RESEARCH BOARD 2008 EXECUTIVE COMMITTEE*
OFFICERS
CHAIR: Debra L. Miller, Secretary, Kansas DOT, Topeka
VICE CHAIR: Adib K. Kanafani, Cahill Professor of Civil Engineering, University of California, Berkeley
EXECUTIVE DIRECTOR: Robert E. Skinner, Jr., Transportation Research Board
MEMBERS
J. Barry Barker, Executive Director, Transit Authority of River City, Louisville, KY
Allen D. Biehler, Secretary, Pennsylvania DOT, Harrisburg
John D. Bowe, President, Americas Region, APL Limited, Oakland, CA
Larry L. Brown, Sr., Executive Director, Mississippi DOT, Jackson
Deborah H. Butler, Executive Vice President, Planning, and CIO, Norfolk Southern Corporation, Norfolk, VA
William A.V. Clark, Professor, Department of Geography, University of California, Los Angeles
David S. Ekern, Commissioner, Virginia DOT, Richmond
Nicholas J. Garber, Henry L. Kinnier Professor, Department of Civil Engineering, University of Virginia, Charlottesville
Jeffrey W. Hamiel, Executive Director, Metropolitan Airports Commission, Minneapolis, MN
Edward A. (Ned) Helme, President, Center for Clean Air Policy, Washington, DC
Will Kempton, Director, California DOT, Sacramento
Susan Martinovich, Director, Nevada DOT, Carson City
Michael D. Meyer, Professor, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta
Michael R. Morris, Director of Transportation, North Central Texas Council of Governments, Arlington
Neil J. Pedersen, Administrator, Maryland State Highway Administration, Baltimore
Pete K. Rahn, Director, Missouri DOT, Jefferson City
Sandra Rosenbloom, Professor of Planning, University of Arizona, Tucson
Tracy L. Rosser, Vice President, Corporate Traffic, Wal-Mart Stores, Inc., Bentonville, AR
Rosa Clausell Rountree, Executive Director, Georgia State Road and Tollway Authority, Atlanta
Henry G. (Gerry) Schwartz, Jr., Chairman (retired), Jacobs/Sverdrup Civil, Inc., St. Louis, MO
C. Michael Walton, Ernest H. Cockrell Centennial Chair in Engineering, University of Texas, Austin
Linda S. Watson, CEO, LYNXCentral Florida Regional Transportation Authority, Orlando
Steve Williams, Chairman and CEO, Maverick Transportation, Inc., Little Rock, AR
EX OFFICIO MEMBERS
Thad Allen (Adm., U.S. Coast Guard), Commandant, U.S. Coast Guard, Washington, DC
Joseph H. Boardman, Federal Railroad Administrator, U.S.DOT
Rebecca M. Brewster, President and COO, American Transportation Research Institute, Smyrna, GA
Paul R. Brubaker, Research and Innovative Technology Administrator, U.S.DOT
George Bugliarello, Chancellor, Polytechnic University of New York, Brooklyn, and Foreign Secretary, National Academy of Engineering,
Washington, DC
Sean T. Connaughton, Maritime Administrator, U.S.DOT
LeRoy Gishi, Chief, Division of Transportation, Bureau of Indian Affairs, U.S. Department of the Interior, Washington, DC
Edward R. Hamberger, President and CEO, Association of American Railroads, Washington, DC
John H. Hill, Federal Motor Carrier Safety Administrator, U.S.DOT
John C. Horsley, Executive Director, American Association of State Highway and Transportation Officials, Washington, DC
Carl T. Johnson, Pipeline and Hazardous Materials Safety Administrator, U.S.DOT
J. Edward Johnson, Director, Applied Science Directorate, National Aeronautics and Space Administration, John C. Stennis Space Center, MS
William W. Millar, President, American Public Transportation Association, Washington, DC
Nicole R. Nason, National Highway Traffic Safety Administrator, U.S.DOT
James Ray, Acting Administrator, Federal Highway Administration, U.S.DOT
James S. Simpson, Federal Transit Administrator, U.S.DOT
Robert A. Sturgell, Acting Administrator, Federal Aviation Administration, U.S.DOT
Robert L. Van Antwerp (Lt. Gen., U.S. Army), Chief of Engineers and Commanding General, U.S. Army Corps of Engineers, Washington, DC
*Membership as of May 2008.
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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM
NCHRP REPORT 617
Accident Modification Factors for
Traffic Engineering and
ITS Improvements
David L. Harkey
Raghavan Srinivasan
Jongdae Baek
THE UNIVERSITY OF NORTH CAROLINA
HIGHWAY SAFETY RESEARCH CENTER
Chapel Hill, NC
Forrest M. Council
Kimberly Eccles
Nancy Lefler
Frank Gross
VANASSE HANGEN BRUSTLIN, INC.
Raleigh, NC
Bhagwant Persaud
Craig Lyon
DEPARTMENT OF CIVIL ENGINEERING
RYERSON UNIVERSITY
Toronto, ON, Canada
Ezra Hauer
CONSULTANT
Toronto, ON, Canada
James A. Bonneson
TEXAS TRANSPORTATION INSTITUTE
College Station, TX
Subject Areas
Safety and Human Performance
Research sponsored by the American Association of State Highway and Transportation Officials
in cooperation with the Federal Highway Administration
TRANSPORTATION RESEARCH BOARD
WASHINGTON, D.C.
2008
www.TRB.org
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NATIONAL COOPERATIVE HIGHWAY NCHRP REPORT 617
RESEARCH PROGRAM
Systematic, well-designed research provides the most effective Project 17-25
approach to the solution of many problems facing highway ISSN 0077-5614
administrators and engineers. Often, highway problems are of local ISBN: 978-0-309-11738-8
interest and can best be studied by highway departments individually Library of Congress Control Number 2008905366
or in cooperation with their state universities and others. However, the © 2008 Transportation Research Board
accelerating growth of highway transportation develops increasingly
complex problems of wide interest to highway authorities. These
problems are best studied through a coordinated program of COPYRIGHT PERMISSION
cooperative research.
Authors herein are responsible for the authenticity of their materials and for obtaining
In recognition of these needs, the highway administrators of the written permissions from publishers or persons who own the copyright to any previously
American Association of State Highway and Transportation Officials published or copyrighted material used herein.
initiated in 1962 an objective national highway research program Cooperative Research Programs (CRP) grants permission to reproduce material in this
employing modern scientific techniques. This program is supported on publication for classroom and not-for-profit purposes. Permission is given with the
understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA,
a continuing basis by funds from participating member states of the
FMCSA, FTA, or Transit Development Corporation endorsement of a particular product,
Association and it receives the full cooperation and support of the method, or practice. It is expected that those reproducing the material in this document for
Federal Highway Administration, United States Department of educational and not-for-profit uses will give appropriate acknowledgment of the source of
any reprinted or reproduced material. For other uses of the material, request permission
Transportation.
from CRP.
The Transportation Research Board of the National Academies was
requested by the Association to administer the research program
because of the Board's recognized objectivity and understanding of
NOTICE
modern research practices. The Board is uniquely suited for this
purpose as it maintains an extensive committee structure from which The project that is the subject of this report was a part of the National Cooperative Highway
Research Program conducted by the Transportation Research Board with the approval of
authorities on any highway transportation subject may be drawn; it the Governing Board of the National Research Council. Such approval reflects the
possesses avenues of communications and cooperation with federal, Governing Board's judgment that the program concerned is of national importance and
state and local governmental agencies, universities, and industry; its appropriate with respect to both the purposes and resources of the National Research
Council.
relationship to the National Research Council is an insurance of
The members of the technical committee selected to monitor this project and to review this
objectivity; it maintains a full-time research correlation staff of
report were chosen for recognized scholarly competence and with due consideration for the
specialists in highway transportation matters to bring the findings of balance of disciplines appropriate to the project. The opinions and conclusions expressed
research directly to those who are in a position to use them. or implied are those of the research agency that performed the research, and, while they have
been accepted as appropriate by the technical committee, they are not necessarily those of
The program is developed on the basis of research needs identified
the Transportation Research Board, the National Research Council, the American
by chief administrators of the highway and transportation departments Association of State Highway and Transportation Officials, or the Federal Highway
and by committees of AASHTO. Each year, specific areas of research Administration, U.S. Department of Transportation.
needs to be included in the program are proposed to the National Each report is reviewed and accepted for publication by the technical committee according
Research Council and the Board by the American Association of State to procedures established and monitored by the Transportation Research Board Executive
Committee and the Governing Board of the National Research Council.
Highway and Transportation Officials. Research projects to fulfill these
needs are defined by the Board, and qualified research agencies are The Transportation Research Board of the National Academies, the National Research
Council, the Federal Highway Administration, the American Association of State Highway
selected from those that have submitted proposals. Administration and and Transportation Officials, and the individual states participating in the National
surveillance of research contracts are the responsibilities of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade
Research Council and the Transportation Research Board. or manufacturers' names appear herein solely because they are considered essential to the
object of this report.
The needs for highway research are many, and the National
Cooperative Highway Research Program can make significant
contributions to the solution of highway transportation problems of
mutual concern to many responsible groups. The program, however, is
intended to complement rather than to substitute for or duplicate other
highway research programs.
Published reports of the
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM
are available from:
Transportation Research Board
Business Office
500 Fifth Street, NW
Washington, DC 20001
and can be ordered through the Internet at:
http://www.national-academies.org/trb/bookstore
Printed in the United States of America
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COOPERATIVE RESEARCH PROGRAMS
CRP STAFF FOR NCHRP REPORT 617
Christopher W. Jenks, Director, Cooperative Research Programs
Crawford F. Jencks, Deputy Director, Cooperative Research Programs
Charles W. Niessner, Senior Program Officer
Eileen P. Delaney, Director of Publications
Ellen M. Chafee, Assistant Editor
NCHRP PROJECT 17-25 PANEL
Field of Traffic--Area of Safety
John S. Miller, Virginia DOT, Charlottesville, VA (Chair)
Jonathan S. Bray, New York State DOT, Albany, NY (retired)
Donald L. Dean, California DOT, Sacramento, CA
Keith R. Gates, Federal Transit Administration, Washington, DC
Rashad Hanbali, Department of Public Works, Cape Coral, FL
Mohammad M. Khan, Ohio DOT, Columbus, Ohio (retired)
Douglas McKelvey, Federal Motor Carrier Safety Administration, Washington, DC (retired)
Do H. Nam, T-CONCEPTS, Madison Heights, MI
Eileen Rackers, Missouri DOT, Jefferson City, MO
Michael D. Freitas, FHWA Liaison
Richard F. Pain, TRB Liaison
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FOREWORD
By Charles W. Niessner
Staff Officer
Transportation Research Board
This report presents the findings of a research project to develop accident modification
factors (AMFs) for traffic engineering and ITS improvements. AMFs are a tool for quickly
estimating the impact of safety improvements. The report will be of particular interest to
safety practitioners responsible for programming and implementing highway safety
improvements.
Accident modification factors (AMFs), also known as crash reduction factors, provide a
computationally simple and quick way of estimating crash reductions. Many states and local
agencies have a set of crash reduction factors that are used for estimating the safety impacts
of various types of engineering improvements, encompassing the areas of signing, align-
ment, channelization, and other traffic engineering solutions. Typically, these factors are
computed using before-and-after comparisons, although recent research also has suggested
the use of cross-sectional comparisons.
Currently, AMFs are often used in program planning to make decisions concerning
whether to implement a specific treatment and/or to quickly determine the costs and ben-
efits of selected alternatives. AMFs are also used in project development for nonsafety as well
as safety-specific projects and could be used by agencies in deciding on policies affecting
general project design (e.g., context-sensitive design solutions and traffic calming). AMFs
are also key components of the latest safety-estimation tools and procedures, including the
Interactive Highway Safety Design Model and the procedures now being developed for the
Highway Safety Manual.
Even though accurate AMFs are critically important to states and municipalities in their
attempts to achieve the greatest return on investment when choosing among alternative
safety treatments, there is no accepted standard set of AMFs. This is because the accuracy
and reliability of many published AMFs is questionable, and no AMFs exist for many impor-
tant safety treatments. The sources of the problem include the lack of AMFs for newer treat-
ments and for common combinations of treatments. AMFs also vary with factors such as
traffic volume, and in some evaluations, crash migration and spillover effects that result
from some treatments are not accounted for in the AMF. However, the major problems
with many existing AMFs result from the poor data and poor evaluation methods used in
their development. Often, AMFs are based on simple before-after studies of high-crash loca-
tions, and the results can be very biased toward overestimating accident reductions.
Under NCHRP Project 17-25, "Crash Reduction Factors for Traffic Engineering and ITS
Improvements," researchers at the University of North Carolina Highway Safety Research
Center developed or modified AMFs for a number of high-priority treatments. The research
team reviewed the literature and ongoing research related to AMF development and pre-
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pared an initial list of treatments deemed to be important in safety decisions. A survey of
state DOTs expanded the list to 100 treatments. The determination of which of the many
possible AMFs should be developed or improved was based on several factors, including the
results of the state survey, the measure of crash-related harm that might be affected by the
treatment, and the availability of data needed in AMF development or improvement.
Two approaches were used in developing the AMFs. The first approach was the rigorous
statistical evaluation of crash data, with priority given to conducting as many empirical
Bayes before-after evaluations of the high-priority treatments as possible. The second
approach to AMF development/modification involved two analysis-driven expert panels.
In summary, this project has verified, modified, or developed 35 AMFs that are deemed
to be of high or medium-high quality. These have been documented in formats that are
usable by both practitioners and researchers. These AMFs are the primary project outputs.
However, the project has also documented both a process that can be used with future
analysis-driven expert panels and the detailed discussions of the two expert panels that were
part of this effort. This material should be helpful in future efforts to develop or improve
AMFs. Finally, the project developed and documented a procedure for ranking needed AMF
research that incorporates not only state DOT user and researcher opinions and knowledge
of the quality of AMFs in the published literature, but also a method for estimating how
crash-related harm might be affected by each treatment. An approach combining these pro-
cedures could also be used in more global efforts to prioritize roadway safety research needs
in general.
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CONTENTS
1 Summary
4 Chapter 1 Introduction
4 The Problem
5 Project Objective and Overview
8 Organization of Report
9 Chapter 2 Status of Existing AMFs and Identification
of AMF Needs
9 Extracting Information on Existing AMFs and Determining AMF Quality
11 Prioritizing Phase II Efforts to Develop Additional AMFs
17 Summary
18 Chapter 3 Development of New AMFs through Analysis
or Reanalysis of Crash Data
18 Introduction
19 Overview of the Empirical Bayes (EB) Methodology
19 Installation of a Rural Traffic Signal
21 Conversion of an Undivided Four-Lane Road to Three Lanes and a Two-Way
Left-Turn Lane--a "Road Diet"
22 Increasing Pavement Friction on Roadway Segments and at Intersection
Approaches
24 Signalized Intersection Treatments in Urban Areas
26 Speed Change and Crashes
27 Effect of Median Width
30 Chapter 4 Development of New AMFs through Expert Panels
30 Introduction
30 Members of the Panels
30 Procedures Followed
33 Results
34 Chapter 5 Compilation of Recommended AMFs
34 Introduction
34 AMF Summaries
68 Chapter 6 Conclusions
70 References
74 Appendices
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AUTHOR ACKNOWLEDGMENTS
The research reported herein was performed under NCHRP Project 17-25. The University of North
Carolina Highway Safety Research Center (HSRC) was the contractor for this study. Vanasse Hangen
Brustlin, Inc. (VHB) and Ryerson University were subcontractors. Authors of this report are David Harkey
(Director, HSRC), Forrest Council (Senior Research Scientist, VHB), Raghavan Srinivasan (Senior Trans-
portation Research Engineer, HSRC), Craig Lyon (Ryerson University), Bhagwant Persaud (Ryerson Uni-
versity), Kimberly Eccles (VHB), Nancy Lefler (VHB), Frank Gross (VHB), Jongdae Baek (HSRC), Ezra
Hauer (Consultant), and James A. Bonneson (Texas Transportation Institute). Mr. Harkey served as the
Principal Investigator for this effort.
The authors wish to acknowledge the assistance and support of others who made this project a success.
Two expert panels were convened to review current knowledge and develop accident modification factors
(AMFs) for both urban/suburban arterials and rural multilane roads. These panels included the following
individuals, and the authors express their thanks to them for their efforts before, during and after the
actual panel meetings.
· Dr. James A. Bonneson, Texas Transportation Institute
· Mr. Doug Harwood, Midwest Research Institute
· Dr. Ezra Hauer, Independent Consultant
· Mr. Loren Hill, Minnesota DOT
· Dr. Dominique Lord, Texas A&M University
· Mr. Brian Mayhew, North Carolina DOT
· Mr. Stan Polanis, City of Winston Salem
· Dr. Simon Washington, Arizona State University
· Mr. Thomas Welch, Iowa DOT
The authors also wish to express additional thanks to Mr. Stan Polanis, Traffic Engineer for the City of
Winston-Salem, North Carolina, for his assistance in efforts to improve AMFs for five key urban treat-
ments. Mr. Polanis provided the authors with detailed information on the specifics of treatment imple-
mentation in Winston-Salem along with before-treatment and after-treatment crash data that had been
manually screened by his staff. His efforts were significant and critical to the success of that project task.
Finally, because the project involved the assessment and improvement of existing AMFs and the devel-
opment of new ones through multiple approaches that could not all be envisioned at the beginning of the
effort, there was considerable interaction with both NCHRP staff and the NCHRP project oversight panel.
The authors wish to express sincere thanks to NCHRP Senior Program Officer Charles Niessner for his
assistance throughout the project and to the individual members of the project panel, who provided
extremely helpful feedback on the many documents they were asked to review.
