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TRANSPORTAT ION RESEARCH BOARD WASHINGTON, D.C. 2008 www.TRB.org N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP REPORT 617 Subject Areas Safety and Human Performance 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 Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the 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 cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program employing modern scientific techniques. This program is supported on a continuing basis by funds from participating member states of the Association and it receives the full cooperation and support of the Federal Highway Administration, United States Department of Transportation. The Transportation Research Board of the National Academies was requested by the Association to administer the research program because of the Boardâs recognized objectivity and understanding of modern research practices. The Board is uniquely suited for this purpose as it maintains an extensive committee structure from which authorities on any highway transportation subject may be drawn; it possesses avenues of communications and cooperation with federal, state and local governmental agencies, universities, and industry; its relationship to the National Research Council is an insurance of objectivity; it maintains a full-time research correlation staff of specialists in highway transportation matters to bring the findings of research directly to those who are in a position to use them. The program is developed on the basis of research needs identified by chief administrators of the highway and transportation departments and by committees of AASHTO. Each year, specific areas of research needs to be included in the program are proposed to the National Research Council and the Board by the American Association of State Highway and Transportation Officials. Research projects to fulfill these needs are defined by the Board, and qualified research agencies are selected from those that have submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Research Council and the Transportation Research Board. 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 NCHRP REPORT 617 Project 17-25 ISSN 0077-5614 ISBN: 978-0-309-11738-8 Library of Congress Control Number 2008905366 Â© 2008 Transportation Research Board COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this 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, FMCSA, FTA, or Transit Development Corporation endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for 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 from CRP. NOTICE 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 the Governing Board of the National Research Council. Such approval reflects the Governing Boardâs judgment that the program concerned is of national importance and appropriate with respect to both the purposes and resources of the National Research Council. The members of the technical committee selected to monitor this project and to review this report were chosen for recognized scholarly competence and with due consideration for the balance of disciplines appropriate to the project. The opinions and conclusions expressed 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 Transportation Research Board, the National Research Council, the American Association of State Highway and Transportation Officials, or the Federal Highway Administration, U.S. Department of Transportation. Each report is reviewed and accepted for publication by the technical committee according to procedures established and monitored by the Transportation Research Board Executive Committee and the Governing Board of the National Research Council. The Transportation Research Board of the National Academies, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, and the individual states participating in the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturersâ names appear herein solely because they are considered essential to the object of this report.
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 TrafficâArea 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 C O O P E R A T I V E R E S E A R C H P R O G R A M S
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, âCrash Reduction Factors for Traffic Engineering and ITS Improvements,â 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- F O R E W O R D By Charles W. Niessner Staff Officer Transportation Research Board
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.
C O N T E N T S 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 Laneâa âRoad Dietâ 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
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. â¢ Dr. James A. Bonneson, Texas Transportation Institute â¢ Mr. Doug Harwood, Midwest Research Institute â¢ Dr. Ezra Hauer, Independent Consultant â¢ Mr. Loren Hill, Minnesota DOT â¢ Dr. Dominique Lord, Texas A&M University â¢ Mr. Brian Mayhew, North Carolina DOT â¢ Mr. Stan Polanis, City of Winston Salem â¢ Dr. Simon Washington, Arizona State University â¢ 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.