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NATIONAL NCHRP REPORT 633 COOPERATIVE HIGHWAY RESEARCH PROGRAM Impact of Shoulder Width and Median Width on Safety

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TRANSPORTATION RESEARCH BOARD 2009 EXECUTIVE COMMITTEE* OFFICERS CHAIR: Adib K. Kanafani, Cahill Professor of Civil Engineering, University of California, Berkeley VICE CHAIR: Michael R. Morris, Director of Transportation, North Central Texas Council of Governments, Arlington 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 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 Debra L. Miller, Secretary, Kansas DOT, Topeka 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, Consultant, Tyrone, GA Steve T. Scalzo, Chief Operating Officer, Marine Resources Group, Seattle, WA 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 Rebecca M. Brewster, President and COO, American Transportation Research Institute, Smyrna, GA George Bugliarello, President Emeritus and University Professor, Polytechnic Institute of New York University, Brooklyn; Foreign Secretary, National Academy of Engineering, Washington, DC James E. Caponiti, Acting Deputy Administrator, Maritime Administration, U.S.DOT Cynthia Douglass, Acting Deputy Administrator, Pipeline and Hazardous Materials Safety Administration, 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 C. Horsley, Executive Director, American Association of State Highway and Transportation Officials, Washington, DC Rose A. McMurry, Acting Deputy Administrator, Federal Motor Carrier Safety Administration, U.S.DOT Ronald Medford, Acting Deputy Administrator, National Highway Traffic Safety Administration, U.S.DOT William W. Millar, President, American Public Transportation Association, Washington, DC Lynne A. Osmus, Acting Administrator, Federal Aviation Administration, U.S.DOT Jeffrey F. Paniati, Acting Deputy Administrator and Executive Director, Federal Highway Administration, U.S.DOT Steven K. Smith, Acting Deputy Administrator, Research and Innovative Technology Administration, U.S.DOT Jo Strang, Acting Deputy Administrator, Federal Railroad 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 Matthew Welbes, Executive Director and Acting Deputy Administrator, Federal Transit Administration, U.S.DOT *Membership as of February 2009.

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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP REPORT 633 Impact of Shoulder Width and Median Width on Safety Nikiforos Stamatiadis Jerry Pigman UNIVERSITY OF KENTUCKY Lexington, KY IN COOPERATION WITH John Sacksteder HMB PROFESSIONAL ENGINEERS, INC. Frankfort, KY Wendel Ruff ABMB ENGINEERS INC. Jackson, MS AND Dominique Lord TEXAS TRANSPORTATION INSTITUTE AND ZACHRY DEPARTMENT OF CIVIL ENGINEERING TEXAS A&M UNIVERSITY College Station, TX Subject Areas Highway Operations, Capacity, and Traffic Control 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. 2009 www.TRB.org

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NATIONAL COOPERATIVE HIGHWAY NCHRP REPORT 633 RESEARCH PROGRAM Systematic, well-designed research provides the most effective Project 15-27 approach to the solution of many problems facing highway ISSN 0077-5614 administrators and engineers. Often, highway problems are of local ISBN: 978-0-309-11782-1 interest and can best be studied by highway departments individually Library of Congress Control Number 2009927863 or in cooperation with their state universities and others. However, the 2009 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 633 Christopher W. Jenks, Director, Cooperative Research Programs Crawford F. Jencks, Deputy Director, Cooperative Research Programs Edward T. Harrigan, Senior Program Officer Eileen P. Delaney, Director of Publications Andrea Briere, Editor Margaret B. Hagood, Editor NCHRP PROJECT 15-27 PANEL Field of Design--Area of General Design Kathleen A. King, Ohio DOT, Columbus, OH (Chair) Duane H. Amos, Russellville, MO Thomas Beeman, Texas DOT, Austin, TX Richard W. Dunne, New Jersey DOT, Trenton, NJ Wayne Gannett, New York State DOT, Albany, NY James R. Kladianos, Wyoming DOT, Laramie, WY John B. Pickering, Mississippi DOT, Jackson, MS Paul Smith, Portland Office of Transportation, Portland, OR Don T. Arkle, Alabama DOT, AASHTO Monitor Ray Krammes, FHWA Liaison Stephen Maher, TRB Liaison AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 15-27 by the Kentucky Transporta- tion Center at the University of Kentucky, HMB Professional Engineers, ABMB Engineers, and the Texas Transportation Institute. The University of Kentucky was the contractor for this study. Dr. Nikiforos Stamatiadis, Professor of Civil Engineering, University of Kentucky, was the principal investigator. The other contributing authors of this report are Jerry Pigman, Research Engineer and co-principal investigator, Don Hartman, Deputy Director, Ken Agent, Research Engineer, and Eric Green, Research Engineer, all of the Kentucky Transportation Center; Professor Richard Kryscio, University of Kentucky; John Sacksteder, Director of Highway Design and Structures, HMB Professional Engineers; Wendel Ruff, Director of Transportation, ABMB Engineers; and Assistant Professor Dominique Lord and Srinivas Geedipally, Graduate Researcher, Texas Transportation Institute, Texas A&M University.

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FOREWORD By Edward T. Harrigan Staff Officer Transportation Research Board This report contains the findings of research performed to quantify the safety and opera- tional impacts of design element trade-offs and their associated risks. The report details the research performed and includes specific recommended crash prediction models and Accident Modification Factors (AMFs) for shoulder width and median width on rural four-lane roads. Thus, the report will be of immediate interest to engineers in state highway agencies responsi- ble for geometric design and traffic operations and safety. Design standards provide a benchmark for the development of elements that compose a highway design. Ideally, every highway design meets the appropriate standards. Realistically, designers are sometimes faced with situations where adherence to standards may not be practical from an engineering, environmental, community, or benefit-cost perspective. In such cases, designers must make decisions regarding the impacts and risks associated with meeting or exceeding the design standards or allowing exceptions to them, for example, in context-sensitive situations. A comprehensive assessment of the safety and operational impacts of trade-offs in design elements is needed to guide designers in weighing appropri- ate trade-offs in design elements against safety and operational concerns for the full range of highway designs, from low volume to high volume, locals to arterials, and 3-R to new construction. This research had two objectives. The first was to quantify the safety and operational impacts of design element trade-offs and their associated risks. The second objective was to develop guidelines to assist designers in making reasonable choices among possible design element trade-offs. The research was carried out in two phases. In Phase I, a literature review and the development of methodology for data collection and analysis were conducted for use in the second phase. In Phase II, extensive data were collected from the literature and individual state databases in the FHWA Highway Safety Information System and analyzed to develop prediction models and AMFs used to understand the safety and operational impacts of the studied design element trade-offs. The original scope of the project encompassed evaluation of design element trade-offs encompassing the full range of highway designs, including context-sensitive solutions and common design exceptions. However, this scope was modified by the NCHRP project panel at the conclusion of Phase I, in order to concentrate on design elements and trade-offs for which there were sufficient data of adequate quality from which to develop well-founded guidance. Specifically, the project panel recommended investigation of the safety impact of design flexibility on rural multi-lane highways of the following: (1) lane width, (2) shoulder width, and (3) median type and width. Final recommended AMFs are presented in the report for shoulder width and median width for four-lane roads with 12-ft lanes.

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Alternate methods are provided for estimating the relative safety of design element choices using either AMFs or prediction models. The research was performed by the University of Kentucky in Lexington, Kentucky. The report fully documents the research leading to the recommended prediction models and AMFs. The recommendations are under consideration for possible inclusion in the future AASHTO Highway Safety Manual.

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CONTENTS 1 Summary 7 Chapter 1 Introduction and Research Approach 7 Problem Statement 7 Research Objectives and Approach 8 Organization of the Report 9 Chapter 2 Literature Review 9 Roadway Design Issues 10 Cross-Section Elements 10 Lanes 11 Shoulders 12 Medians 14 Rural Two-Lane Conversions to Multilane 14 Summary 17 Chapter 3 Data Analysis 17 Methodology 18 Data Base 21 Data Analysis 22 Prediction Models 23 Divided Roads, All Crashes 23 Undivided Roads, All Crashes 23 Divided Roads, Injury Crashes 24 Trade-Offs from Models' AMFs 24 Divided Highways 25 Undivided Highways 25 Injury Models 26 Summary 27 Chapter 4 Design Elements Recommendations 27 Average Shoulder Width 27 Recommendation 28 Supportive Background 29 Median Width 29 Recommendation 29 Supportive Background 30 Median Barrier 30 Recommendation 30 Supportive Background 31 Applications

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32 Chapter 5 Conclusions and Suggested Research 32 Conclusions 32 Suggested Research 34 References A-1 Appendix A Using Prediction Models to Determine Relative Safety of Design Element Choices