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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
