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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 794 Median Cross-Section Design for Rural Divided Highways Jerry L. Graham Douglas W. Harwood Karen R. Richard Mitchell K. OâLaughlin MRIglobal Kansas City, MO Eric T. Donnell Sean N. Brennan PennsylvanIa TRansPoRTaTIon InsTITuTe State College, PA Subscriber Categories Designâ â¢â SafetyâandâHumanâFactors TRANSPORTAT ION RESEARCH BOARD WASHINGTON,âD.C. 2014 www.TRB.orgâ 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 794 Project 22-21A ISSN 0077-5614 ISBN 978-0-309-30819-9 Library of Congress Control Number 2014954196 Â© 2014 National Academy of Sciences. All rights reserved. COPYRIGHT INFORMATION 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. The members of the technical panel selected to monitor this project and to review this report were chosen for their special competencies and with regard for appropriate balance. The report was reviewed by the technical panel and accepted for publication according to procedures established and overseen by the Transportation Research Board and approved by the Governing Board of the National Research Council. The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board, the National Research Council, or the program sponsors. The Transportation Research Board of the National Academies, the National Research Council, and the sponsors of 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 the report.
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. Upon 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. C. D. Mote, Jr., 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, upon its own initiative, to identify issues of medical care, research, and education. Dr. Victor J. Dzau 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 Academyâs 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 Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. C. D. Mote, Jr., are chair and vice chair, respectively, of the National Research Council. The Transportation Research Board is one of six major divisions of the National Research Council. The mission of the Transporta- tion Research Board is to provide leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Boardâs varied activities annually engage about 7,000 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 individu- als interested in the development of transportation. www.TRB.org www.national-academies.org
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 CRP STAFF FOR NCHRP REPORT 794 Christopher W. Jenks, Director, Cooperative Research Programs Christopher Hedges, Manager, National Cooperative Highway Research Program B. Ray Derr, Senior Program Officer AndreÃ¡ Parker, Senior Program Assistant Eileen P. Delaney, Director of Publications Hilary Freer, Senior Editor NCHRP PROJECT 22-21A PANEL Field of DesignâArea of Vehicle Barrier Systems Thomas M. Welch, Leidos, Fairfield Bay, AR (Chair) Drew Boyce, Delaware DOT, Dover, DE Rene Garcia, Texas DOT, Austin, TX Michael D. Hurtt, Clough Harbour & Associates LLP, Albany, NY Rodney D. Lacy, Burns & McDonell, Kansas City, MO Richard D. Powers, KLS Engineering, LLC, Herndon, VA James D. Young, Ohio DOT, Columbus, OH Karl H. Zimmerman, Valparaiso University, Valparaiso, IN Martin W. âMartyâ Hargrave, FHWA Liaison Richard A. Cunard, TRB Liaison
F O R E W O R D ByâB.âRayâDerr StaffâOfficer TransportationâResearchâBoard This report presents guidelines for designing typical cross-sections for medians on new and existing rural freeways and divided highways. The report examines the interrelation- ships between median width, median slope, and the use of median barrier on crash risk and severity. The report should be useful to designers and safety analysts, particularly those responsible for agency standards. The AASHTO Policy on Geometric Design of Highways and Streets contains general median width and median side-slope design guidance that has remained unchanged for many years. However, changes have occurred in the vehicle fleet, travel speeds, and traffic volumes that warrant further examination of this guidance. Concern with rollover crashes has caused many state departments of transportation (DOTs) to flatten their depressed medians. More recently, highly visible cross-median crashes have caused many state DOTs to increase their use of median barrier beyond the recommendations in the AASHTO Road- side Design Guide. There is speculation that flatter medians have contributed to the cross- median problem, but the data do not present a clear picture. Installation of median bar- rier reduces the number of cross-median crashes but increases the number of fixed-object crashes. Understanding how different median cross-section designs influence different types of crashes is vital in making safe and cost-effective decisions for state design standards and for project design. In NCHRP Project 22-21, MRIGlobal, in association with the Pennsylvania State Univer- sity, updated the survey of state practice developed in NCHRP Project 17-14, including the types of barrier being installed and the policies for their installation. The research team also compiled information on typical median cross-sections for new construction and recon- struction projects. Based on the literature, the researchers identified design, traffic, and human factors that influence median and roadside safety. The research team then collected field data to assess the safety and cost-effectiveness of various median cross-section designs. Simulations of median encroachments were also made to evaluate the contributions of the various factors to cross-median crashes.
C O N T E N T S Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions. 1â Summaryâ 3 Chapter 1â âIntroduction 3 1.1 Background 3 1.2 Research Objectives and Scope 3 1.3 Organization of This Report 5 Chapter 2â âLiteratureâReview 5 2.1 AASHTO Median Design Guidelines 8 2.2 Review of Median Safety Studies 18 2.3 Other State Highway Agency Median Safety Research 22 2.4 Median Barrier Effectiveness Evaluations 25 Chapter 3â âStateâSurveyâResults 25 3.1 Survey Method 25 3.2 Response Rate 25 3.3 Survey Summary 43 3.4 In-Service Performance Evaluations and Unpublished Reports 43 3.5 Analysis of Trends in the State of Median Design 46 Chapter 4â âSafetyâAnalysisâofâMedianâCross-SectionâDesign 46 4.1 Target Crashes 47 4.2 Analysis of Traversable Medians 63 4.3 Analysis of Medians with Barriers 83 Chapter 5â âMedianâEncroachmentâSimulation 83 5.1 Introduction 83 5.2 Brief History of Vehicle Dynamics Simulations 84 5.3 Methodology for Highway Median Safety Analysis 86 5.4 Data Post-Processing and Analysis 87 5.5 Influence of Median Geometry 93 5.6 Bumper Height During the Off-Road Trajectory 96 5.7 Influence of Driver Intervention 96 5.8 Implications of Simulation Results for Median Design 103 Chapter 6â âInterpretationâofâResultsâandâDesignâGuidelines 103 6.1 Median Width 103 6.2 Median Slope 104 6.3 Median Barriers 105 6.4 Design Guidelines 107 Chapter 7â âConclusionsâandâRecommendations 109â References