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SPECIAL REPORT 254 MANAGING SPEED Review of Current Practice for Setting and Enforcing Speed Limits Committee for Guidance on Setting and Enforcing Speed Limits TRANSPORTATION RESEARCH BOARD National Research Council NATIONAL ACADEMY PRESS WASHINGTON, D.C. 1998
Transportation Research Board Special Report 254 Subscriber Categories IVA highway operations, capacity, and traffic control IVB safety and human performance Transportation Research Board publications are available by ordering individual pub- lications directly from the TRB Business Office, through the Internet at http://www.nas.edu/trb/ index.html, or by annual subscription through organizational or individual affiliation with TRB. Affiliates and library subscribers are eligible for substantial discounts. For further information, contact the Transportation Research Board Business Office, National Research Council, 2101 Constitution Avenue, N.W., Washington, D.C. 20418 (telephone 202-334-3214; fax 202-334- 2519; or e-mail email@example.com). Copyright 1998 by the National Academy of Sciences. All rights reserved. Printed in the United States of America. NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special com- petencies and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to the procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The views expressed in the individually authored papers that are included in this report are those of the authors and do not necessarily reflect the views of the committee, the Transportation Research Board, the National Research Council, or the project's sponsors. The study was sponsored by the National Highway Traffic Safety Administration and the Federal Highway Administration of the U.S. Department of Transportation and by the Centers for Disease Control and Prevention of the U.S. Department of Health and Human Services. Library of Congress Cataloging-in-Publication Data Managing Speed : review of current practice for setting and enforcing speed limits / Committee for Guidance on Setting and Enforcing Speed Limits, Transportation Research Board, National Research Council, National Academy Press. p. cm.--(Special report (National Research Council (U.S.). Transportation Research Board) ; 254) ISBN 0-309-06502-X 1. Speed limits--United States. 2. Roads--United States--Safety measures. I. National Research Council (U.S.). Transportation Research Board. Committee for Guidance on Setting and Enforcing Speed Limits. II. Series. HE5620.S6M36 1998 388.3 144 0973--dc21 98-40323 CIP
Committee for Guidance on Setting and Enforcing Speed Limits John G. Milliken, Chairman, Venable, Baetjer & Howard, McLean, Virginia Forrest M. Council, University of North Carolina, Chapel Hill Terrance W. Gainer, Metropolitan Police Department, Washington, D.C.* Nicholas J. Garber, University of Virginia, Charlottesville Kristine M. Gebbie, Columbia University School of Nursing, New York Jerome W. Hall, University of New Mexico, Albuquerque Charles A. Lave, University of California, Irvine John M. Mason, Jr., The Pennsylvania State University, University Park Frederick Mosteller, Harvard University, Cambridge Sharon D. Nichols, Western Highway Institute, Casper, Wyoming Clinton V. Oster, Jr., Indiana University, Bloomington Richard A. Retting, Insurance Institute for Highway Safety, Arlington, Virginia Thomas B. Sheridan, Massachusetts Institute of Technology, Cambridge William C. Taylor, Michigan State University, East Lansing George Tsebelis, University of California, Los Angeles David C. Viano, General Motors Corporation, Warren, Michigan Richard P. Weaver, California Department of Transportation (retired), Sacramento *Formerly Illinois State Police, Springfield
MANAGING SPEED iv Liaison Representatives Stephen F. Campbell, Sr., American Trucking Associations, Alexandria, Virginia Richard P. Compton, National Highway Traffic Safety Administration, Washington, D.C. Mark Edwards, American Automobile Association, Heathrow, Florida David J. Hensing, American Association of State Highway and Transportation Officials, Washington, D.C. Henry Jasny, Advocates for Highway and Auto Safety, Washington, D.C. Richard Pain, Transportation Research Board, Washington, D.C. Transportation Research Board Staff Nancy P. Humphrey, Study Director
Preface With repeal of the National Maximum Speed Limit (NMSL) of 55 mph (89 km/h) in 1995, states once again have the responsibility for setting appropriate speed limits on major highways. Much has changed in the 20 years during which the NMSL was in effect. Passenger vehicles have become more crashworthy, and vehicles are equipped with more safety features like airbags. Improvements in highway design, roadside safety, and emergency medical services have made the highways safer and assistance to crash victims more rapid. Drivers and other vehicle occupants are buckling up more, and drunk driving is less widespread than it used to be. Together these improve- ments have contributed to a national reduction in highway fatality rates, although rates have stabilized in recent years and total numbers of fatalities and injuries have crept up. States have raised speed lim- its on many major highways, and several states report that driving speeds are up, particularly for those who drive well in excess of speed limits. Some drivers appear to have reacted to safer conditions on the highways by altering their perception of the riskiness of driving and engaging in more risk-taking behavior. Methods for setting speed limits have essentially remained unchanged since before the NMSL came into effect. In the wake of the repeal of the NMSL, many states and some local governments are reexamining speed limit policies; most have already raised speed lim- its. Thus, it is an appropriate time to reevaluate speed limit and relat- v
MANAGING SPEED vi ed enforcement policies not only for Interstate highways but for all road classes. The Transportation Research Board (TRB) undertook a major evaluation of the NMSL in 1984 to provide Congress with an assessment of the costs and benefits of speed limit policies in effect at that time. The primary objective of this study--requested and funded by the National Highway Traffic Safety Administration (NHTSA), the Federal Highway Administration, and the Centers for Disease Control and Prevention--is to review current practice for setting and enforcing speed limits on all types of roads. There are numerous strategies for managing driving speeds. The charge of this study, however, is focused primarily on regulating speed through speed limits and enforcement. More specifically, and in response to the charge, the study reviews existing methods of setting and enforcing speed limits, taking into consideration relevant research, opportunities provided by new technology, and expected changes in highway travel. The findings of the study are presented in the form of guidance, rather than standards, to those who must make decisions about appropriate speed limits and related enforcement policies. To conduct the study, TRB formed a panel of 17 experts under the leadership of John G. Milliken, Partner at the firm of Venable, Baetjer & Howard. The study committee includes experts in traffic engineering, highway design, traffic operations and highway safety, vehicle design and biomechanics, human factors, public health, traf- fic enforcement, highway users, economics, statistics, political sci- ence, and public policy. The committee was assisted during its deliberations by the input and advice of several liaison representa- tives. The committee also supplemented its expertise with invited presentations by state and local traffic engineers, local law enforce- ment officers, and a circuit court judge. The report that follows, how- ever, represents the consensus view solely of the study committee. The committee wishes to acknowledge the work of many individ- uals who contributed to the report. Nancy P. Humphrey managed the study and drafted the final report under the guidance of the commit- tee and the supervision of Stephen R. Godwin, Director of Studies and Information Services. The committee also commissioned three literature reviews to inform its deliberations. The papers are append- ed to the report to make the information available to a broad audi-
vii Preface ence. The interpretations and conclusions reached in the papers are those of the authors; the key findings endorsed by the committee appear in the main body of the report. David Shinar of Ben-Gurion University of the Negev, Israel, reviewed the theoretical literature on the relationship between speed and safety. The major findings of that paper are included in Chapter 2 with supporting detail in Appendix B. Patrick S. McCarthy of Purdue University reviewed empirical evi- dence of the effect of speed limits on vehicle speeds and highway safety. The major findings of that paper are included in Chapter 3 with the full review in Appendix C. William D. Glauz of the Midwest Research Institute reviewed experience with automated technologies for speed management and enforcement. The major findings of that paper are included in Chapter 4 with the full detail in Appendix D. The committee also wishes to thank Suzanne Schneider, Assistant Executive Director of TRB, who managed the report review process. The report has been reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report: R. Quinn Brackett, Brackett & Associates; Gary Byrd, Alexandria, Virginia; James H. Hedlund, NHTSA (retired); Lester A. Hoel, University of Virginia; Joseph Hummer, North Carolina State University; Gerald W. Hyland, Fairfax County Board of Supervisors; Herbert S. Levinson, Herbert S. Levinson Transportation Consultant; Bradley L. Mallory, Pennsylvania Department of Transportation; John A. Rice, University of California, Berkeley; Thomas C. Schelling, University of Maryland. While the individuals listed above have provided constructive comments and suggestions, it must be emphasized that responsibili-
MANAGING SPEED viii ty for the final content of this report rests solely with the authoring committee and the institution. The report was edited and prepared for publication under the supervision of Nancy A. Ackerman, Director of Reports and Editorial Services, TRB. Special appreciation is expressed to Norman Solomon, who edited the report, and to Marguerite Schneider, who assisted in meeting arrangements, travel plans, and communications with the committee and provided word processing support for prepa- ration of the final manuscript.
Contents Executive Summary 1 1 Introduction 15 Scope of Study and Charge, 20 Responsibility for Setting Speed Limits, 21 Purpose of Setting Speed Limits, 22 Overview of Issues Related to Setting Speed Limits, 24 Factors Affecting Determination of Appropriate Speed Limits, 29 Organization of Report, 33 2 Effects of Speed 36 Determination of Appropriate Driving Speeds-- Making Trade-Offs, 37 Relation of Speed to Safety, 38 Relation of Speed to Travel Time, 66 Relation of Speed to Fuel Use and Other Vehicle Operating Costs, 68 Relation of Speed to Emissions, 70 Summary, 71 3 Managing Speeds: Speed Limits 77 Regulating Speed--A Theoretical Justification, 78
Methods of Setting Speed Limits, 80 Application of Speed Limits, 103 Effectiveness of Speed Limits, 110 Summary, 131 4 Speed Enforcement and Adjudication 139 Lessons from Deterrence Theory, 140 Application of Deterrence Theory to Speed Enforcement, 144 Alternative Methods for Increasing Effectiveness of Speed Enforcement, 150 Sanctions and Adjudication, 159 Summary, 161 5 Other Speed Management Strategies 166 Roadway Design, Infrastructure Improvements, and Traffic Control, 167 Vehicle- and Highway-Related Technologies, 173 Special Driving Populations, 178 Summary, 181 6 Guidance on Setting and Enforcing Speed Limits 186 Why Regulate Driving Speeds? 187 What Is the Purpose of Speed Limits? 189 What Information Should Speed Limits Convey to the Driver? 191 How Should Speed Limits Be Set? 192 Can Drivers Be Induced To Obey Speed Limits Through Enforcement? 209 How Can the Judicial System Assist in Achieving Desired Driving Speeds? 211 What Potential Does Technology Offer To Improve Methods of Determining and Enforcing Speed Limits? 212 Concluding Comment, 213
Appendixes 215 A: Trade-Off Decisions in Selecting Driving Speeds, 215 B: Speed and Crashes: A Controversial Topic and an Elusive Relationship, 221 David Shinar C: Effect of Speed Limits on Speed Distributions and Highway Safety: A Survey of the Literature, 277 Patrick McCarthy D: Review of Automated Technologies for Speed Management and Enforcement, 359 William D. Glauz E: Glossary, 391 Study Committee Biographical Information 419