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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
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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 aarcher@nas.edu).
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
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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
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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
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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
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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-
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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-
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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.
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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
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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
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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
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