Date: Oct. 1, 1998
Contacts: Craig Hicks, Media Relations Officer
Jennifer Cooke, Media Relations Specialist
(202) 334-2138; e-mail <news@nas.edu>

Publication Announcement

Road Conditions Vary Too Widely
For Single Approach to Speed Limits

When Congress repealed the national maximum speed limit in 1995, state governments again became responsible for setting limits on major highways. Since then, legislatures in every state but Hawaii have raised speed limits on interstate highways and other major roads, renewing interest in how speed limits are established.

A new National Research Council report reviews current practice and offers guidance for setting and enforcing speed limits on all types of roads. But it stops short of recommending specific numeric speed limits, because road and traffic conditions vary too widely to justify a "one-size-fits-all" approach.

There is no single "right" answer in setting appropriate speed limits, the report says. Every speed limit represents a trade-off between safety and travel time, and reflects attempts to achieve an appropriate balance between these two competing goals for different road classes, such as freeways, local roads, and residential streets. While a strong link between vehicle speed and crash severity supports the need for setting maximum limits on high-speed roads, legislators setting specific limits may assign different priorities to key factors such as safety, travel time, enforcement costs, and community concerns. Moreover, the available data do not provide an adequate basis for precisely quantifying the effects that changes in speed limits have on driving speeds, safety, and travel time on different kinds of roads.

Legislators who are considering speed-limit changes for broad classes of roads should consult with traffic engineers, law enforcement officials, judges, public health officials, and the general public in their deliberations, the report says. The process for determining an appropriate speed limit should include consideration of road design characteristics, free-flowing traffic speeds, safety experience, and enforcement levels -- although the relative importance given to each of these factors depends on the type of road.

For example, on rural interstate highways, vehicle-operating speeds, determined from data gathered in spot surveys of free-flowing traffic, often are a major factor in setting speed limits. Drivers typically can anticipate appropriate driving speeds, because these highways usually are built to the highest design standards, access is limited, and roadside activity is minimal. The risks to other road users from higher speeds are apt to be small, because traffic generally is not congested and bicyclists and pedestrians are not on the road. Additionally, maintaining high levels of enforcement is difficult on long stretches of rural interstate highways.

On urban residential streets, however, driver misjudgment about appropriate speeds poses high risks to other road users, so travel efficiency should be given a lower priority. In some cases, neighborhood pressures may result in setting very low speed limits on residential streets, but often they are not strictly enforced and compliance is poor, even by some neighborhood residents. Generally, speed limits on these and other roads should be set at levels that most people will obey, or at the lowest level that police are able to enforce. On residential streets where low speeds are desirable and strict enforcement is impractical, alternatives to reduce speeding such as speed humps may be necessary.

Determination of alternative speed limits in "speed zones" -- where general limits don't fit specific road or traffic conditions -- should be made on the basis of traffic engineering studies and in consultation with law enforcement officials. In cases where the community has expressed concerns about driving speeds, elected officials and citizen groups should have a voice in such decisions. Speed zones should be reviewed periodically to determine whether a change in speed limits or boundaries of the speed zone is warranted, particularly in areas where conditions are changing rapidly, such as developing suburbs.

Enforcement is an integral part of any speed-limit policy. The main difficulty with traditional enforcement methods is their short-lived effect on deterring speeding, the report says. Automated enforcement technologies such as photo-radar are available, but require careful introduction to ensure public support.

The study was sponsored by the National Highway Traffic Safety Administration, Federal Highway Administration, and Centers for Disease Control and Prevention. A committee roster follows.

Read the full text of Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits (TRB Special Report 254) are available at www.nap.edu or by calling 202-334-3313  or 1-800-624-6242.  Reporters may obtain a pre-publication copy from the Office of News and Public Information at the letterhead address (contacts listed above).


NATIONAL RESEARCH COUNCIL
Transportation Research Board
Studies and Information Services

Committee for Guidance on Setting and Enforcing Speed Limits
John G. Milliken (chair)
Partner
Venable, Baetjer & Howard
McLean, Va.

Forrest M. Council
Director
Highway Safety Research Center
University of North Carolina
Chapel Hill

Terrance W. Gainer
Executive Assistant Chief of Police
Metropolitan Police Department
Washington, D.C.

Nicholas J. Garber
Professor and Chair
Department of Civil Engineering
University of Virginia
Charlottesville

Kristine M. Gebbie (1)
Associate Professor of Nursing
Columbia University School of Nursing
New York City

Jerome W. Hall
Professor
Department of Civil Engineering
University of New Mexico
Albuquerque

Charles A. Lave
Professor
Department of Economics
University of California
Irvine

John M. Mason Jr.
Associate Dean of Graduate Studies
and Research, and Professor of Civil Engineering
College of Engineering
Pennsylvania State University
University Park

Frederick Mosteller( 1,2)
Roger I. Lee Professor Emeritus of
Mathematical Statistics
Harvard University
Cambridge, Mass.

Sharon D. Nichols
Executive Director
Western Highway Institute
Casper, Wyo.

Clinton V. Oster Jr.
Professor
School of Public and Environmental Affairs
Indiana University
Bloomington

Richard A. Retting
Senior Transportation Engineer
Insurance Institute for Highway Safety
Arlington, Va.

Thomas B. Sheridan( 3)
Ford Professor of Engineering and Applied Psychology, and Professor of Aeronautics and Astronautics
Massachusetts Institute of Technology
Cambridge

William C. Taylor
Professor of Civil Engineering
Department of Civil and Environmental Engineering
Michigan State University
East Lansing

George Tsebelis
Professor
Department of Political Science
University of California
Los Angeles

David C. Viano
Principal Research Scientist
Research and Development Center
General Motors Corp.
Warren, Mich.

Richard P. Weaver
Deputy Director and Chief Engineer (retired)
California Department of Transportation
Sacramento

RESEARCH COUNCIL STAFF

Nancy P. Humphrey
Staff Officer

1 Member, Institute of Medicine
2 Member, National Academy of Sciences
3 Member, National Academy of Engineering