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News from the National Academies
Date: Feb. 21, 2002
Contacts: Jennifer Burris, Media Relations Associate
Corbin Arberg, Media Relations Assistant
(202) 334-2138; e-mail <news@nas.edu>

FOR IMMEDIATE RELEASE

PUBLICATION ANNOUNCEMENT

ROLLOVER RESISTANCE RATINGS FOR CAR, LIGHT TRUCK, AND
SUV BUYERS NEED IMPROVEMENT

Motor vehicle rollovers involving cars, vans, pickup trucks, and sport utility vehicles result in about 10,000 deaths and 27,000 serious injuries in the United States each year. Rollover accounts for nearly one-third of light-vehicle occupant deaths, even though it occurs in fewer than one in 10 crashes involving light vehicles.

The U.S. government began rating vehicle resistance to rollover in 2001 as a way to inform consumers about vehicle performance. These rollover ratings are based on a measure of a vehicle's "top-heaviness," called the static stability factor. Automobile manufacturers and some consumer groups expressed concern about the decision by the National Highway Traffic Safety Administration to base the five-star rating system on the static stability factor alone, without any consideration of how vehicles handle when they are in motion.

A new report by the National Academies' National Research Council finds that the static stability factor is a useful indicator of a vehicle's propensity to roll over, but that U.S. government ratings for new cars, light trucks, and sport utility vehicles do not adequately reflect differences in rollover resistance shown by available crash data. The five-star system should be revised to allow better discrimination among vehicles and incorporate results from road tests that measure vehicle control and handling characteristics. Moreover, the limited procedures used by NHTSA to develop the ratings and evaluate consumers' ability to understand them raise questions about the system's effectiveness.

The static stability factor is a number generated by dividing a vehicle's track width, or distance between wheels from side to side, by twice its center-of-gravity height -- essentially a measure of how top-heavy the vehicle is. A five-star rating indicates the highest static stability factor, and a one-star rating the lowest. In a single-vehicle crash, a vehicle with a rating of five stars has a less than 10 percent risk of rollover; four stars, between 10 percent and 20 percent; three stars, between 20 percent and 30 percent; two stars, between 30 percent and 40 percent; and one star, greater than 40 percent. According to the agency's analyses of 220,000 actual single-vehicle crashes, taller, narrower vehicles such as sport utility vehicles are more likely than lower, wider vehicles such as passenger cars to roll over. In general, sport utility vehicles receive between one and three stars for rollover resistance; pickup trucks, between one and four stars; vans, two or three stars; and passenger cars, four or five stars.

But the choice of only five broad categories for the rating system does not take advantage of what available crash data show about differences among vehicles, making the system less helpful than it could be for consumers, the report says. For example, the rating categories are so broad that two vehicles given the same star rating may have significantly different rollover tendencies. A rating system with more categories, or a numerical score, could provide more help to consumers who want to choose the safest vehicle within each vehicle class. NHTSA should develop and test different ways of communicating rollover risk with consumers to find the system most effective in leading to informed choices by vehicle buyers.

In addition, the agency should vigorously pursue ongoing research on driving-maneuver tests for rollover resistance, with the goal of developing one or more tests that can be used to assess factors leading to rollover, the committee said. In the longer term, the agency should develop revised consumer information on rollover that incorporates the results of these tests to supplement information such as the static stability factor.

Rollover information from the NHTSA's Web site (http://www.nhtsa.dot.gov) includes guidance on interpreting the five-star ratings, as well as information on what consumers can do to reduce rollover risk, particularly wearing a seat belt to lower the risk of death or serious injury in a rollover crash. The static stability factor is listed for each rated vehicle, forming the basis for the agency's ratings for rollover resistance.

The report was requested by Congress to inform an investigation on the potential role of vehicle characteristics and related consumer information in reducing rollover-related deaths and injuries.

The committee's work was sponsored by the U.S. Department of Transportation. The National Research Council is the principal operating arm of the National Academy of Sciences and the National Academy of Engineering. It is a private, nonprofit institution that provides independent advice on science and technology issues under congressional charter. A committee roster follows.

Read the full text of An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance (TRB Special Report 265) for free on the Web at http://www.nap.edu/. Printed copies will soon be available for purchase from the Transportation Research Board; tel. (202) 334-3213, fax (202) 334-2519, or e-mail <TRBSales@nas.edu>. Reporters may obtain a pre-publication copy from the Office of News and Public Information (contacts listed above).


NATIONAL RESEARCH COUNCIL
Transportation Research Board

COMMITTEE FOR THE STUDY OF A MOTOR VEHICLE ROLLOVER RATING SYSTEM

DAVID N. WORMLEY (CHAIR)
Dean
College of Engineering
Pennsylvania State University
University Park

KARIN M. BAUER
Principal Statistician
Midwest Research Institute
Kansas City, Mo.

JAMES E. BERNARD
Anson Marston Distinguished Professor of Engineering, and
Director
Virtual Reality Applications Center
Iowa State University
Ames

ANN BOSTROM
Associate Professor
School of Public Policy
Georgia Institute of Technology
Atlanta

SUSAN A. FERGUSON
Senior Vice President
Insurance Institute for Highway Safety
Arlington, Va.

B. JOHN GARRICK*
Independent Consultant
Laguna Beach, Calif.

PAUL A. GREEN
Senior Research Scientist
Human Factors Division
University of Michigan Transportation Research Institute, and
Adjunct Associate Professor
University of Michigan
Ann Arbor

DAVID L. HARKEY
Manager of Engineering Studies
Highway Safety Research Center
University of North Carolina
Chapel Hill

J. KARL HEDRICK
James Marshall Wells Professor and Chair
Department of Mechanical Engineering, and
Director
Partners for Advanced Transit and Highways Research Center
University of California
Berkeley

DAVID C. HOLLOWAY
Professor of Mechanical Engineering, and
Director
Center of Graduate Automotive Technology Education
University of Maryland
College Park

L. DANIEL METZ
President
Metz Engineering and Racing, and
Professor Emeritus
Department of General Engineering
University of Illinois
Urbana-Champaign

N. EUGENE SAVIN
Professor
Department of Economics
University of Iowa
Iowa City

KIMBERLY M. THOMPSON
Assistant Professor of Risk Analysis and Decision Science
School of Public Health
Harvard University
Boston


RESEARCH COUNCIL STAFF

JILL WILSON
Study Director


* Member, National Academy of Engineering