TRB Special Report 265 - An Assessment of the National Highway Traffic Safety Administration's Rating System for Rollover Resistance 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. According to the report, 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. Following the National Highway Traffic Safety Administration's (NHTSA's) issuance of vehicle ratings to inform consumers about rollover risk, Congress requested a TRB study to evaluate the appropriateness of the rating system. Motor vehicle rollovers involving passenger cars, vans, pickup trucks, and sport utility vehicles result in approximately 10,000 deaths and 27,000 serious injuries each year in the United States. NHTSA developed a five-star rating system to inform consumers about the rollover resistance of passenger cars and light-duty passenger vehicle trucks. After thoroughly evaluating NHTSA's development of the rating system, the committee that conducted this study concurred with the agency's reliance on a static measure of vehicle stability but pointed out some inadequacies of the statistical model used to relate this static measure to rollover risk. Alternative statistical approaches would provide a better approximation of risk. The rating system itself was found wanting. The procedures used to develop and test the ratings with consumers through focus groups did not provide credible evidence that consumers understood the message about the actual risk associated with a given vehicle. By being limited to only five levels, the system also discarded valuable information. The data developed by NHTSA could be refined to enable consumers to discriminate better among vehicle models with regard to their rollover experience.