A Statistical Analysis of Seat Belt Effectiveness in 1973–1975 Model Cars Involved in Towaway Crashes. Volume 1

Reinfurt, Donald W.; Silva, Claudio Z.; Seila, Andrew F. · 1976 · ROSA P / United States. National Highway Traffic Safety Administration

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Summary

This study addresses the inconsistency in previous estimates of seat belt effectiveness, which varied due to differing reporting thresholds, injury criteria, and inadequate control for confounding variables. To resolve these issues, the authors conducted a statistical analysis of seat belt effectiveness using "Level 2" data from the National Highway Traffic Safety Administration’s Restraint Systems Evaluation Program. This dataset combines police reports with detailed occupant interviews, hospital records, and vehicle investigations, offering higher quality and national representativeness than prior studies. The analysis focused on a weighted probability sample of 15,818 occupants in 1973–1975 model passenger cars involved in towaway crashes across five U.S. regions. The study examined three restraint categories: unrestrained, lap belt only, and lap-and-shoulder belts. To account for confounding factors, the researchers controlled for crash configuration, vehicle damage severity, vehicle weight, and occupant age. They employed two multivariate estimation techniques—weighted least squares (GENCAT) and Mantel-Haenszel-type estimation—to derive standardized injury rates and effectiveness measures, defined as the relative decrease in injury severity when moving to a more restrictive belt system. The study also incorporated direct injury costs derived from insurance data. The results indicated that 58.5% of occupants were unrestrained, 16.1% wore lap belts, and 25.4% wore lap-and-shoulder belts. Unadjusted injury rates for moderate or worse injuries (AIS > 2) were 12.1% for unrestrained, 7.4% for lap-belted, and 4.7% for lap-and-shoulder-belted occupants. After adjusting for confounding variables, the GENCAT estimates showed that lap belts reduced the risk of moderate injury by 30.9% compared to no restraint, while lap-and-shoulder belts reduced it by 56.5%. For serious injuries (AIS > 3), the effectiveness estimates were 46.3% and 56.8%, respectively. Sensitivity analysis revealed that controlling for vehicle damage and crash configuration significantly altered effectiveness estimates, whereas age had the least impact. The study also calculated standardized non-fatal injury costs, finding that lap-and-shoulder belts reduced costs by 37.7% compared to no restraint. The study concludes that rigorous statistical control for confounding variables is essential for accurate effectiveness estimates, as unadjusted rates can be misleading. The findings provide precise, evidence-based measures of seat belt efficacy, supporting the superior performance of lap-and-shoulder systems over lap belts alone. These results offer policymakers reliable data for evaluating restraint systems and highlight the importance of using high-quality, nationally representative accident data in safety research.

Key finding

Controlling for crash configuration and vehicle damage severity reduced the estimated effectiveness of lap belts relative to unrestrained occupants from 38.8% to 30.9%.

Methodology

field_study

Sample size: 15818

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified_with_issues.

Topics

Ranked by relevance to this paper. Hover a topic for its definition.

• incidence prevalence
• seat belt use
• demographic disparities
• induced exposure
• comparative international
• fatality injury trends

Information type

What kind of knowledge this paper contributes, grouped by family — independent of topic (what it is about) and method (how it was studied).

• Empirical Findings: crash risk outcomes, observational prevalence
• Methodological Resource: dataset resource