The Effectiveness of the 55 MPH National Maximum Speed Limit as a Life Saving Benefit: Executive Summary

Johnson, P.; Klein, Terry M.; Levy, P.; Maxwell, D. · 1981 · ROSA P / United States. National Highway Traffic Safety Administration. Office of State Vehicle Programs

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Summary

This report evaluates the life-saving benefits of the 55 mph National Maximum Speed Limit (NMSL) implemented in the United States from 1974 to 1979. The study was motivated by conflicting estimates in existing traffic safety literature regarding the magnitude of fatalities prevented by the law and a lack of comprehensive national evaluations covering the full period since implementation. The authors aimed to provide a consensus estimate, arguing that previous analyses had underestimated the safety benefits of the speed reduction. The researchers employed a statistical time series model using monthly national fatality data from 1970 to 1979. The model isolated the impact of the NMSL by accounting for confounding variables, including changes in vehicle miles traveled (VMT), the introduction of vehicle and highway safety improvements, and the repeal of mandatory motorcycle helmet laws in many states. Because data could not be segregated by posted speed limit for all roads, the analysis used total national fatalities as the impact measure. To refine the estimates, the authors conducted a secondary analysis using Fatal Accident Reporting System (FARS) data to assess compliance levels and speeding involvement on 55 mph roads. This allowed them to adjust the baseline model for eroding compliance observed in later years. Additionally, FHWA speed monitoring data was analyzed to validate trends in average and 85th percentile speeds against fatality rates. The study found that the 55 mph NMSL resulted in significant annual life savings, though these benefits varied over time due to compliance levels. The model estimated that 7,532 lives were saved annually in 1974 and 1975. However, as compliance eroded, the estimated savings declined to 7,216 in 1976, 6,794 in 1977, 6,423 in 1978, and 6,454 in 1979. Over the six-year period, the total estimated lives saved was 41,951. The analysis attributed the increase in fatalities observed from 1976 to 1979 primarily to decreased compliance with the speed limit, particularly on Interstate systems. Speed data corroborated this finding, showing that average and 85th percentile speeds, which had dropped significantly in 1974, began to rise again in 1977. The authors conclude that the 55 mph NMSL is one of the most effective countermeasures for reducing highway fatalities, provided that compliance is maintained. The study demonstrates that while safety improvements in vehicles and infrastructure helped offset fatality increases from rising VMT, the speed limit itself accounted for the dramatic drop in deaths beginning in 1974. The findings emphasize that the effectiveness of the law is directly dependent on enforcement and public adherence to the lower speed limits.

Key finding

The 55 mph National Maximum Speed Limit saved a total of 41,951 lives between 1974 and 1979, with annual estimates ranging from 6,423 to 7,532 lives saved.

Methodology

modeling

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Topics

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• fatality injury trends
• comparative international
• regulatory evaluation

Information type

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• Empirical Findings: crash risk outcomes