Journal of Transportation and Statistics: Volume 4, Number 1: April 2001

NHTSA · 2001 · ROSA P / United States. Department of Transportation. Bureau of Transportation Statistics

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Summary

This study investigates the relationship between speed-limit increases and the incidence of fatal crashes on U.S. rural and urban interstates, challenging the conventional wisdom that higher speed limits directly cause higher fatalities. Motivated by legislative changes, including the 1987 allowance for 65 mph limits on rural interstates and the 1995 National Highway System Designation Act which returned speed-limit authority to states, the authors aim to determine if these policy shifts resulted in a statistically significant increase in fatal accidents. The researchers employed structural time-series modeling to analyze monthly data on fatal crashes from January 1975 to December 1998, sourced from the Fatality Analysis Reporting System (FARS). The analysis was conducted separately for each state and for rural versus urban interstates. The methodology utilized logarithmic transformations to assess percentage changes and included intervention variables to detect permanent shifts in the crash level corresponding to the timing of speed-limit changes. This approach allowed for the explicit modeling of evolving level, seasonal, and irregular components, providing a more nuanced assessment than previous studies that relied on simple linear trends or historical projections. The results indicate that the impact of speed-limit increases varied significantly by location and time. For rural interstates, 19 of 40 states experienced a significant increase in fatal crashes following the 1987 speed-limit changes, while 10 of 36 states saw significant increases after the 1995–1996 changes. In contrast, the effect on urban interstates was weaker, with only 6 of 31 states showing a significant increase. Aggregate analysis using a "Super t-Test" confirmed a significant national increase in fatal crashes on rural interstates for both periods, but the effect on urban interstates was not statistically significant at conventional levels. The study also identified strong seasonal patterns in crash data, with peaks often occurring in summer months or during holiday seasons, which varied by state. The significance of this research lies in its refutation of the blanket claim that higher speed limits universally lead to higher fatalities. The findings suggest that the relationship is complex and context-dependent, with rural interstates being more susceptible to increases in fatal crashes than urban ones. Additionally, the study highlights the importance of accounting for seasonal variations and state-specific trends when evaluating transportation safety policies. The authors conclude that while speed-limit increases did correlate with higher fatal crash rates in many rural areas, the magnitude and consistency of this effect vary, necessitating further investigation into factors such as driver adaptation and enforcement practices.

Key finding

Speed-limit increases on rural interstates resulted in statistically significant increases in fatal crashes in 19 of 40 states after the 1987 changes and 10 of 36 states after the 1995 changes, whereas only 6 of 31 states showed significant increases on urban interstates after the 1996 changes.

Methodology

dataset

Provenance

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

Topics

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• regulatory evaluation
• exposure measurement
• incidence prevalence
• fatality injury trends

Information type

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• Applied Guidance: countermeasure evaluation
• Empirical Findings: crash risk outcomes, observational prevalence