Effects of 65‐MPH Speed Limit on Traffic Safety

Abstract

In April 1987 the United States Congress passed the Surface Transportation and Uniform Relocation Assistance (STURA) Act that permitted the states to increase the speed limit on rural Interstate highways up to 65 mph. This paper investigates the safety impacts of the new speed limit using nationwide fatality data. The study focuses on those 32 states that raised their speed limit prior to June 30, 1987. Monthly fatality data for each state were obtained from the Fatal Accident Reporting System maintained by the National Highway Traffic Safety Administration. The long‐term patterns (January 1975–September 1988) of rural interstate fatalities were first examined through a systematic trend analysis using the autocorrelation function and the U‐statistic. Box‐Jenkins modeling techniques were then used to forecast the number of fatalities that would have occurred if the speed limit remained at 55 mph. As expected, given the limited amount of “after” data, the effects of 65‐mph speed limit on Interstate highway fatalities cannot be determined definitively at the current stage. From the available data, it was found that the predicted fatalities were greater than the actual fatalities in 14 of the 15 months after the speed‐limit change, but in only two of these months was the difference statistically significant.