Janice Daniel

Although many studies have been performed to determine the impacts of the 65-mph speed limit on roadway safety, little is known about the impact of the 65-mph speed limits on truck safety. Previous studies have shown that increasing the speed limit can result in greater speed differentials between trucks and other vehicles on the roadway. The impact of these differentials on truck crashes, however, has not been fully explored. The objective of this paper is to demonstrate the impact of the 65-mph speed limit on crashes involving trucks. An Autoregressive Integrated Moving Average (ARIMA) time series intervention analysis was performed of monthly truck accidents in the 65-mph speed zones to determine whether there was a detectable change in truck crashes after the 65 mph speed limit was implemented. Models were developed for all crashes, truck crashes and truck-car crashes using monthly crashes within the 65-mph speed zones. The results show detectable changes in monthly truck and truck-car crashes after the change in speed limit. Using a step function, the intervention parameter, I, was modeled as 0 prior to the speed limit change, and 1 after the speed limit change. In each case the coefficients for the interaction term is positive suggesting an increase in the monthly crashes after the speed limit change.