Longitudinal Analysis of Rural Interstate Fatalities in Relation to Speed Limit Policies

Existing research literature has shown that traffic fatalities increase at higher speed limits. A related issue is the establishment of maximum speed limits for trucks and buses. As of 2014, eight states have a differential speed limit in place that establishes a higher limit for passenger vehicles than for trucks and buses. This study aimed to inform the continuing debate regarding the safety impacts of speed limits by comparing states with various speed limit policies. The study included a longitudinal comparison of state-level rural Interstate fatalities in the United States from 1999 through 2011. In addition to an examination of differences in traffic fatalities as a function of maximum speed limits, comparisons were also made between states with differential limits for truck and buses. Random parameter negative binomial models were estimated for annual total and truck-involved fatalities. A random parameter framework allowed for consideration of temporal correlation in annual fatality counts within states as well as for unobserved heterogeneity across states. The results of this study provided further evidence that both overall and truck-involved fatalities increased with maximum speed limits. States with differential speed limits were found to have marginal differences in total fatalities as compared with states with uniform speed limits. However, truck-involved fatalities were significantly lower in states where differential limits were in place. The effects of speed limit policies as well as other covariates were found to vary significantly across states. The random parameter models demonstrated significantly improved goodness of fit as compared with standard Poisson and negative binomial models.