The impact of higher speed limits on the frequency and severity of freeway crashes: Accounting for temporal shifts and unobserved heterogeneity

• Impacts of speed limits increases on the likelihood and severity of crashes is assessed. • Random parameters crash count and injury severity models are estimated before and after speed limit increase. • Temporal shifts in the effects of explanatory variables complicate the assessment of speed limit increases. • The overall effects of the speed limit are modest with only a significant increase in likelihood of rollover crashes. In recent years, US States have raised their maximum interstate speed limits from 70 mi/h to 75 mi/h, 80 mi/h and even 85 mi/h. However, understanding the effect that these higher speed limits have had on the frequency and severity of crashes using traditional before and after analyses has been difficult due to possible temporal shifts in driver behavior, and potential changes in vehicle safety technology and highway safety features. Using multi-year data from before and after higher speed limits were instituted on Kansas freeways, random parameters models of crash frequency and resulting injury severity were estimated. Regarding the frequency of crashes, the findings showed that the higher speed limits did not have a significant effect in the mean number of crashes on the 253 studied roadway segments. For injury severity, model-estimation results in one- and two-vehicle crashes show that the factors affecting driver-injury severities have changed before and after the speed limit increase, but changes were also observed in the years before the speed limit increases and the years after. However, using pre-speed-limit-increase model estimation results to predict post-speed-limit-increase injury-severity distributions it was found that the aggregate effect of the changing influences of explanatory variables on average injury severities was relatively small. While the injury-severity estimation results make it difficult to attribute any temporal shifts in parameter values to the increased speed limit, there was a significant increase in the probability of rollover crashes that suggests the higher speed limits may have had some contributory effect on injury severities in single-vehicle crashes.