A Hierarchical Framework to Examine the Safety Role of Speed Limit on Road Segments

Speed management is essential to road safety. This study assesses the safety role of speed limit as an alternative in understanding the speed impacts on road segments. Crash occurrence and crash severity are modelled under a hierarchical framework. For crash occurrence, a negative binomial (NB) model is used instead of a Poisson model to deal with the over-dispersion problem. For crash severity, a hierarchical binomial-logistic approach is adopted. The study places special focus on road segments with speed limit transitions. The results show that all four explanatory factors are significantly related to the number of crashes on the road segment: speed limit, speed limit difference, natural log of vehicle-kilometer-travelled [ln(VKM)], and number of junctions. These four factors can be categorized into three quadrants according to their relationship with crash occurrence and severity. The findings in this study are helpful for better transportation planning, road design, and emergency service facility planning.