A comparative study on crash characteristics between urban and intercity expressway basic segments

This study aims at identifying the differences of crash characteristics at basic segments between urban and intercity expressways. Crash rate (CR) model is firstly developed as a function of traffic density. The causes leading to different CR characteristics by expressway type are then examined considering the interaction of geometry, traffic flow and ambient conditions. Affecting mechanisms of those factors are finally analyzed through using principle component analysis. The results reveal that, urban expressway has significantly higher CR in low-density uncongested flow due to its compact design relative to intercity expressway. As traffic flow increases, the significance of traffic-related variables is increasing as opposed to the decreasing significance of geometry affecting crashes. Meanwhile, intercity expressway then starts to have higher CR compared to urban expressway, which is possibly related to its vehicle composition characterized by more heavy vehicles (HV). Since HV can interrupt their surrounding traffic, more HV may result in higher frequency of “disruptive” traffic.