Probability of Secondary Crash Occurrence on Freeways with the Use of Private-Sector Speed Data

Abstract

A percentage of crashes on freeways are suspected to be caused in part by congestion or distraction from earlier incidents. Identification and prevention of these secondary crashes are major goals of transportation agencies, yet the characteristics of secondary crashes—in particular the probability of their occurrence—are poorly understood. Many secondary crashes occur when a vehicle encounters nonrecurring congestion, yet previous efforts to identify incident queues and their secondary crashes have relied either on deterministic queuing theory or on data from uniformly spaced dense loop detectors. This study is the first analysis of secondary crash occurrence to integrate incident timelines and traffic volumes with widely available and legally obtained private-sector speed data. Analysis found that 9.2% of all vehicle crashes were secondary to another incident and that 6.2% of these crashes were tertiary to another primary incident. Secondary crashes occurred on average once every 10 crashes and 54 disabled vehicles. The findings support a fast incident response, because the probability of a secondary crash occurrence increases approximately 1 percentage point for every additional 2 to 3 min spent on the scene in high-volume scenarios.