Exploring the impacts of traffic flow states on freeway normal crashes, primary crashes, and secondary crashes

This study aims to explore the relationship between traffic flow states and crash type/severity in the scenarios of normal crashes, primary crashes, and secondary crashes using the association rules mining approach. The crash data and real-time traffic data were collected from the I-880 freeway for five years in California, USA. The secondary crashes were identified using a speed contour plot approach. Traffic flow states were identified by the three-phase flow theory. The results showed that the free flow is associated with the proportion of the sideswipe normal crash, the hit object primary crash, and the injury primary crash. The synchronized flow, the wide moving jams, and the transitional state from synchronized flow to wide moving jams are associated with the proportion of the rear-end secondary crash. The transitional state from synchronized flow to free flow is associated with the proportion of the rear-end primary crash and the property damage only primary crash. In addition, the unsafe speed behaviour can increase the proportion of the rear-end normal, primary, and secondary crashes. The unsafe lane change behaviour can increase the proportion of the sideswipe normal, primary, and secondary crashes. These results have the potential to reduce the secondary crash probability.