Analysis of Cascading Incident Event Durations on Urban Freeways

Incident-induced traffic congestion is a major source of travel uncertainty. Sometimes multiple incidents occur sequentially because of queue backups, which substantially increase uncertainty. Such cascading incidents can be grouped into one event because of their spatial and temporal proximity. Events consisting of a primary and its secondary incidents are expected to have longer durations than single incidents and therefore to result in larger impacts on traffic. Though relatively rare, such cascading events are a major concern for transportation operations managers, and they are the focus of this paper. A unique event database, based on incident and road inventory data from Hampton Roads, Virginia, is created. Single-pair events (one primary and one secondary incident) and large-scale events (one primary and multiple secondary incidents) are identified and analyzed. "Event duration" is defined as the time elapsed from the notification of a primary incident to the departure of the last responder from the event scene after removal of the primary and associated secondary incidents. Events are further categorized as either contained or extended. If the primary incident is the last one being cleared during such an event, then it is a contained event; otherwise, it is an extended event. Correlates of contained and extended event durations are identified through a set of rigorous statistical models. The findings of this study provide knowledge that can aid in mitigating the impacts of cascading incidents.