Queuing Delays Associated with Secondary Incidents

Incidents cause substantial travel delays on urban freeways. To evaluate the effectiveness of potential incident management strategies, travel delays are typically calculated for incidents independently. This study examines delays due to primary-secondary incident pairs, which occur on the same stretch of freeway within a short time gap. Depending on their correlation in time and space, they can have varying degrees of impacts on the traffic flow. This paper assesses the total delays induced by primary-secondary incident pairs by jointly modeling their occurrences. First, incident data combined with roadway inventory data from Hampton Roads, Virginia were analyzed to understand the attributes of primary-secondary incident pairs, e.g., durations, lane blockages, and time gaps (between start times of the primary and its secondary incidents). Using microscopic simulation as a modeling tool, three critical parameters are evaluated: time gap, physical distance between primary and secondary incidents, and traffic demand levels. The results show that primary-secondary incident pairs have substantially larger durations than single incidents, on average. Furthermore, for those secondary incidents that end after their associated primary incidents, total delays increase as time gap increases; and furthermore, increasing distance is associated with declining delays. Additional results and the implications of the findings for incident management are presented.