Characterization of Freeway Capacity Reduction Resulting from Traffic Accidents

Incidents, defined as unplanned events that temporarily reduce roadway capacity, contribute significantly to urban freeway congestion. Transportation agencies have developed incident management programs in order to support the effective identification and response to incidents. An important element of an effective incident management program is the ability to accurately estimate the freeway capacity remaining following an incident. The objective of this research was to characterize urban freeway capacity reduction resulting from traffic accidents using a large set of traffic flow and accident data. Traffic accidents, a subcategory of incidents, were investigated since they, in particular, result in significant urban freeway congestion due to their frequency and generally substantial reduction of capacity. The results of this effort lead to the following conclusions: (1) Accidents significantly reduce remaining capacity on freeway segments, well beyond the physical blockage of lanes. This research found that an accident blocking one of three freeway lanes resulted in a mean capacity reduction of 63%, while an accident blocking two of three freeway lanes resulted in a mean capacity reduction of 77%; and (2) There is evidence to suggest that accident capacity reduction is best modeled as a random variable, not a deterministic value. The results of this research indicate that the beta distribution provides a good representation of accident capacity reduction for one or two lanes of three lanes blocked.