Secondary accident rates on Los Angeles freeways.

There is a prevalent assumption that Freeway Service Patrol (FSP) programs improve safety for unassisted motorists by reducing the likelihood of secondary accidents. This research identifies 84,684 accident records from the California Highway Patrol's First Incident Response Services Tracking system and subjects the records to a sequence of filters that check for incrementally more stringent conditions consistent with secondary accidents. The paper shows that secondary accidents on Los Angeles freeways are much less frequent than suggested in the transportation engineering literature. Avoiding secondary accidents provides only a small incentive to deploy FSPs, but the expected benefits associated with reducing already low secondary accident rates may be sufficient to justify accounting for these costs.