Epidemiological aspects of fatal and severe injury urban freeway crashes.

Two types of crashes are common on urban freeways: in-lane rear-end impacts with stopped or slowed vehicles and those where a vehicle leaves the roadway (and road shoulder) and impacts with obstacles outside traffic lanes. Crashes resulting in a fatal or severe injury to a vehicle occupant were identified from crash records in the regional office of the California Department of Transportation for the 1,500 miles of freeways in Los Angeles, Ventura, and Orange counties, California, for 1984 and 1985. Selected road characteristics were assessed and traffic-volume data obtained for these freeways to determine the relationship of these types of crashes to freeway features. Fatal- or severe-injury crash rates were highest for off-road crashes compared with in-lane crashes, with certain lanes having more frequent in-lane collisions and with a higher rate of in-lane crashes in freeway segments without a left shoulder. Off-road collisions were significantly more frequent in the absence of a right shoulder and on freeways with two or three lanes. Multivariate analyses that control for selected confounding factors such as traffic volume show similar findings including strong association with hour of crash.