ACCIDENT RATES FOR HEAVY TRUCK-TRACTORS IN MICHIGAN

Accident rates of heavy truck-tractors are modelled using log-linear m ethods. The accident data used are a census of truck-tractor involveme nts in Michigan from May 1987 to April 1988. Travel data used to calcu late the rates were produced by a survey of truck-tractors in Michigan covering the same time period. Both the accident and travel data were limited to Michigan-registered tractors operating in Michigan. Log-li near models of casualty and property-damage-only accident rates were d eveloped using number of trailers, road type, area type, and time of d ay as predictor variables. Overall, differences between tractors with one and two trailers were not significant. Tractors with no trailers ( bobtails) have significantly higher accident rates. Characteristics of the operating environment were found to have larger effects on the ac cident rate than tractor configuration (except for the bobtail). Rates varied by a factor of up to 6.8, depending on the road type. Casualty accident risk at night was 1.4 times the risk during the day. The ris k of a casualty accident in rural areas was 1.6 times that of urban ar eas.