Abstract:A time-dependent logistic regression model has been formulated to assess the safety of motor carrier operations. The model estimates the probability of having an accident at time interval t, subject to surviving (i.e., not having an accident) before that time. Using accident and nonaccident data for 1984 from one national less-than-truckload carrier, nine logistic regression models are estimated that include time-independent effects (i.e., age, experience, multiday driving pattern, and off-duty time before the trip of interest), time main effects (the driving time), and a series of time-related interactions. Driving time has the strongest direct effect on accident risk. The first 4 hr consistently have the lowest accident risk and are indistinguishable from each other. Accident risk increases significantly after the fourth hour, by approximately 65% until the seventh hour, and approximately 80% and 150% in the eighth and ninth hours. The most experienced drivers, those driving more than 10 yr, had the lowest accident risk. All other groups had risks at least 67% higher than these safest drivers. There was little difference among the remaining driver groups, although drivers with 1 to 5 yr experience were marginally elevated in risk. Multiday driving patterns had a marginal effect on subsequent accident risk. Daytime driving, particularly in the three days before the day of interest, results in the lowest accident risk. Four driving patterns have an accident risk about 40 to 50% higher than Pattern 2: one representing infrequently scheduled drivers; the remaining three involving some type of night driving.