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) until that time. Three logistic regression models are estimated, which include time main effects (the driving time), time-independent effects (experience), time-dependent effects (time of day), 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 4th hr, by approximately 50% or more, until the 7th hour. The 8th and 9th hr show a further increase, approximately 80% and 130% higher than the first 4 hr. Drivers with more than 10 years of driving experience retain a consistently low accident risk; all other categories of driving experience have a significantly higher risk. Daytime driving, particularly at the noon time (10:00 a.m. to 12:00 noon), results in a significantly lower risk of an accident. Drivers at one time of day (4:00 to 6:00 p.m.) have an accident risk about 60% higher than those driving during the baseline; drivers during the other three significant times of day also experience accident risks about 40% higher than drivers during the baseline. All three times of day involve night or dawn driving; two are associated with circadian rhythms. Rest breaks, particularly those taken before the 6th or 7th hr of driving, appear to lower accident risk significantly for many times of day.