This permits the calculation of a "risk factor" for each class, which is a measure of accident involvement per vehicle-mile. Using this measure, we show that at any time of day drunk drivers face a risk of a fatal accident on the order of 100 times the risk that sober drivers face. We also show that the sober driver's risk of being involved in a fatal accident with a drunk driver is some 50 times as large in the period 1-3 a.m. as during the period 7 a.m. to noon. In conclusion we indicate how information about the general driving population may be obtained from accident data. (Author abstract)