Driving and riding behaviors were assessed by the following questions:
During the last two weeks, how many times have you driven a car, truck, or motorcycle after … drinking alcohol? … having five or more drinks in a row? … smoking marijuana? … using other illicit drugs?
A second set of questions asked:
During the last two weeks, how many times (if any) have you been a passenger in a car … when the driver had been drinking? … when you think the driver had 5 or more drinks? … when the driver had been smoking marijuana? … when the driver had been using other illicit drugs?
Response categories were none, once, twice, 3 to 5 times, 6 to 9 times, and 10 or more times. We collapsed these into binary values (0 = none; 1 ≥ once). The questions did not assume that behaviors were mutually exclusive. For example, an individual could have driven after smoking marijuana and drinking heavily on the same occasion.