Forty-nine per cent of 1271 incoming college freshmen reported using marihuana during the previous year. A majority of marihuana users simultaneously consume alcohol at least on occasion, and many of them have enough alcohol at those times to be impaired by the alcohol alone. Among users who smoke weekly or more often one-quarter of driving while "high" occurs under the combined effect of marihuana and medium to heavy alcohol use. Most users reported marihuana effects on driving judged to be hazardous, such as altered attention, vision or time perception. In an estimated 15,000 driving trips while "high" on marihuana two crashes occurred definitely attributable and two possibly attributable to marihuana use. Also, 42 persons (13 per cent of drivers) reported near crashes while "high". Since this is not a comparison study, it is not known whether or not this reported experience is excessive for this age group. Drivers who encountered trouble while driving after marihuana said 59 per cent of the time that the incidents were caused by the marihuana, 27 per cent by alcohol, and in 14 per cent that they couldn't assess which drug was responsible.
Abstract