A recent review of the harms from marijuana use (Hall W, Lancet 374, p 1383-1391, 2009) reported that marijuana users are 2-3 times more likely to be in a car accident. This rate is smaller than that for heavy alcohol drinkers (6-15 times more likely), in part, because marijuana users appear to be more aware of their impairment and drive slower and take fewer risks. In laboratory studies, the typical doses of marijuana do impair driving ability, especially when long boring driving is involved.
–Dr. John R. Hughes