Experimental research measures the potential risk of an accident using a driving simulator or driving course.
3.2.1 Studies that do not show impairment
Surprisingly, given the alarming results of cognitive studies, most marijuana-intoxicated drivers show only modest impairments on actual road tests.
37,
38 Experienced smokers who drive on a set course show almost no
functional impairment under the influence of marijuana, except when it is combined with alcohol.
39
Many investigators have suggested that the reason why marijuana does not result in an increased crash rate in laboratory tests despite demonstrable neurophysiologic impairments is that, unlike drivers under the influence of alcohol, who tend to underestimate their degree of impairment, marijuana users tend to
overestimate their impairment, and consequently employ compensatory strategies. Cannabis users perceive their driving under the influence as impaired and more cautious,
40 and given a dose of 7 mg THC (about a third of a joint), drivers rated themselves as impaired even though their driving performance was not; in contrast, at a BAC 0.04% (slightly less than two “standard drinks” of a can of beer or small 5 oz. glass of wine; half the legal limit in most US states), driving performance was impaired even though drivers rated themselves as unimpaired.
41 Binge drinkers are particularly likely to rate themselves as unimpaired, possibly because they tend to become less sedated by high doses of alcohol.
42
This awareness of impairment has behavioral consequences. Several reviews of driving and simulator studies have concluded that marijuana use by drivers is likely to result in decreased speed and fewer attempts to overtake, as well as increased “following distance”. The opposite is true of alcohol.
43 One review of eight driving simulator studies and seven on-road studies
44 found that cannabis use was associated with either poor lane control
41,
45–
48 or slower driving that successfully maintained lane control.
49–
51 In seven of ten studies cited, cannabis use was associated with a decrease in driving speed despite explicit instructions to maintain a particular speed, whereas under the influence of alcohol, subjects consistently drove faster. Two simulator studies showed that the tendency to overtake was decreased with cannabis use but increased with alcohol.
52,
53 One simulator study and two on-road studies examining car-following behavior concluded that cannabis smokers tend to increase the distance between themselves and the car in front of them.
41,
45 Other studies have found no adverse effects of marijuana use on sign detection,
49 a sudden lane-changing task,
43 or the detection of and response to hazardous events.
48