Attentiveness, vigilance, perception of time and speed, and use of acquired knowledge are all affected by marijuana;
18–
21 in fact, a meta-analysis of 60 studies concluded that marijuana causes impairment in every performance area that can reasonably be connected with safe driving of a vehicle, such as tracking, motor coordination, visual functions, and particularly complex tasks that require divided attention,
22 although studies on marijuana’s effects on reaction time have been contradictory.
23 Similar conclusions have been reached by other reviewers.
2 Worse still, marijuana and alcohol, when used together, have additive or even multiplicative effects on impairment.
24 Consequently, on the basis of cognitive studies, it seems reasonable to propose that smoking marijuana may increase the risk of having a fatal traffic accident.
Alcohol at 0.75 g/kg (slightly less than four standard drinks) causes high levels of impairment in psychomotor performance and medium-to-high levels of impairment in such tasks as critical flicker fusion and short-term memory.25 Alcohol impairs pursuit tracking, divided attention, signal detection, hazard perception,26–28 reaction time, attention, concentration, and hand-eye coordination.29, 30
Alcohol also reduces the perceived negative consequences of risk-taking,31 which can increase willingness to take risks after drinking,32 the amount of risk-taking behavior while driving, even at low alcohol doses,33and the incidence of road traffic accidents while driving drunk.34, 35 However, there is considerable variability in the effects that alcohol can have on people—the same dose may have different effects not only on different individuals, but also in the same individual on different occasions, because of other factors such as gender, body mass index, age, drinking habits, time of day, stomach contents, genetics, stage of the menstrual cycle, and environmental factors.
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