It is because they know they are stoned and compensate for it when driving.
Real-world data from auto accidents indicate that a drunk driver is approximately 10 times more likely to cause a fatal accident than a stoned driver. In most studies, smoking one-third of a joint or less has virtually no impact on a driver’s performance. A couple of studies even suggest that pot smokers are less likely to cause an accident than sober drivers.
…Participants in one study who smoked one-third of a joint perceived themselves as being impaired, even though the experiment suggested they were not. By contrast, subjects who had two drinks thought they were fine, despite performing poorly in driving tests. In the driving simulators, pot smokers drove significantly slower than the drunk drivers, even with researchers reminding them to speed up. They also gave the car in front of them a lot more room and were less likely to pass. Alcohol, on the other hand, increases risk-taking behavior. Drunk drivers drive faster, tailgate, and pass recklessly.
The most consistent result of the driving studies is that taking marijuana and alcohol together creates a much greater hazard than taking either one alone. Drivers who are drunk and high seem to suffer from the worst effects of both drugs: They meander, pass recklessly, drive too fast, take unnecessary risks, and are unaware of their incapacity.
Full story at Slate
And, to confirm that this is real, US states that have legalized marijuana have seen a 9% decrease in road fatalities. Perhaps many drinkers would prefer to be stoned, if only it were legal and readily available…
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