Is It More Dangerous To Drive Under the Influence of Alcohol or Marijuana?

Answers to your questions about the news.
Nov. 30 2011 5:40 PM

DWI Versus DW-High

Is it more dangerous to drive drunk or stoned?

Woman smoking marijuana.
Marijuana may not impair driving as much as alcohol

Medioimages/Photodisc.

A new study suggests that legalizing medical marijuana reduces traffic fatalities. The authors noted that legalizing marijuana reduces alcohol consumption, and people are more wary of driving high than drunk. Which drug is actually more dangerous on the road?

Alcohol, and it’s not even close. It’s hard to directly compare alcohol and marijuana, because driving impairment depends on dosage and the two drugs tend to affect different skills. (Pot makes drivers worse at mindless tasks like staying in a lane, while alcohol undermines behaviors that require more attention like yielding to pedestrians or taking note of stop signs.) Nevertheless, Yale psychiatrist Richard Sewell reviewed the academic literature on driving while intoxicated in a 2009 article, and found that alcohol is significantly more dangerous. 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.

It’s a little surprising that THC has such a small effect on driving. In experiments testing the skills required for driving—coordination, visual tracking, and reaction time—rather than driving itself, subjects under the influence of pot fare significantly worse than sober people. But when you put them behind the wheel of a driving simulator, tokers perform okay. Those who have taken in a moderate dose of the drug show minimal impairment, and very experienced smokers show almost no deficit at all. (Interestingly, habitual stoners are also better at driving drunk than ordinary people.)

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No one’s entirely sure how to explain these results. The dominant hypothesis is that people are more likely to be aware of—and, more importantly, compensate for—their intoxication when high than when drunk. 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.

Driving while high can be dangerous, though. At larger doses of THC, problems emerge. Very high drivers can’t stay in a lane, react more slowly to yellow lights and unexpected obstacles, and are unaware of their speed. Epidemiological studies show that drivers with a blood THC level of more than 10 ng/mL—about one-half of a joint—are far more likely than sober drivers to cause an accident. It’s more difficult to estimate your blood THC concentration than your blood alcohol concentration, because of inhalation irregularities and the metabolism patterns of the drug, which aren’t as well understood as those of alcohol.

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.

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