Supporters of legal marijuana, medical or otherwise, are fond of pointing out that marijuana is less deadly than alcohol. Even President Obama's deputy drug czar admitted as much in February. That doesn't mean pot is harmless: USA Today reported Tuesday that marijuana’s role in traffic fatalities tripled between 2000 and 2010, according to Columbia University researchers. But driving stoned is still safer than driving drunk, right? Don't be so sure.

In 2000, researchers at Maastricht University in the Netherlands gave driving tests to subjects who had consumed various amounts of alcohol and/or marijuana. While all subjects both drank and smoked in each round of the study, some were given placebos, so that the researchers were able to test the effects of each substance on its own as well as their combined effect. They measured drivers’ “standard deviation of lateral position” (SDLP), or the distance they drifted out of their lane, and also the time out of lane (TOL).

The study found that alcohol on its own increased SDLP by 2.2 centimeters (as compared to double-placebo conditions). Marijuana, depending on the dosage of THC (100 or 200 micrograms per kilogram of body weight), increased SDLP from placebo conditions by 2.7 and 3.5 centimeters respectively. In other words, drivers who had smoked pot were less able to drive in a straight line than drivers with an elevated BAC. (Most drivers’ BACs fluctuated around 0.04 grams per deciliter, below the legal limit of 0.08.)

The researchers concluded that the percentage of TOL was not significantly affected by either alcohol or marijuana alone, but that it was much higher when both substances were used together. Similarly, in a test that involved following other cars, drivers’ reaction times under the influence of either THC or alcohol was not significantly different from the double placebo, but they did react slower when both substances were used together.

Driving instructors were also asked to score subjects on their performance in separate tests. On average, subjects scored slightly lower when they had only smoked pot than they did when they had only drunk alcohol. Participants also had to rate their own performance. The good news is that while they rated themselves better than their instructors did, the drivers were, for the most part, aware of their own impairments.