How Safe Is Driving Under the Influence of Cannabis?

Blood-level cannabis up in fatally injured drivers — is cannabis the cause?

Posted May 06, 2014

For example, in what it calls a review of the scientific evidence, the marijuana advocacy website Norml writes that, “Marijuana has a measurable yet relatively mild effect on psychomotor skills, yet it does not appear to play a significant role in vehicle crashes.” And other studies show that many marijuana users think that use of the drug actually improves driving skill. But then when you look at the brain chemistry, the journal Clinical Chemistry writes that, “Blood THC concentrations of 2–5 ng/mL are associated with substantial driving impairment, particularly in occasional smokers.”

Which is it? Does marijuana help, hurt or leave unchanged a person’s driving ability?

A study in progress at the Journal of Safety Research pulled 72 young, male marijuana users into the driving simulator to try to answer some of these questions. First, the study asked how often subjects “drove within the hour following cannabis use in the previous 12 months?” Then it asked participants about their risky driving habits: how risky did these people consider their own driving and how many traffic tickets had they gotten for things like speeding or failing to stop at a light or sign? Finally, the study put these 72 subjects in a driving simulator to see how “risky” they really were in simulated everyday situations.

The more often a person had driven under the influence of cannabis, the riskier were their driving behaviors and the more traffic tickets they had earned. The researches write that, “Taken together, these results indicate that self-reported driving under the influence of cannabis is associated with a risky driving style including a broad range of reckless on-road behaviors and support the problem driving behavior theory.”

Then another driving simulator study gave subjects THC cigarettes – how would they perform while high compared to how they performed sober? The results are, well, sobering. Even with low consumption, “increase in THC dosage alone influences perception of what is a safe distance to leave between cars” (among other risky driving behaviors). This study also happens to show that the addition of alcohol to THC is especially dangerous – the sum of these two drugs created about 20 percent more dangerous driving behaviors than either drug alone.

So the answer to whether marijuana impairs driving ability is yes, it does. Not only are people who drive under the influence of marijuana more likely to drive in risky ways, but it’s the marijuana and not their personalities alone that create this risky driving.

Now in the era of legal recreational marijuana, stay tuned for research like that in the wake of medical marijuana exploring the percentage of fatally injured drivers with measureable blood THC.


Richard Taite is founder and CEO of Cliffside Malibu, offering evidence-based, individualized addiction treatment based on the Stages of Change model. He is also co-author with Constance Scharff of the book Ending Addiction for Good.