Is Stoned Driving Really That Dangerous?

By Zachary Siegel 06/29/15

Debates continue about how high one can be to drive safely.

stoned driver laughing.jpg
She doesn't think so. Photo via

Results from a new double-blind study confirm that marijuana has a less dramatic effect on driving than alcohol does, casting further doubt on the current standard in Colorado and Washington that determines when someone is too buzzed to drive.

The study involved 18 occasional pot smokers who vaped marijuana at either high or low THC concentrations, drank alcohol, or took a placebo and then drove in a simulator. The study focused on three main parameters of driving ability such as weaving within the lane, the number of times the car left the lane, and the speed of the weaving.

It was found that a THC level of 13.1 ug/L (micrograms per liter of blood) slightly impacted the driver's performance, for instance, weaving within the lane. The 13.1 ug/L of THC in the blood is similar to a blood alcohol concentration (BAC) of 0.08%.

The current limit in Washington and Colorado is 5 ug/L, meaning 13.1 ug/L of THC is more than twice the legal limit. The occasional pot smoker was defined as those who used marijuana at least once in the previous month, but no more than three days a week. It is then likely that people who regularly use marijuana and therefore have a higher tolerance can withstand even higher levels of THC in their bloodstream and still perform as well as someone with a BAC of 0.08%.

Dr. Marilyn Huestis, senior investigator at NIDA and the principal investigator in this particular study, argues that the 5 ug/L limit is not a strict enough cutoff, especially for people who don’t have a tolerance, according to TIME.

A senior editor at Reason took issue with Dr. Huestis’s remark stating that the, “cutoff in practice means that many regular cannabis consumers can never legally drive, even when they’re not impaired, which hardly seems sensible or fair.”

Because this study measured the concentration of THC in the user's system while driving, the next step for the NIDA team is to measure how fast THC levels drop in the bloodstream, said Dr. Huestis, because most of the time a driver is not tested until hours after an arrest or accident.

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Zachary Siegel is a freelance journalist specializing in science, health and drug policy. His reporting has also appeared in Slate, The Daily Beast, Salon, Huffington Post, among others. He writes often about addiction, sometimes drawing from his own experience. You can find out more about Zachary on Linkedin or follow him on Twitter.