Stoners don’t make the best drivers, even the ones who aren’t stoned. At least that’s the contention of a new study published in the journal Drug and Alcohol Dependence.

When they’re sober, heavy cannabis users were found more likely to speed, swerve into the wrong lane, run red lights, and get into accidents than their non-stoner counterparts, according to researchers.

All drivers in the study had abstained from cannabis for at least 12 hours and were sober, according to urine tests.

“Heavy use was defined by daily or near daily use, a minimum of four or five times a week, with a lifetime exposure of 1,500 times,” said Staci Gruber, as reported by CNN. Gruber is director of the Marijuana Investigations for Neuroscientific Discovery (MIND) program of McLean Hospital at Harvard Medical School.

The worst drivers in the study were found to be those initiated into the ways of weed before the age of 16.

“Prior to age 16, the brain is especially neurodevelopmentally vulnerable, not just to cannabis but to other drugs, alcohol, illness, injury,” Gruber said. 

“And when we looked at the cannabis users and separated those into early (before age 16) versus later onset of use, almost exclusively these differences between the two groups were attributed to the early onset group. So it’s really early exposure to cannabis that appears to confer greater difficulty with complex cognitive tasks like driving.”

There was, however, one group of cannabis users who actually saw improvement in their driving capabilities, as opposed to a decline.

“In our medical cannabis patients we don’t see that at all,” Gruber said.

The researchers also noted that their findings don’t actually conclude that cannabis use causes bad driving. It could just be that the kind of person who would drive recklessly would also be the kind of person who would smoke weed every day.

Photo via Flickr/katesheets