More Drugged Driving

Last week, another study on marijuana and driving surfaced out of Canada. It made some pretty serious claims:

U.S. drivers who tested positive for cannabis over a 10-year period had a 29 per cent higher risk of causing a fatal crash than motorists not taking the drug, a new Canadian-led study suggests.

Michel Bédard, director of public health at Lakehead University in Thunder Bay, Ont., and his team studied the effects of cannabis on driving using test results from 32,543 drivers in the U.S.

Between 1993 and 2003, 1,567 of those tested, or five per cent, were positive for cannibis but not alcohol, the researchers report in Wednesday's edition of the Canadian Journal of Public Health. The journal's publisher has launched a campaign against drug-impaired driving.

[emphasis mine]

A number of studies have actually shown that marijuana is nowhere near as dangerous a drug as alcohol when it comes to driving; and anyone who's familiar with driving a car while stoned knows that it might make you more likely to get lost, but it doesn't make you more likely to kill someone. Stoned drivers tend to be overly cautious, which is why a recent study showed that marijuana actually reduces your likelihood of causing a fatal crash.

As I've done in the past, I emailed the researcher, but didn't hear anything (and I was nice, I swear!). But Pete Guither also emailed Dr. Bédard, and managed to get a reply. He gets a little closer to finding out why Dr. Bédard's study ended up so far from what other studies have shown.

First, all those who tested positive for alcohol were eliminated from the study, so the sample is dangerously skewed.

This is definitely suspect, but I'm not really sure exactly how that affected his results.

Second, the study counted anybody who tested positive for marijuana, not just people who were under the influence of marijuana (people can test positive for marijuana for over a week after smoking). In addition, the person did not have to have caused the fatal crash, just be involved in one. So as Pete points out:

If an alcohol impaired driver crashed into someone sober who used cannabis days earlier, and both drivers were cited for speeding, the drunk driver would not be included in the study, but the sober driver would be classified as positive for cannabis and having an unsafe driving action associated with a fatal crash.

I was wondering if age had something to do with this, but Dr. Bédard reported that he corrected for that. Is it possible that since most fatal accidents occur at night - and that people who drive at night are more likely to test positive for marijuana - that we see this skew? Are people who use marijuana more likely to die in a crash whether they caused it or not? Is it possible that Dr. Bédard's study is somehow discovering something that all the other studies missed? I have no idea, but it may be some time before we know since the full study is still not available online.