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Increased collision risk, skill impairment
Allison Gerrard - February 14, 2012
You may have come across someone claiming they can drive just fine after marijuana use. But new Dal research suggests that if you’re the sober passenger in that scenario, you should really ask for the keys.
Dr. Mark Asbridge, associate professor in Dalhousie Medical School's Department of Community Health & Epidemiology, completed a systematic review and meta-analysis of nine studies to determine if cannabis use increases the risk of a motor vehicle collision (MVC).
His review, consisting of a total sample of 49,411 people, found that drivers who consume cannabis within three hours of driving are nearly twice as likely to cause an MVC as those who aren't under the influence of drugs or alcohol.
Dr. Asbridge's findings were published last week in the British Medical Journal.
"This is the first review looking at observational studies concerned with the risk of vehicle collision after the recent consumption of cannabis," says Dr. Asbridge, who also has an appointment in the Dalhousie/Capital Health Department of Emergency Medicine.
"This research clearly demonstrates that recent cannabis consumption impairs the skills necessary for safe driving and increases collision risk."
All MVCs involved in the study took place on a public road and involved one or more moving vehicles such as cars, vans, sports utility vehicles, trucks, buses and motorcycles. Cannabis use was identified through blood samples or direct self-report.
Rates of cannabis use high among young drivers
Rates of driving under the influence of cannabis have increased in Canada in recent years, with about four per cent of adult drivers reporting use prior to getting behind the wheel. The rate among youth and young adult drivers is higher, with between 14 and 21 per cent reporting that they have driven after using the substance.
"This research supports existing policies, in Canada and abroad, that prohibit driving under the influence of cannabis," says Dr. Asbridge. "While public health efforts aimed at drinking and driving have been highly successful, greater attention needs to be directed to cannabis and other substances that may affect safe driving performance."
Cannabis is the most widely used illicit substance globally, and recent statistics have shown a significant increase in use across the world.
Between 2000 and 2007, cannabis was the second most commonly found drug among fatally injured drivers in Canada. A 2010 roadside survey in British Columbia showed that 5.8 per cent of the 2,442 drivers stopped had cannabis in their systems.
Readers Say
February 14, 2012 7:43 PM
February 15, 2012 10:01 AM
I'm all for being critical of science and studies and such, but I really think the message is is precautionary (and rightly so): we have reasonable belief that cannabis affects your ability to safely drive, so just don't do it before you drive! I understand you trying to correct the study by saying "correlation is not causation", but I think your intentions were not to offer criticism of the scientific method employed here, but to defend cannabis use and driving. Which, again, I think is not wise.
February 15, 2012 11:57 AM
Like T says, driving is safest when done without being under any influences. I am not going to argue that cannabis use while driving is safe or advisable.
However, I think J makes a great point that alcohol and other factors need to be taken into consideration. Perhaps they were, and this article simply does not mention it - but in order to draw a conclusion about collision risk after marijuana use, alcohol needs to be ruled out.
I say alcohol specifically because, like J said, the two substances are commonly consumed together. I would find it very strange if researchers did not account for this in their research, so I would assume this was based on drivers that were under the influence of marijuana and no other substances , but if that is the case it should be clarified.
February 15, 2012 12:41 PM
J: I agree with you that other substances must be taken into account. But, I would think that the researchers would have thought of that, or at least the peer-reviewers at the medical journal. I haven't read the actual journal article, though, so I'm not sure.
February 15, 2012 3:19 PM
February 16, 2012 7:40 PM
February 17, 2012 9:09 AM
"Across studies, the main bias related to exposure measurement was the concurrent use of alcohol or other drugs in addition to cannabis... Since this bias would overestimate the effect of cannabis on collision risk, we calculated odds ratios using “cannabis only” cases and controls—that is, only those motorists whose system contained cannabis without any other drugs or alcohol present."
In other words the study explicitly excludes "'somebody [who] consumes 20 beers and a joint then wrecks their car."
February 17, 2012 11:35 AM
February 17, 2012 11:56 AM
Upon reading the BMJ it looks like they've covered all their bases as far as bias and confounding factors. The exclusion of alcohol impairment from this study is absolutely crucial to it's validity. But, we should not be quick to draw anything too conclusive from this, as the effects of cannabis are widely variable among individuals. Also, numerous studies in the past have shown an overwhelming amount of results in direct conflict with these findings.
Check these out!
http://www.erowid.org/plants/cannabis/cannabis_driving6.pdf
http://miqel.com/reading_library/archived_stories/feel-free-to-drive-on-weed.html
February 17, 2012 7:20 PM
This report suggests that young people use marijuana at a higher rate than older people. We know that younger people have higher accident rates. So we would expect that marijuana users would have higher rates of accidents due to their ages alone. Was the study controlled for this?
What about relative risks? How much does marijuana raise the risk of accidents compared to, for example fatigue?
March 5, 2012 11:41 PM
March 8, 2012 12:38 AM
I wonder if the sample group of people were tested while driving sober as well, to see if their driving aptitude would have caused them to have twice the chance of getting into the accident, without any foreign substances in their blood.
Also, a list of chance of accident ratios to compare the findings to would be paramount, as to not sensationalize the study.
Example:
Chances of crashing a car while driving while crash victims reported:
1. Being Sleepy = 4 times
2. Taking Ibuprophin = 1.234 times
3. Daydreaming = 7 times
4. Smoking Crack = 34 Times
5. Reading this article while driving = 10 times
April 4, 2012 7:43 PM
After reading many news articles, and the Methods section of the study itself, I'm still stumped as to how the study's author got the number to compare the risk against. Twice the risk compared to WHAT? Is it assumed that all drivers who don't in car accidents are "sober"? That's obviously not true, so how do you measure the drivers who have used cannabis within the last three hours and DON'T get in car accidents?
It seems they'd have to make random stops of drivers and ask them whether they'd used cannabis in the last three hours, in order to make a comparison. And c'mon: how many people would answer "yes" to that question, even if they had? (In the US, at least, forcing someone to answer that question or take a drug test would be against the law if there's no reason to believe they're under the influence.)
Also, according to the Methods described at the link above, this study excluded simulator studies which is really the only way to eliminate the correlation vs causation problem, and the problem of unreliable data for people NOT involved in collisions. (Simulator studies, btw, show no difference in "sober" vs "stoned" drivers.)
Meta-studies are of questionable relevance anyways. Of course that doesn't stop the police from citing this study as they lobby for...oops, I mean..."enforce"...tougher laws against people making a choice to use something safer than alcohol, healthier than fast food, and less addictive than coffee.