Cannabis found in twice as many injured drivers since legalization: B.C. study

A man holds a joint while smoking marijuana to celebrate the legalization of recreational cannabis, in Vancouver, on Wednesday October 17, 2018. (THE CANADIAN PRESS/Darryl Dyck)

VANCOUVER -

A new study out of the University of British Columbia suggests that cannabis is being detected in twice as many injured drivers since its 2018 legalization.

Dr. Jeffrey Brubacher, an associate professor at UBC and the principal investigator of the study, says before cannabis was legalized, 3.8 per cent of injured drivers had THC concentrations above the Canadian legal driving limit of two nanograms/ml. That percentage rose to 8.6 per cent after cannabis was legalized.

The amount of injured drivers with a THC concentration of above five nanograms/ml in their system also went up, from 1.1 per cent pre-legalization to 3.5 per cent after legalization.

"It's concerning that we're seeing such a dramatic increase," Brubacher says in a news release. "There are serious risks associated with driving after cannabis use. Our findings suggests more is needed to deter this dangerous behaviour in light of legalization."

Brubacher and his colleagues analyzed blood samples from 4,339 "moderately injured" drivers who received treatment at four B.C. trauma centres between 2013 and 2020.

Researchers found that those over the age of 50 made up the largest increase of drivers injured while under the influence of cannabis.

The study, published in the New England Journal of Medicine, also suggests there weren't any significant changes in drivers testing positive for alcohol, on its own, or in combination with THC.

The federal government amended the Criminal Code when cannabis was legalized, which gave police more powers to test drivers who they suspected might be driving under the influence of drugs.

But Brubacher says the findings of the study show that it wasn't enough to deter cannabis impaired driving.

“We hope that policymakers will use our findings to design public information campaigns and enforcement measures that encourage drivers, especially older drivers, to separate cannabis use from driving. At the same time, it is important not to lose sight of alcohol impaired driving, which is extremely high risk, especially when combined with cannabis.”

The study suggests that while cannabis use is often associated with cognitive deficits, the presence of THC in the blood is not always an indicator that a collision was caused by cannabis impairment.

"Detecting cannabis, especially at low concentrations, doesn't necessarily mean a driver is impaired," Brubacher adds. "But the risk is real with higher THC levels, which is why it’s so important that we continue to assess and respond to the impact that legalization is having on road safety.”

Brubacher says his team is now expanding their research efforts to 15 trauma centres across Canada.

They plan to collect more information on the prevalence of cannabis, alcohol and other substances in injured drivers, which they hope could help guide traffic policies both nationally and provincially moving forward.