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Younger drivers are more at risk of crashing a vehicle up to five hours after they consume cannabis, according to a new clinical trial at McGill University that researchers say underscores the need for Canadians to be aware of how marijuana can hurt one’s driving ability.

The study, published Monday in the CMAJ Open, found the drug had no effect on the way participants – casual cannabis users ranging from the age of 18 to 24 – were able to complete simple driving tasks such as braking or maintaining speed. But it made them significantly worse at identifying and reacting to distractions that can lead to crashes. The peer-reviewed study also found 80 per cent or more of the young drivers reported feeling less safe to drive after using the drug, including five hours later.

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“More complex and new tasks are more affected and that’s really what driving is all about,” said Isabelle Gélinas, a study co-author from McGill’s School of Physical and Occupational Therapy. “We never know – even in a neighbourhood that is quiet – when something unexpected can happen, so we have to always be able to react to the unexpected.”

The researchers selected a potentially dangerous segment of Canada’s drivers – the sizable population of young adults who are also recreational cannabis users. They did this by choosing 45 participants that had consumed the drug at least once in the past three months, but no more than four times a week. Every province or territory has set the minimum age for buying legal cannabis at 18 or 19, while Quebec’s incoming provincial government has signalled it will soon raise it to 21.

Jeff Walker, chief strategy officer of the trial’s sponsor Canadian Automobile Association, said these new findings are especially relevant given regular polling by the organization has found one in five young Canadians believe consuming cannabis has no effect on or even improves their driving. Dr. Gélinas said the results also back up the Centre for Addiction and Mental Health’s recommendation that people wait at least six hours after using cannabis before they get behind the wheel.

The research follows a survey by Statistics Canada this summer that found roughly one in seven cannabis users said they had recently got behind the wheel soon after using the drug – which is a higher rate than people report driving soon after drinking in other studies. Police forces across the country have said they are not rolling out the widespread use of cannabis screening devices this week and that they are speeding up efforts to train more officers to detect stoned drivers to combat a potential rise in drug-impaired driving.

The McGill study involved having each participant take five long puffs from a vaporizer containing less than a joint’s worth of medical-grade cannabis with a THC concentration of 12.9 per cent – akin to the average strength of marijuana commonly found on the street. They then waited in a room where they had snacks, music, reading material and Netflix – but no video games – until they performed field-of-view and driving simulation tests one, three and five hours after taking the cannabis.

“It was easy to recruit for, I will say that,” Dr. Gélinas said.

The baseline data on participants’ driving performance and self-perceptions were captured by having them take these tests without any cannabis. The study also found participants showed higher levels of vigilance and caution in the driving simulator at the one-hour mark after vaping, which researchers said might be due to them being able to focus before coming down from their high and growing tired or more easily distracted. Still, Dr. Gélinas said, those high drivers' overall skills at the one-hour mark were impaired and they presented a greater potential danger to others on the road than when they were sober.

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With more and more older Canadians turning to cannabis, new research is needed on how the drug affects the driving of seniors – another segment of the population statistically more at risk of crashing, Dr. Gélinas said. “The impact will be different than in young drivers because of the aging process.”