Young people who use cannabis and drive are at greater risk of being involved in a vehicular collision even if five hours have elapsed since inhaling it, according to a McGill University study published Monday.

The research, published just two days before cannabis is legalized across the country, found that performance declined significantly in key areas such as reaction time after inhaling the equivalent of less than one typical joint.

"This new trial provides important Canadian evidence that cannabis can affect the skills needed to drive safely even five hours after consuming," Jeff Walker, chief strategy officer for the Canadian Automobile Association, said in a statement.

The CAA funded the clinical trial by the Montreal-based Research Institute of the McGill University Health Centre and McGill University.

"If you consume, don't drive," he said. "Find another way home or stay where you are."

The clinical trial examined the effects of cannabis on driving reflexes among occasional consumers aged 18 to 24 years.

A total of 45 study participants, 21 of whom were women, were put in a driving simulator and exposed to "the kinds of distractions common on the road." Research participants also took computerized tests that assess attention abilities.

Participants completed simulations at one, three and five hours after inhalation of a standard 100-mg dose of cannabis through a vaporizer (a typical joint is 300-500 mg of dried cannabis). Participants were also tested with no cannabis in their system.

'Significant impairment' complex driving tasks

While the cannabis dose did not affect simple, distraction-free driving, there was "significant impairment on complex and novel driving-related tasks," according to the peer-reviewed findings, which were published in CMAJ Open, an open-access journal published by the Canadian Medical Association.

The complex or novel tasks included situations such as avoiding sudden obstacles, like a child crossing the street unexpectedly or driving through a busy intersection.

"This is really what driving is all about: you always have to be on your toes," the study's co-author Isabelle Gélinas told CBC Montreal's Daybreak Monday. She is a researcher in McGill's School of Physical and Occupational Therapy.

In addition, a large percentage of participants reported they didn't feel as safe to drive after consuming cannabis, even five hours after use.

The study says the findings substantiate Canada's Lower-Risk Cannabis Use Guidelines, developed by the Canadian Research Initiative in Substance Misuse in 2017 and endorsed by the Canadian Public Health Association, which recommend waiting six hours after cannabis use before driving.

Increasing evidence

Gélinas says the clinical trial's findings add to the growing amount of scientific evidence proving cannabis does affect driving ability.

"The findings provide new evidence on the extent to which driving-related performance is compromised following a typical dose of inhaled cannabis, even at five hours after use," Gélinas said in the statement.

It wasn't a surprise that driving is impaired by cannabis consumption, she said, but how long drivers are affected was an important finding.

She said she has heard of ongoing studies that are looking at cannabis's affect on driving up to 24 hours after consumption.

Young drivers 'more at risk'

Young drivers are already more likely to be involved in a collision, Gélinas said, and they are "smoking more cannabis. So this combination makes them even more at risk in terms of accidents."

The study, however, did not look at how it affects more mature drivers. Because of age-related changes seen in older, experienced drivers, Gélinas said the study could produce different results.

"They are totally different groups, but [it would be] interesting to look at that in the future," she said.

With legalization around the corner, she said the study should help people, especially young drivers, be more aware of the dangers of driving after consuming cannabis.

Drivers, she said, should wait a "significant amount of time" before getting behind the wheel.

CAA is committed to furthering "this important road safety issue, but governments must step up too," Walker concludes, pushing for more funding to study the effects of cannabis on driving.

With files from CBC Montreal's Daybreak