Pot effects last up to 5 hours, driving study finds
Reaction times can decrease significantly
A considerable proportion of people who smoke pot have rather blasé attitudes toward driving while high, with surveys showing many users are convinced the drug doesn’t impair their ability to drive safely.
Now, a new Canadian study shows using pot before driving increases the risk of a crash even five hours after inhaling.
McGill University researchers found that performance in key areas like reaction time decreased significantly, and that the effects lasted up to five hours after inhaling the equivalent of one typical joint.
Simple driving tasks were largely unaffected, but once the situation became even remotely complicated with normal distractions, that’s when the wheels started coming off, according to the paper published in CMAJ Open Monday, two days before Canada’s prohibition on recreational marijuana is lifted.
The Canadian Automobile Association, whose polling has found that one in five millennials (18- to 34-year-olds) believe they can drive as well — or even better — stoned as they do sober, funded the research.
With the legalization of recreational weed, “young adults, who are already at risk of automobile crashes, may increase their use of cannabis, which may further increase the risk of crashes,” the authors write in CMAJ Open.
Until now there’s been confusion over whether people can safely consume any amount of cannabis before driving.
While alcohol has been shown to impair a user more than cannabis, some studies have found cannabis can have various effects on driving, including increased breaking, more weaving within lanes and distorted time perception.
Less clear was how long any effects might last.
The McGill study involved 45 recreational users ages 18 to 24. All used cannabis at least once a week in the past three months, but not more than four times per week.
Each completed four test sessions — without cannabis, and at one, three and five hours after inhaling a standard, 100mg dose of weed. They were tested on driving simulators as well as a computerized test that measures, among other things, divided attention, how fast a person can detect an object in the periphery and distractibility.
Participants were also asked about their perceived ability to drive, and drive safely, with questions such as: “How confident are you in your ability to drive right now?”
Overall, while pot appeared to have no significant effects on driving when there were no distractions, “complex driving-related performance” — for example, a car ahead suddenly braking or a child crossing the street — “was affected at all time points after cannabis use,” the authors write.
“When we’re driving we have to be prepared to react to unexpected situations,” said co-author Isabelle Gélinas, a researcher in McGill’s School of Physical and Occupational Therapy. “The ability to react to novel, more complex tasks was altered.”
The effects were most pronounced at three and five hours after inhaling pot. One possibility is that, during the acute “post-cannabis” phase, people “are indeed able to effectively focus on tasks,” the authors said, but that, three and five hours later, a different kind of impairment sets in as people come down from the “high” and become more tired and more easily distracted.
Even up to five hours after inhalation, volunteers said they didn’t feel safe to drive, Gélinas said.
According to data collected by Statistics Canada over the first half of 2018, about 1.4 million Canadians reported that they had been a passenger in a vehicle driven by someone who had consumed cannabis in the previous two hours. In addition, one in seven cannabis users with a driver’s licence reported that they got behind the wheel at least once within two hours of using the drug in the past three months.
The new study’s findings fit with recommendations under Canadian guidelines for “lower risk cannabis use,” which recommend waiting six hours after using pot before driving.
So far, police forces across Canada have been slow to deploy roadside saliva testing that can check a driver for recent drug use. They’re also grappling with how to reliably and quickly get blood samples from suspected drug-impaired drivers in order to use new criminal charges created ahead of the legalization of recreational pot.
Still, police can rely on field sobriety tests — which can involve standing on one leg or tracking an object with your eye — to screen for drug-impaired driving at the roadside. Anyone who fails can be taken in for further testing.
THE ABILITY TO REACT TO NOVEL, MORE COMPLEX TASKS WAS ALTERED.