As cannabis legalization ripples across Canada, some Canadians may be tempted to consume cannabis and get behind the wheel of their car, perhaps thinking a cannabis high won’t disrupt their driving behavior compared to drinking a few beers. But such assumptions can be dangerous, if not lethal.
Dozens of studies have found motor-skills, reaction times and perceptions of stoned drivers were negatively affected by cannabis.
After consuming cannabis, drivers may have slower reaction times and impaired peripheral vision and lane control. In a 2015 study  , researchers found that high THC levels posed “challenges in concentration-based effects interpretation” in relation to driving.
Such studies have noted cannabis use increased the risk of traffic accidents by 30 percent, and “there remains a high risk of accidents under the influence of cannabis, and the evidence indicates that recreational cannabis users should not drive while intoxicated. 
In Canada, a 2017 Health Canada survey found  that 39 percent of cannabis users had at some point driven within two hours of consuming cannabis, and half of cannabis users felt the drug affected their driving ability.
We’ve often heard how cannabis consumers claim they drive slower than the legal limit, preferring be more cautious than erratic. There is no evidence to support claims that slow driving contributes to fewer traffic accidents. But it should be noted that cannabis tends to make their users less aggressive with the gas pedal, compared to alcohol users.
What shouldn’t be ignored is how often cannabis is consumed with alcohol, which can be a deadly combination for motorists. A French study on fatal crashes found that drivers who consumed both alcohol and cannabis were 14 times more likely to be found responsible for the crash when compared with sober driving and much higher than those who had used either drug on its own.
A challenge for law enforcement, though, will be testing THC levels in a driver’s bloodstream. There are no universally agreed-upon blood-THC levels at which a driver can be objectively considered “impaired,” since THC is quickly eliminated from the bloodstream, a markedly different trait than alcohol.
When the federal government decides on a saliva-testing technology to administer to drivers, Canada will get a clearer sense of the legal THC limits allowed on the road. But it’s safe to assume that even just a few puffs of a joint or half an edible cookie can influence a driver’s reaction time and perception behind the wheel, and could quickly become a danger to both fellow drivers and pedestrians.
Another issue worth considering relates to the legality of being on the road high on THC. Lawyer Nathan Baker brought up  the point that unlike alcohol “a person eating a cookie, a chocolate bar or gummy bears at a party may not realize they have consumed THC.”
He went on to write: “Since this drug can affect inhibitions, a person may operate a motor vehicle without appreciating the danger they pose. Courts will have to carefully scrutinize evidence in these cases in order to protect the innocent while still punishing those with a guilty intent.”
Several researchers stress the importance of educating drivers, especially young motorists, about the consequences of driving under the influence of cannabis. A report published in Pediatrics & Child Health  found that “plans for legalization of recreational use should anticipate the costs of preventive education efforts that present an accurate picture of potential risks for driving. Youth also need to understand risks for dependence, and screening for and treatment of marijuana use disorders is needed.”