I don't regularly smoke weed, but I'll probably smoke casually when it gets legalized. How much marijuana can I have and be safe to drive? – Bryce, Toronto

The evidence is still hazy on how much 420 is safe before you hit the 401, experts say.

"The big variable is the THC content – it can range from very little to up to 30 per cent, so you really can't do calculations like you can for alcohol," says Andrew Murie, CEO of Mothers Against Drunk Diving (MADD) Canada.

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Because the THC [tetrahydrocannabidinol, the active ingredient in cannabis] may vary, it's tough to say how impaired you could actually get from a few puffs.

"It would be great to be able to say, 'If you smoke one joint, you're okay, if you smoke two you're impaired,' but it's not that simple," says Dr. Robert Mann, a social and epidemiological research scientist at the Centre for Addiction and Mental Health (CAMH). "When you have a bottle of beer, it says how much alcohol is in it – when you're smoking cannabis, you don't really know what you're getting."

How much THC you get varies too depending on how you smoke – how deeply you inhale and how long you hold it in.

The actual effects are tough to test. Canadian researchers who test the effects of marijuana on driving – how quickly and how badly people become impaired – have had to use medical marijuana and not illegal pot off the street. Medical marijuana is generally weaker –12 per cent THC – than what most people may actually be consuming.

"In many ways, the question of cannabis and driving is where alcohol and driving was 40 or 50 years ago," Mann says.

Weed versus alcohol

So how do high drivers fare on the roads? There's research showing that, unlike drunk drivers who think they're fine to drive, high drivers are usually aware they're high. They're afraid of getting in an accident – and so they drive too carefully.

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"There's a fair amount of research where they pay people to get high and they pay people to get drunk and then they get behind the wheel in a simulated environment," says Benjamin Hansen, an economist at the University of Oregon who has studied marijuana legalization in relation to driving accidents. "When you look at physical impairment, the two groups are not too different – but people who are high tend to become more risk averse and more afraid of bad outcomes so they go slower and follow cars at greater distances."

In contrast, drunk drivers become more aggressive and take more risks – they speed and follow cars too closely.

In a study by University of Iowa, researchers found that drivers using marijuana were more likely to weave within their own lane than people who were sober, but they were not more likely to speed or weave out of their lane."

But that doesn't mean stoned drivers are safe, says MADD's Murie.

"It's a bit like that old Cheech and Chong sketch, 'How's my driving?' because they're driving so slow," Murie says. "They make gross turns, they overcompensate, they drive really slow – but they're still a danger on the roads."

Mann says since driving is "a life or death situation and one of the most complex psychomotor tasks you can do," it's a good idea for marijuana to be treated the same way alcohol is treated under Canada's low-risk drinking guidelines. "I think its reasonable to say, if you're planning to smoke than the safest thing to do is not to drive," he says.

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And how long should you stay off the road? Again, the research is hazy.

"It's difficult to say," Mann says. "I saw one paper that said if you've smoked to the point that you feel it, you should wait maybe 10 hours."

"The jury is still out" in U.S.

Marijuana has been legalized in five states: Alaska, Colorado, the District of Columbia, Washington and Oregon.

One study by the U.S. National Highway Traffic Safety Administration (NHTSA) found that cannabis consumption was not associated with increased probability of getting in an accident. But, a report by the Rocky Mountain High Intensity Drug Trafficking Area (RMHIDTA), a federal agency that's against marijuana legalization, stated that marijuana-related traffic deaths in Colorado increased 32 per cent in 2014. That's an increase from 71 to 94 deaths.

"If someone who used marijuana two days ago is driving safely and gets hit by someone, the fact that they had detectable marijuana in their system is irrelevant," says Morgan Fox with the Marijuana Policy Project. "That's like referring to every accident involving a person who recently drank coffee or a soda a 'caffeine-related accident.'"

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John Urquhart, a Washington State sheriff, says "the jury is still out," on whether legalization has made roads more dangerous. "The statistics I've seen show that driving fatalities in Washington are about the same as they were before the legalization of marijuana," Urquhart says in an email. "There are more positive blood tests for marijuana, but that could be because the police are looking for more marijuana and testing more for it."

How do police enforce?

At least 17 states make it illegal to drive with certain levels of THC in the blood. In Colorado and Washington, that amount is 5 nanograms per millilitre (ng/ml).

There's some concern that some drivers might not actually be impaired at 5 ng/ml, Hansen says.

"Suppose you were involved in fatal crash and you weren't responsible, but they test your blood and it comes back at 5.2 ng/ml," Hansen says, "You could be charged with vehicular homicide despite the fact that you're not really impaired."

Canada does not have a blood-THC limit for drivers, instead, police must make drivers perform a field sobriety test.

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Murie says Canada should establish a limit before Ottawa goes ahead with plans to legalize pot here.

"I'm just signing a letter to Trudeau suggesting that they give police the rights to use the new saliva-based tests – the RCMP has just finished their testing of them," Murie says. "We don't want police officers taking blood at the roadside – most Canadians would be horrified at that and it's not a solution.

Murie says he supports the 5 ng/ml limit.

"Anything lower than that is a drug policy, not a safety policy," Murie says.

Police have been dealing with drugged driving for decades, but if marijuana is legalized, part of the challenge will be changing attitudes – the same way we did for alcohol.

"Back in the '70s and '80s, you used to hear 'one for the road,' and now you never hear that and you should never, ever hear that," says Ottawa Police Acting Superintendent Paul Johnston, who sits on the Canadian Association of Chiefs of Police drug abuse committee. "We need to have the equivalent understanding for marijuana – if you're consuming marijuana, you shouldn't be driving, period."

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