Two U.S. states that have introduced recreational marijuana sales, as prime-minister-designate Justin Trudeau plans to do, have both seen significant increases in the proportion of fatal accidents involving drivers who tested positive for the drug.

But officials south of the border say it is too early to draw concrete conclusions about the effects of pot on traffic deaths.

Colorado started allowing adults to buy the drug at pot shops two years ago, and Washington's licensed cannabis stores began sprouting up in July, 2014.

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In both states, the overall number of people killed in crashes increased by only a handful, but the percentage of those accidents in which the drivers tested positive for marijuana increased considerably.

In Colorado, about 10 per cent of drivers involved in fatal accidents in 2010 tested positive for the drug. By 2014, just over a year after recreational sales were legalized, that percentage nearly doubled to 19 per cent.

In Washington over that same period, the percentage of drivers involved in fatal accidents who tested positive for THC, the psychoactive compound in marijuana, doubled to 12 per cent from about 6 per cent, according to the Washington Traffic Safety Commission.

Sergeant Rob Madden, a spokesman for the Colorado State Patrol, said reports of marijuana-impaired driving could also be going up because all officers on the force are now trained properly to look for stoned drivers and are "pro-actively doing our best."

Still, he said it is difficult to draw conclusions from such a small amount of data.

Mr. Trudeau has promised to legalize and regulate the sale of marijuana, but has not said what that would look like. The Liberal platform called for a task force involving multiple levels of government, public health officials, law enforcement agencies and addictions experts.

A recent study of impaired drivers who ended up in B.C. emergency rooms noted that cannabis was the most common recreational drug among the injured drivers, with 7.3 per cent consuming it in the hours before their crashes.

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Studies have shown a heavy dose of pot nearly doubles a driver's risk of a crash that results in serious injury or death.

Cannabis affects a person's ability to drive by impairing depth perception, attention span and concentration, slowing reaction time and decreasing muscle strength and hand steadiness, according to the Toronto-based Centre for Addiction and Mental Health.

Traces of THC can stay in a user's system days or weeks after the psychoactive effects are felt.

In Colorado and Washington, a person can be charged with impaired driving if a blood test shows at least 5 nanograms per millilitre of THC. Authorities there say this is analogous to the 0.08-per-cent limit set out in drunk driving laws across many jurisdictions.

Paul Doroshenko, a lawyer based in Vancouver, said getting the blood samples is very difficult under Canadian law, and police often must prove marijuana impairment with a field sobriety test, which rarely holds up in court.

Dan Werb, a researcher with the Toronto-based International Centre for Science in Drug Policy, said drivers impaired on cannabis are definitely a concern, but added "the question really is whether regulation or prohibition is a better way of ensuring that people don't drive or use heavy machinery and hurt themselves or others while they're using drugs."

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He said there is no evidence people use cannabis less frequently when it is illegal, noting Canada has some of the highest consumption rates in the world.

To reduce the number of stoned drivers, authorities would do best to take a nuanced approach, as with alcohol, he said.

"You can see on TV that there are ads around drinking saying, 'don't drink and drive,'" Dr. Werb said. "There aren't ads that say, 'drinking is evil, you shouldn't do it.'

"Under a system where cannabis is illegal, it's really hard to get information about its potential harms."

Last year, the Washington Traffic Safety Commission aired ads produced by Colorado's department of transportation advising young people that getting high and barbecuing, playing basketball or installing a TV was now legal. But driving high would still get them a DUI.