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As the country undergoes the thorny process toward cannabis legalization, a key aspect has been settling on what’s known as a per se limit, which specifies how much of the drug has to be in a driver’s system to constitute an offence.Eduardo Verdugo/The Associated Press

There will be zero tolerance for cannabis among young, novice and commercial drivers, the Ontario government announced last week, with the province's Premier and Transportation Minister standing before a carefully curated group of police and safety advocates to deliver the uncompromising message.

"We are proposing to strengthen our impaired driving laws," said Steven Del Duca, who heads the Transportation Ministry. "Let me be clear, driving while impaired is not acceptable and will not be tolerated."

Ministerial handouts spelled out the proposed new regime, saying that young, novice and commercial "drivers should not get behind the wheel if they have any detectable presence of drugs or alcohol in their system."

However, there may be less to the tough soundbite than it appears. The provincial proposal comes before the federal government, which has promised to legalize cannabis by next July, has even approved standards for roadside screening devices. And since the technology measures the presence of a drug, not the level of impairment, too rigorous a screening approach would risk penalizing people who smoked long before driving. Because marijuana can linger in the body, there have been instances in other countries of drivers busted as long as nine days after smoking, only to have the case thrown out.

Steven Laskowski, president of the Ontario Trucking Association, noted that under the current approach, someone with a blood-alcohol content of 0.02 or less is deemed to be at zero. "So what they're going to have to do is develop a marijuana equivalent of point-zero-two," he said.

"If somebody uses cannabis in the evening, do you want them testing positive the next day?" asked Robert Mann, a senior scientist at Toronto's Centre for Addiction and Mental Health.

Ontario Ministry of Transportation spokesman Bob Nichols said in an e-mail that it is expected that federal screening rules would be established to turn up "recent use" of cannabis. Mr. Del Duca said that the ministry is working with Ottawa on "the appropriate way to deal with the notion of zero tolerance."

But Mr. Del Duca also has a stern warning for young, novice or commercial drivers who consider mixing marijuana and driving.

"I think you need to think long and hard about exactly how you're going to behave," he said later in the week, after a separate road-safety announcement. "I think the responsibility we have, the obligation I have as minister, is to err on the side of making sure we have a regime in place that protects everybody who's using the roads."

The proposed provincial legislation is part of a flurry of preparation by jurisdictions that is ratcheting up across the country, even as police warn that the change is coming too fast for them to get ready. Among the many hurdles still to be passed is figuring out the right roadside screening devices and training police to use them.

As the country undergoes the thorny process toward cannabis legalization, a key aspect has been settling on what's known as a per se limit, which specifies how much of the drug has to be in a driver's system to constitute an offence. Ottawa has decided that people with at least two but less than five nanograms of THC per millilitre of blood would be subject only to a fine, to a maximum of $1,000. Those with at least five nanograms would be treated more seriously.

But this approach isn't perfect, according to experts who warn that there is a key difference between the presence of cannabis and a recognized level of impairment.

The Canadian Association of Chiefs of Police stated in its submission this week to the federal Standing Committee on Justice and Human Rights that oral fluid screening devices "are not determinative of blood drug levels or impairment."

And a report this month from the Drugs and Driving Committee of the Canadian Society of Forensic Sciences, which has been tasked by the federal government with developing standards for screening devices, noted that is "a controversial exercise" to set a per se limit.

"One of the challenges for many potentially impairing drugs is that there is not currently substantive and consistent scientific evidence on which to base per se limits," their report states. "Setting a per se limit does not mean that all drivers below that concentration are not impaired and all drivers above that concentration are impaired."

At the province's zero-tolerance announcement, MADD Canada chief executive Andrew Murie acknowledged that the best approach to dealing with drug-impaired driving remains a work in progress.

"This is a big first step," he said. "It's not going to be the last step."

New federal legislation to legalize marijuana would include allowing police to test drivers’ saliva for drugs. Justice Minister Jody Wilson-Raybould says three new offenses related to driving while on drugs would also be added.

The Canadian Press