It would be unrealistic to bar drivers from having any trace of marijuana in their system when it becomes legal in Canada, as B.C. has suggested, because the drug can be detected long after a person is impaired, public-health experts say.

The federal government's task force on the future of legalization has highlighted impaired driving as one of the core issues it must address before making its recommendations by the end of this month.

It's an issue that several provinces have also raised as a major concern. Most recently, British Columbia's Solicitor-General, Mike Morris, told the B.C. Liberal Party's convention on the weekend that he favours such a zero-tolerance policy when it comes to drivers and cannabis. Mr. Morris noted that Australia has implemented such a policy.

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But M.J. Milloy, an infectious-disease epidemiologist who is studying the therapeutic effects of marijuana at the B.C. Centre for Excellence in HIV/AIDS, said enforcing a zero-tolerance approach would be problematic because roadside testing could capture traces of tetrahydrocannabinol (THC), the psychoactive compound in cannabis, long after the driver feels any sort of high.

"THC sticks around for a long time," he said, adding that blood tests can reveal traces of the compound two weeks after marijuana is consumed.

Instead, Dr. Milloy said, the federal government should invest in researching how and to what degree cannabis impairs drivers. Then policy makers can create an objective limit of THC impairment, he said. The best option for testing that limit may be by swabbing someone's saliva, which could reveal use of the substance hours – not days – beforehand, he added.

Still, he said, Washington and Colorado's established limits of five nanograms of THC per millilitre of blood are arbitrary because different people can metabolize the drug to widely varying degrees, depending on their body type and history of using cannabis.

There are other barriers to a testing and enforcement regime. Roadside cannabis tests are still in their pilot stages, and researchers are trying to determine how, and at what level, marijuana affects drivers. The Toronto-based Centre for Addiction and Mental Health could release results as early as next month on its three-year study of how marijuana impairs one's driving ability.

David Hammond, an associate professor of public health at the University of Waterloo, said it is accepted that marijuana can impair one's co-ordination, ability to pay attention as well as reaction time, but more research is needed to assess the role marijuana use plays in fatal traffic accidents.

"There's no evidence that it's more dangerous [than alcohol] and requires stiffer penalties," Prof. Hammond said. "If a jurisdiction wants to have stiffer penalties and have zero tolerance, that's fine, but I think they should be consistent across substances."

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The federal task force on legalization reported that 97 per cent of impaired-driving incidents reported by police in 2013 involved alcohol, while only 3 per cent involved drugs, including cannabis.

Prof. Hammond said that having different impairment standards creates the risk of a cultural bias that penalizes those who prefer a toke over those who like a cocktail, the latter of which has been normalized in Canadian society – especially among wealthier citizens.

Under the current system of enforcement, it can be difficult for police to get cannabis-impaired driving charges to stick.

An August, 2012, Saskatchewan Provincial Court ruling illustrates how difficult it can be for police to get cannabis-impaired driving charges to stick under the current system of enforcement.

In that case, a judge ruled that police at a roadside check could not prove a woman was impaired from a joint she admitted to smoking 2 1/2 hours earlier. That was despite her failing several field sobriety tests – such as walking in a straight line and touching her nose – and being given a blood test at the police station that was positive for THC.

The judge stated that there was no doubt the accused had used the drug before getting in her car, but there was no proof that it negatively affected her performance behind the wheel.