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Former Toronto police chief and current Liberal MP Bill Blair takes part in a news conference in Ottawa April 27, 2015. REUTERS/Chris Wattie

CHRIS WATTIE/Reuters

Marijuana policy can look a little like a Russian nesting doll: crack open an issue, and all it reveals is more issues.

Ottawa's point man on pot, MP and former Toronto police chief Bill Blair, recently told the CBC that the federal government is considering instituting roadside saliva tests for stoned driving. At first blush, this sounds like a no-brainer, given that impaired driving is a scourge that requires a strong deterrent. But even though such drug testing exists in Europe and Australia, it's not a straightforward proposition.

For one thing, there is no broad scientific consensus on the baseline level of THC (cannabis' principal psychoactive substance) that tips someone into impairment. For another, the testing currently available can be imprecise. Given the legal consequences, which can include vehicle seizure and criminal charges, that's worrisome.

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It would be ideal if the drunk-driving model could be applied to pot – with an agreed-upon limit of the substance permitted in the blood system, and cheap and reliable testing to determine if a driver was near or over the line. Unfortunately, that may not be possible when it comes to marijuana, at least not yet.

In fact, there is conflicting research on whether using pot results in more erratic driving. Some studies suggest it does not; others say it does. Two weeks ago, a study out of the state of Washington showed the number of roadway fatalities involving marijuana has doubled since recreational consumption was legalized in 2012.

It made for snazzy headlines, but the overall number of fatalities is essentially flat; just 17 per cent were found to have marijuana in their system and two-thirds of those also had taken alcohol or other drugs.

Not incidentally, Washington has established a legal limit of five nanograms of THC per millilitre of blood. Colorado posts the same limit, but pot states Alaska and Oregon have none.

As it stands, the federal Liberals, who have pledged to legalize marijuana next year, have offered no indication of what the maximum acceptable level of THC will be, or if there is to be one.

Mr. Blair's idea for roadside testing may turn out to be sound policy, but it's hard to say so conclusively because of gaps in the research. Before legislating on a topic this complex, the federal government is going to have to fill them.

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