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The busywork of readying the country for legalized cannabis continues apace, and Ottawa has gotten around to mapping out the murky territory that is stoned driving.

Proposed legislative changes would create a two-tiered approach to setting the maximum amount of THC, cannabis's main psychoactive element, allowable in drivers' blood. Motorists measured with two to five nanograms of THC per millilitre of blood within two hours of being stopped would be subjected to a fine; those above would be treated criminally, mirroring the way many provinces deal with alcohol and driving.

The approach raises various problems, not least of which are the iffy reliability of roadside testing for weed and a paucity of statistics to accurately depict the scale of the problem.

Then there's the question of where to set the limit for medicinal users, who by definition have a higher baseline THC level, long after it's been metabolized.

Thus, a group of defence lawyers is urging Ottawa to put a hold on changes to the Criminal Code, on the grounds that the current proposals unfairly single out legal medical-marijuana consumers.

It's a thin argument.

Limiting how much cannabis one may consume before driving does not stigmatize medicinal users any more than a 0.08 per cent blood-alcohol content stigmatizes legal drinkers. Many medications carry warnings against operating machinery or driving; if taking them results in impairment, you can be arrested – and you should be.

When it comes to marijuana, the law may be moving faster than the mechanism of enforcement, but focusing the debate on technicalities obscures the broader goal. There is nothing wrong with erring on the side of caution. A restrictive standard is preferable to an overly lenient one.

While we're at it, Canada should consider dropping the bar for booze. Drunk driving is a large-scale and well understood threat, and this country has higher legal limits than many others.

As a society we should be extremely prudent about what condition one is allowed to be in when getting behind the wheel. That means aggressively fighting impaired driving on our roads, whatever the source of the impairment.