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When marijuana use becomes legal next July, more people will drive drugged. Because there is no breath-analysis machine available, police will have a hard time detecting these drivers and securing convictions. Proposed measures to make it easier for police to lay a charge are cumbersome, expensive and possibly unconstitutional.

That doesn't mean the Trudeau government should abandon plans to legalize marijuana. But we should face facts.

"If you're saying to me, 'With these new measures, drug-impaired driving enforcement won't be at a level to have a dramatic deterrent impact on behaviour,' I agree with you," says Robert Solomon, a law professor at the University of Western Ontario who is director of legal policy for Mothers Against Drunk Driving Canada.

The confusion around policing drug-impaired driving is one of the many challenges government will face as marijuana is legalized.

Adult marijuana use increases once it's legal. In Colorado, where consumption has been legal since 2014, use among adults went from 17 per cent of the population before legalization to 20 per cent afterward, according to a 2016 federal study. It follows that increased use will lead to increased abuse, such as driving while impaired.

Under Bill C-46, the criminal-justice legislation that accompanies marijuana legalization, impaired driving due to cannabis use will occur when a person's blood drug concentration (BDC) is above a certain limit. But no breath-analysis machine exists that accurately measures the amount of THC – the mind-altering chemical in cannabis – in a person's blood.

There are machines that can detect the presence of THC in a person's saliva, which under C-46 the police would legally be able to obtain. But this new law may prove to be unconstitutional.

"It is a highly invasive technique and a serious intrusion on privacy, safety and integrity," criminal defence lawyer Jordana Goldlist said in an e-mail exchange. " It is not a matter of the individual blowing into a mouthpiece but police actually extracting a sample from one's mouth."

Furthermore, "saliva contains DNA, which ordinarily requires a search warrant for the police to obtain," she said. "Bill C-46 would allow police to collect a sample of a suspect's DNA under the false pretense of a sobriety test."

Prof. Solomon is much more confident of the measure's constitutionality. He compared handing over a swab of your saliva with security screening at an airport, where you may be subjected to a full body scan while your luggage is searched. "I think many people would find that more intrusive," he believes.

The real problem, Prof. Solomon says, is that the saliva test, the field sobriety test to determine impairment, and ultimately the blood test that may accompany a charge, take a long time to administer and cost a lot of money. "We don't have any cheap, highly accurate, quick mechanism for screening large numbers of drivers for drugs. We just don't." And it's unlikely we will any time soon.

For police forces, coping with the expected increase in drugged driving will be like "trying to drain an Olympic-sized pool with a garden hose in a rainstorm," as Saint John police chief John Bates put it. The key to preventing drinking and driving was the fear of getting caught: the RIDE program, the Breathalyzer test. For marijuana, none of that is available.

Of course, people are toking and driving now. They are taking legal drugs such as painkillers and then getting behind the wheel high. They are mixing drugs and alcohol, and otherwise behaving irresponsibly. Legalization may not increase levels of impaired driving all that much.

Robert Mann, of Toronto's Centre for Addiction and Mental Health, has been conducting research on how marijuana use affects driving ability. "They're different drugs, and so not surprisingly the effects are different," he said in an interview. But his research shows that both drugs increase the driver's risk of being involved in a car crash.

Dr. Mann looks to provincial measures, such as licence suspensions, to keep people who toke out of their cars. And he says he's optimistic that improved technology will make it easier to detect and deter drugged driving. In the meantime, it's up to us. Before you light up, hand over your keys. Even if it's harder for the cops to catch you, it's just not worth the risk.

Prime Minister Justin Trudeau is proposing a federal tax on recreational cannabis, once it becomes legal. Premieres at a Tuesday first ministers meeting argued provinces should get most of the revenue due to expected legalization costs.

The Canadian Press