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Dr. Robert Mann demonstrates a driving simulation used to study how marijuana affects a person's ability to drive at the Centre for Addiction and Mental Health in Toronto.Kevin Van Paassen/The Globe and Mail

Police would be given sweeping new authority to combat drunk driving by demanding a breath sample from any driver at roadside, as the Liberal government prepares for the legalization of marijuana with a major overhaul of impaired-driving laws.

A proposed law introduced in Parliament on Thursday drops the requirement that police need a reasonable suspicion that a driver has been drinking before demanding a breath sample. It will almost certainly be challenged as a violation of the constitutional right to be free from unreasonable searches, legal observers say.

For marijuana, police would have the right to take "oral fluid samples" from drivers at roadside – but only those they have reason to suspect are using the drug (or other drugs). The sample would not be proof of drug use, but would help create reasonable grounds to believe a crime was committed, so police could conduct further testing.

In pairing the liberalization of marijuana laws with tough-on-crime legislation featuring enhanced police powers and stronger mandatory punishments, the federal government said its aim is to create among the world's strongest impaired-driving regimes, especially compared with jurisdictions around the world where cannabis is legal. It also says that the current set of laws on drunk driving are difficult to prosecute, and the new law would restrict or eliminate some defences.

"If it passes Parliament [it] will be one of the strongest impaired-driving pieces of legislation in the world," Justice Minister Jody Wilson-Raybould told reporters, adding that she is confident the new authority given to police to conduct roadside breath tests does not violate any rights in the Constitution. "Ensuring that we have safety on our roads and our highways is of paramount concern."

Western Ontario law professor Robert Solomon says the new era brings with it major dangers, adding that marijuana legalization will mean more use by young people, and inevitably more driving while high, and more fatal collisions. He says much remains to be done by the provinces, which would have the authority to raise the minimum age of use from 18. But he sees the government's overall approach to impaired driving as a strong message to drivers.

"Our courts have tended to interpret the impaired-driving law in a very pro-accused way. A lot of people never viewed it as a crime – 'boys will be boys,'" Prof. Solomon, the legal adviser to MADD Canada, an anti-impaired driving group, said in an interview. That interpretation, he said, manifested itself in the recognition of defences that strain credulity, such as contending that the last drink came just before or even during driving, which made it too soon to be absorbed into the bloodstream. (The government says the new law would remove this defence by making it a crime to be over the legal limit for alcohol in the blood within two hours of driving, rather than simply while driving.)

But "mandatory breath screening of drivers … is going to probably make the most significant difference in reducing deaths and injuries." He said 121 countries already have such mandatory screening, and every jurisdiction has achieved sharp and sustained reductions in crashes.

Criminal defence lawyer Joseph Neuberger of Toronto is among legal observers who say the dropping of the reasonable-suspicion standard for a roadside breath test may be unconstitutional because it requires the giving of a bodily sample, which he describes as invasive, despite no sign of danger. Reasonable suspicion generally has meant an odour of alcohol on the driver's breath, an admission of alcohol consumption or other indicators – a low threshold police have met without difficulty, he said.

"The premise of the legislation is that many drivers have escaped detection, who in fact were impaired, because of this requirement. Or it was difficult to prosecute cases because of this suspicion requirement. I don't think that's true. There is generally a high rate of conviction."

He said he does not understand why police would need a reasonable suspicion to test for marijuana, but not need one to test for alcohol. "One is not worse than the other. Significant impairment by a drug can be as dangerous as alcohol impairment. So I don't see why we should have two different standards."

A spokesman for Ms. Wilson-Raybould said the different standards for policing in detecting marijuana and alcohol consumption reflect differences in the technology of screening devices.

"Unlike the approved screening device for alcohol, which gives results in seconds that reliably indicate blood-alcohol concentration, drug screeners take eight to 10 minutes to indicate presence of a drug in oral fluid. In addition, unlike with alcohol, the concentration in oral fluid cannot be converted to concentration of the drug in the blood."

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