The Liberal government has mounted a pre-emptive defence of its contentious new impaired driving bill, arguing that mandatory breath samples will serve as a strong deterrent and will not violate constitutional rights.

A charter statement from Justice Minister Jody Wilson-Raybould, tabled in the House of Commons today, outlines why the Liberals believe Bill C-46 upholds the Charter of Rights and Freedoms.

Defence lawyers have said the proposed reforms expose the government to a number of constitutional challenges, including a new section that gives police the power to demand a breath sample even if there's no sign the driver has been drinking. Currently, an officer must have reasonable suspicion the person behind the wheel is impaired before administering the breathalyzer.

Today's statement insists any interference in privacy is justified to meet the government's "compelling objective" of enhanced road safety.

"The Supreme Court of Canada has recognized as reasonable the authority, under provincial law and common law, of police officers to stop vehicles at random to ensure that drivers are licensed and insured, that the vehicle is mechanically fit, and to check for sobriety," Wilson-Raybould's statement reads.

'Highly regulated context'

"The information revealed from a breath sample is, like the production of a driver's licence, simply information about whether a driver is complying with one of the conditions imposed in the highly regulated context of driving. It does not reveal any personal or sensitive information and taking the sample is quick, and not physically invasive."

A "fail" in the test does not constitute an offence, but could lead to further testing at a police station.

Mandatory breath tests are already the practice in a number of countries, including Australia, New Zealand, Ireland, France, Belgium and the Netherlands. The statement says they have led to a measurable reduction in accidents and deaths on roads and highways.

Jody Wilson-Raybould on the government's proposed reforms to impaired driving testing. 6:49

In Ireland, the tests have been credited with a 23 per cent reduction in road deaths in the 11 months after introduction. In New Zealand, visible mandatory-screening checkpoints were credited with a 32 per cent reduction in crashes, according to the statement.

Toronto defence lawyer Norm Stanford questioned that data, and called giving police unfettered power to demand a breath test "dangerous."

"It allows for police abuse. Now, police for whatever reason they want, can make you do a breathalyzer. If you talk back to them or they feel you're disrespecting them, they have the power to do that," he said.

Accompanies pot legalization bill

Bill C-46 was tabled last month at the same time as legislation to legalize marijuana, and includes tougher penalties and new powers for police. To deal with drivers impaired by pot or other drugs, the legislation allows police to demand a driver provide a saliva sample if they suspect he or she is impaired by drugs.

A positive reading could lead to further testing, including a blood test.

Some lawyers expect that provision will also face court challenges based on privacy and because there is no universally accepted levels for pot impairment with available tools.

Justice Minister Jody Wilson-Raybould says Canadians have limited privacy rights in the highly regulated context of driving. (Adrian Wyld/Canadian Press)

"I don't know there's a lot of solid research linking impairment to the level of drugs in a person's system," Stanford said.

Unlike the breathalyzer, an officer must have a reasonable suspicion the driver has consumed drugs before asking for a saliva sample.

But the charter statement said the drug screener is "a quick, non-intrusive search method" to provide information, and that individuals have "a limited expectation of privacy" in the highly regulated roadway context.

'Endless' legal challenges expected

Despite the minister's assurances the legislation is charter-proof, Ottawa defence lawyer Brett McGarry predicts the bill will produce "endless" constitutional challenges. Saliva tests are unreliable because science doesn't yet accurately measure impairment, and they give a high rate of false positives.

McGarry also expects to see challenges new provisions intended to close defence loopholes, including one he says would allow someone to be convicted of impaired driving even if they consumed alcohol after driving.

"That's a total reversal of the presumption of innocence," he said. "You're going to have to prove that you're innocent even though you were out of the car sitting in your house or a restaurant."

Stanford said he had a client who was sober until he got in an accident, at which time he consumed alcohol that was stashed in his trunk.

"It was a very unusual situation, but unusual situations happen," he said. "And I think at the end of the day, the problem with this legislation is you're taking power and discretion away from judges to judge cases on the facts of the case."