If Canada's new impaired driving laws are passed police could show up on your doorstep — up to two hours after you arrive home — to demand a breath or saliva sample.

It's one of the things that most concerns Calgary defence lawyer Dale Fedorchuk. who says the proposed law "begs" for a constitutional challenge.

Fedorchuk says officers would have the power to charge someone with impaired driving, even after they park their vehicle.

Calgary defence lawyer Dale Fedorchuk is concerned police would be able to show up at your front door up to two hours after you arrive home to demand a breath or saliva sample. (Bryan Labby/CBC)

"I don't understand the reasoning," said Fedorchuk. "Why do we have to be concerned about people who are already home?"

The Trudeau government introduced sweeping changes last month that include, among other measures, mandatory roadside breath samples and much harsher penalties for some offences that could result in up to 10 years in prison.

Police can also demand a roadside saliva sample — but they must first have reasonable suspicion that a driver has drugs in their body.

The changes, which were announced the same day the federal government unveiled its bill to legalize marijuana, are stirring a lot of debate among local lawyers, legal experts and victims groups who are lining up to either praise or pan the legislation.

"I'm a little puzzled, actually," said Calgary law professor Lisa Silver.

Along with Fedorchuk, Silver also predicts the law will be challenged on constitutional grounds because police will no longer require "reasonable suspicion" before demanding a breath sample.

Officers can simply demand a test and drivers will have no choice, since refusing to provide a sample is a Criminal Code offence.

"There's going to be litigation around this for sure," says Silver.

University of Calgary law professor Lisa Silver says Canada's proposed impaired driving laws appear to contravene the Charter of Rights and Freedoms and will face a constitutional challenge. (Bryan Labby/CBC )

Section eight of the Canadian Charter of Rights and Freedoms states "everyone has the right to be secure against unreasonable search or seizure."

Silver says once a sample is unreasonably seized, a driver would essentially be "conscripted against themselves."

"What you've given those officers will be used against you to convict you, ultimately, in court," said Silver.

Shifting the burden of proof

Fedorchuk says the proposed law could shift the burden of proof onto an accused. As an example, he says someone who is approached by police at their residence would have to prove they consumed alcohol or drugs at home after they exited their vehicle and were not driving drunk.

Silver says the changes are likely meant to eliminate what's known as the bolus or intervening drinking defences.

Calgary Police Service Acting Sgt. Mudassir Rana pulls over a driver who failed a breath test and was subject to an immediate 72-hour licence suspension. The driver's vehicle was also impounded for three days. (Bryan Labby )

Currently, drivers can avoid fines or a criminal conviction by claiming they consumed alcohol just before or while driving, and thus were not over the legal limit at the time they were driving because the alcohol was not yet fully absorbed in their system.

They can claim it was only later, at the time of testing, that they reached an illegal blood-alcohol concentration.

Additionally, they can attempt to dash away from the scene of an accident and claim they consumed alcohol once home.

The government says it would close that loophole by changing the timeframe for blowing over the legal limit from "at the time of driving" to within two hours of driving.

It's a big problem

Silver says the issue may run contrary to the presumption of innocence.

Calgary police officers who conducted a recent checkstop on Mission Road in southwest Calgary appear supportive of the new changes — particularly the section that drops the requirement that officers must first have reasonable suspicion before demanding a breath test.

Sgt. Mudassir Rana supports the change to Canada's impaired driving laws, which include dropping the requirement officers must first have 'reasonable suspicion' before demanding a breath test. (Bryan Labby/CBC)

"To keep everybody safe it's a good idea they should also be checked randomly for any alcohol consumption or drugs," said Acting Sgt. Mudassir Rana. "So checking their sobriety should be a basic part, a condition of their license."

Calgary ranks 20th among Canadian metropolitan areas in the number of impaired driving charges laid according to a report from Statistics Canada.

Good first step

Mothers Against Drunk Driving says the proposed changes are a good first step. Karen Harrison says there's a link between tougher impaired driving laws and reduced fatalities and injuries in other jurisdictions.

Her brother, Tony Harrison, was killed by a repeat drunk driver in 2012 who was then sentenced to six years in prison.

Karen Harrison is the president of the Calgary chapter of Mothers Against Drunk Driving. She supports the overhaul of Canada's impaired driving laws and says it could deter some people from driving drunk. (Bryan Labby/CBC)

Harrison isn't sure the new tougher laws still stop chronic, repeat offenders.

"It will deter some people from drinking and driving," she said.