OTTAWA -- The federal government’s drug-impaired driving bill has taken a backseat so the Senate Legal and Constitutional Affairs Committee can focus on passing its sister bill on marijuana legalization, as uncertainty remains over how to legislate and enforce ways to measure marijuana impairment.

Determining marijuana impairment has been a major issue raised by a series of expert witnesses, including law enforcement and toxicologists who have testified at the Senate committee.

Senators on the committee heard there is no consensus on how to determine scientifically whether a person is high behind the wheel, and what level of THC -- the main compound that causes a high -- is required to constitute impairment.

“Drugs, and cannabis in particular, is very different from alcohol. Alcohol we know that 0.08 you’re impaired, and that’s been scientifically proven. Drugs, cannabis, it’s not clear,” Independent Sen. André Pratte, a committee member, said on CTV’s Power Play.

In April 2017 the federal government introduced its suite of marijuana legalization legislation. Two bills tackle the major policy overhaul:

Bill C-45, the main piece, sets out the parameters around the production, possession, safety standards, distribution, and sale of marijuana

Bill C-46 changes impaired driving laws to give police new powers to conduct roadside intoxication tests, including oral fluid drug tests, and would make it illegal to drive within two hours of being over the legal limit.

Both bills have passed the House of Commons and have been before the Senate for months. Senators have agreed to extend the study of Bill C-45, meaning a legalized marijuana regime won’t be up and running across Canada until after the government’s initial July 2018 target.

Drug-impaired driving bill takes backseat

The agreed-upon timeline for C-45 means that Bill C-46 has been delayed for at least a few weeks, because it’s the same Senate committee studying both bills.

Sen. Pratte said he anticipates the bills will pass around the same time in June, ahead of Parliament rising for the summer. He estimates this will mean the impaired driving clauses will be implemented in the early fall.

But first, he said the Senate has to make sure it’s confident in what they pass, calling the marijuana legalization policy “one of the most important changes in criminal law for decades.”

The measurement of impairment remains a big question.

“The government has decided to put in the bill what they call ‘per se limits,’ which is that even though you’re not proven to be impaired, if you’re over that threshold… that’s an offence, that’s a criminal offence,” said Pratte. “So, we have to take a bit of time.”

Experts cast doubt on THC tests, constitutionality

The proposed legislation states that having between two and five nanograms of THC per milliliter of blood would be a summary criminal conviction, punishable by a fine of up to $1,000.

Experts said that the level of concentration each person might have when tested largely depends on the individual and their tolerance.

“Unlike alcohol, however, the effects of THC do not correlate directly with THC blood concentrations. Instead, THC impairment demonstrates variability between individuals but is related to the amount, the route of administration and the time elapsed since use,” said Chair of the Canadian Society of Forensic Sciences’ Drugs and Driving Committee Amy Peaire at committee.

“It’s a very difficult exercise to try and determine a per se blood level for THC. Unlike alcohol, in which you can have blood concentrations which have links to impairment, that’s not the same for THC because there’s not a good correlation between impairment and blood concentrations,” she said.

Criminal Defence Lawyer Michael Spratt said this could result in people who have smoked a joint on a Monday facing criminal charges when pulled over a week later and a roadside test indicates they have above acceptable levels of THC still in their system.

“I think the probability is that a large number of people will be charged with being impaired while driving through the consumption of THC, who aren't impaired at all,” Spratt said.

He predicts constitutional challenges by people who are facing the consequences of having a criminal record for drug offenses without scientific certainty and said the bill could have the opposite impact of what the government intends.

“Someone's going to get charged. Someone's going to face those consequences and go through the expense of challenging the laws as being unconstitutional, it will make its way up to the Supreme Court and five years from now, seven years from now, we'll have our answer,” Spratt said.

During her testimony at the committee in January, Justice Minister Jody Wilson-Raybould said the government is able to make amendments if there’s an update in the science.

“We are closely following the science in terms of drug-impaired driving. We take, as a base position within this legislation, a very precautionary approach that any level of drugs in a body is impairing,” she said.

Timeline, training still big concerns for police

Throughout the parliamentary process, Canadian law enforcement agencies have been vocal in asking for more time to make sure police officers are properly trained. They argue that police will not be ready to properly apply legalized marijuana laws by the time the government wants.

A key aspect of this is enforcement of the new impairment policy through roadside testing devices.

There is currently no government-approved roadside testing technology for marijuana impairment. Currently, specially-trained police “Drug Recognition Experts” (DREs) use a 12-step system to detect drug impairment in drivers, including examining one’s eyes, attention, blood pressure and pulse.

The RCMP, in conjunction with other Canadian police departments and Public Safety Canada, has recently experimented with roadside saliva tests. While these have proven to successfully test for the presence of marijuana in a driver’s system within several hours of use, they cannot quantify a driver’s level of impairment the way roadside alcohol tests do.

Once cannabis becomes legal, police forces across the country will likely be utilizing roadside saliva tests before moving onto assessments by DREs to determine if a driver is impaired.

“We expect more situations, more people driving under the influence, so that’s why we have to train more officers to be able to deal the demand on the roads,” Canadian Association of Chiefs of Police President Mario Harel said.

“Our concern is the timeframe that we’ll have to train about 65,000 officers across country, so this will be a task we’ll have to prioritize … We’ll have to really, really step up with the training… and be as ready as possible,” Harel said.

Public Safety Minister Ralph Goodale said he is committed to making sure the municipalities and police forces have the funding they need and is confident that the training will be completed in time.

“There’s still a long way to go before the summer, so we’re working on all fronts to get this adopted in the right order. We are also working on the accreditation of testing machines so that the roadside testing devices will be available and properly accredited,” Goodale said on CTV’s Power Play.

“We’re still on track and we’re determined to work with all our partners to get this done on time and get it done right.”

With a report from CTV News' Ottawa Bureau Chief Joyce Napier and with files from Daniel Otis