Criminal defence lawyers in Manitoba are calling for changes to how roadside tests for cannabis are administered before new bills are passed that will both legalize the drug and strengthen impaired driving laws.

"There's no true, reliable way to test for the presence or absence of recent drug consumption," said Scott Newman, a spokesperson for the Criminal Defense Lawyers Association of Manitoba.

"When they say they're going to be testing for the levels of marijuana in your system or the active ingredients, how are they going to do it? And how reliable is it? Because that's not something they've been doing to date."

Federal Liberals tabled long-awaited changes to Canada's marijuana laws Thursday.

The sweeping legislation includes two bills — one that amends the Controlled Drugs and Substances Act, making recreational pot legal, and another that cracks down on driving while intoxicated.

The latter will give police the power to demand a saliva sample which could lead to further testing, such as a blood test, in an effort to assess whether a person is high.

Under the new law, penalties for driving while impaired by drugs would range from a $1,000 fine to up to 10 years in jail, where a large amount of THC is found in the driver's blood. In cases where a person has been injured or killed, drivers could face life imprisonment.

Ottawa has yet to recommend a method of testing for drugs but other jurisdictions use the DrugWipe system to test for signs of drug use in spit and sweat.

No reliable way to test for drugs

Newman said testing for marijuana in salvia is notoriously unreliable.

"The saliva test doesn't really tell you a lot because the effects of marijuana can stay in the system of anyone up to 30 days," he said.

What's more, Ottawa's legislation measures marijuana using nanograms in the blood, an imperfect measure because users metabolize the drug differently.

Liberals Pot 20170327

Federal legislation tabled on Thursday allows people to possess up to 30 grams of dried or fresh cannabis and sets the minimum at 18 years of age. Amendments to the Criminal Code, also introduced Thursday, deliver harsher penalties for impaired driving. (Sean Kilpatrick/Canadian Press)

Police already have the power to collect saliva samples and rarely do charges result in impaired driving convictions, unless a driver admits to using pot or is evidently intoxicated and incapable of driving, said Newman.

In Manitoba, dozens of police officers are trained as drug recognition evaluators, which RCMP say enables them to spot telltale signs that a person is high.

Trained evaluators can check for the physiological signs people exhibit if they are intoxicated by drugs — by conducting divided attention tests and taking a driver's blood pressure, for example.

But defence lawyers have been able to poke holes in those types of roadside evaluations as well.

"It doesn't seem to be terribly effective. It's based on observations made by the officer," said Newman.

One way around that is to start videotaping evaluations made by trained officers, he said, so courts can see how a driver was acting after being pulled over by police.

"We should be able to go back and review that," said Newman.

While courts may see more drivers fighting new impaired driving charges, Newman thinks legalizing pot will be a net positive for the criminal justice system.

"At the end of the day, we're going to have fewer charges before the court. Judges will have more time to deal with serious issues."