With 200 officers trained to detect inebriated drivers already — and more on the way in time for marijuana's expected legalization next spring — Sgt. Brett Moore of Toronto police's Traffic Services is confident in law enforcement's ability to prevent stoners from getting away with driving high.

But Moore admits the drug presents its share of challenges.

"It is a shift from alcohol," he told CBC's Metro Morning earlier this week.

But testing for cannabis impairment, he says, follows the same principles as testing for alcohol, despite the lack of a handy breath test.

Moore says he's got officers preparing for pot's release on Canadian drivers the only way they can: they're training for "standard field sobriety testing," which checks for red or glassy eyes and the driver's ability to balance.

If suspected impaired drivers don't perform well on those initial exams, says Moore, they're taken in to see an evaluator, who performs a medical exam.

And if that goes poorly, a urine test is requested.

But unlike blood-alcohol levels, which are easily determined, "there is no number for drugs and that is the challenge," Moore said.

"That's what the courts and the legal folks are going to have to figure out," Moore continued. "There's going to have to be some level or degree of concentration in your blood," and, he said, "it's not set yet."

'Subjective opinion,' no test for pot

Paul Burstein, a lawyer with Burstein Bryant Barristers, pointed out the same problem, but says it represents a significant concern with impaired driving offences once pot becomes widely available.

With cannabis, he says, "it's not such a clear-cut standard. With alcohol there's a defined limit."

He points to the Breathalyzer, "a scientifically-reliable test" that provides a standard result. The number that appears on the test can be compared to the legal limit.

Toronto police say they're ready to snag the city's stoned drivers, but one legal expert points out the pitfalls of their evaluation plans. (Bryan Labby )

Burstein says that for cannabis, which can remain in a person's system long after its effects wear off, an officer may only rely on "a subjective opinion" to determine if someone's too high to drive. "It's really just the famed standard test: stand on one leg, walk the line."

Thus any test that does detect traces of the drug "doesn't prove impairment, it only proves you've consumed it at some point in the past," he said.

"The result is there are many other explanations for all of these apparent symptoms, so it doesn't establish anything."

Move first offenders out of criminal courts?

Burstein believes the legal system should focus on cases "close to the line."

He gives an example of a regular cannabis smoker tired after leaving a long shift at work. They might have red, glassy eyes, a reduced ability to balance, and fail their urine test — even though they aren't actually impaired. Burstein says, in this hypothetical, the driver risks falling victim to a false accusation.

Possible cases like that one should compel legislators to move first offences out of the criminal courts, Burstein said, pointing to British Columbia as an example.

Offenders wouldn't risk jail time or a criminal record, but the threat of licence suspension should be enough of a deterrent, he said.

Until someone develops a reliable test, Burstein adds, it's a stopgap measure to protect Ontario drivers.

Test 'not a magic solution'

Moore says urine, saliva or breath tests aren't a "magic solution" to determining who's impaired.

Officers ultimately rely on the entirety of their arsenal, including witness statements, field tests and diligent notes, to determine if somebody is fit to drive, Moore said.

And legal weed, he added, won't change that.

"The message for the public to know is...we have the skills and abilities to prosecute, to watch, to train, to arrest."