"The biological tests are not useful for identifying people that represent a safety risk," Macdonald says. "What we’re left with is behavioural symptoms. We’re still working on developing tests to assess whether an individual who consumes cannabis is a safety risk. It’s hard to do.

However, THC can keep showing up on tests long after any impairment has ended — unlike alcohol, which would stop being detectable as impairment went away.

The study shows that similarly to alcohol, cannabis impairment disappears within a day of consumption at most, if not much earlier.

"We found significant evidence of difference in driver behaviour, heart rate and self-reported drug effects 30 minutes after smoking cannabis, but ... we found little evidence to support residual effects," the authors wrote.

But in further tests 24 and 48 hours later, they still had detectable levels of THC, but performed normally in the driving simulator.

Immediately after smoking, the THC group showed signs of impairment, centring the imaginary car poorly in its lane and driving inappropriately slowly.

The driving simulator's scenario involved a nine-kilometre drive on a stretch of rural highway signed at 80 km/h, with a few simple problems to solve, such as a slow-moving vehicle.

Because they were allowed to control their own consumption, THC blood levels in the group varied. They ranged from zero to 42 nanograms per millilitre, which would be nearly 10 times the legal limit to drive.

Researchers at the Toronto-based Centre for Addiction and Mental Health gave participants 10 minutes to smoke cannabis to a level they chose, then tested them in a driving simulator.

"I consider it one of the biggest myths about cannabis, that there are 24-hour hangover effects that are measurable," he said. "When people smoke cannabis, they’re only impaired for a short, short period of time. You could have THC in your bloodstream, but you’re not a danger."

The research has implications for laws and workplace rules that require no trace of THC, which Scott Macdonald, a retired professor at the University of Victoria, calls "not scientific."

People using a driving simulator showed no signs of impairment a day after they smoked cannabis, though they still tested positive for THC, its main psychoactive component, a recently-published paper says.

"Cannabis is not in the same class as alcohol, in terms of safety risk. Alcohol is much, much worse."

Several provinces forbid young or new drivers from having any detectable levels of THC.

In Ontario, for example, drivers violating a zero-tolerance rule for THC face a three-day licence suspension, $250 fine and $281 licence reinstatement fee for the first offence. Penalties increase for repeat offenders.

Saskatchewan took what may be Canada's strictest approach: new drivers in the province with any level at all of THC face a 60-day licence suspension, a three-day vehicle seizure and four demerit points. A young driver can face serious consequences for driving on THC.

Ontario's transport ministry defends the policy.

"In October 2018, the Canadian Society of Forensic Science released a report stating that impairment from cannabis begins almost immediately and can last up to six hours or more, depending on factors such as THC levels and how it is consumed," spokesperson Kristine Bunker wrote in an e-mailed response.

"Since the effects of cannabis vary, there is no way to know exactly how long to wait before it is safe for you to drive."

Zero- tolerance rules work as a "back-handed way of prohibiting cannabis use for young people," argues Jenna Valleriani, executive director of Hope for Health Canada.

"Most young people drive, and have day to day responsibilities like school and work, so when we have data demonstrating that individuals would test over the limits 24 and 48 hours after use, this does little to make our roads safer from impaired drivers and deserves a reassessment," she wrote in an exchange of e-mails.

"Driving is a privilege, but at the same time, cannabis is legal now, and young people over the age of access should be able to consume on the weekend and drive one full day later to work or school without risking a DUI charge."

READ MORE: Zero-tolerance THC restrictions for new drivers open to Charter challenge: lawyer

Across Canada, some police forces have effectively banned cannabis for their own members.

Police in Montreal are allowed to consume on their own time, so long as they show up for work "fit for duty."

But Toronto police and the RCMP ban officers from consuming less than 28 days before reporting for duty, while Calgary police officers are forbidden to consume at all. (Since THC can be detected up to 28 days after consumption, these policies are more or less the same.)

Police unions in Toronto and Calgary objected to the policy when it was announced, with Toronto's calling it “ill-contrived" and “arbitrary,” and Calgary's saying that members had said they were "certainly not really happy with the idea of my employer telling me what I can and cannot do with a legal substance on my own time.’”

"I think [28-day bans] are excessive and not scientifically based, especially now that cannabis is legal," Macdonald says. "If cannabis was illegal, I could see it from that perspective. It’s not scientifically based in terms of them being a risk for the public if they’ve used cannabis in the last 28 days."

Toronto police are open to revisiting the 28-day ban, spokesperson Const. Victor Kwong wrote in an e-mail.

"(The rule) was made thoughtfully and based on sound advice and evidence, considering the critical role members play in ensuring a safe workplace and a safe community. We will continue to research and explore this procedure. If at any time there is new science or research to cause us to re-evaluate our processes, we will do so."

A B.C. labour arbitrator recently ruled against TransLink, the Vancouver-area transit authority, over its handling of an employee's positive test for THC.

David Solomon, a train attendant, was forced to undergo drug tests twice a month for a year, despite a doctor's opinion that his cannabis use wasn't a problem. Solomon had to call the testing service daily to find out whether he was going to have a test that day or not, and go to a lab to be tested if the answer was yes.

READ MORE: TransLink ordered to stop drug testing pot-smoking SkyTrain attendant

The military allows cannabis use at some times for some members, depending on their jobs. Some, like divers, aircrew, members of submarine crews and drone operators, have a 28-day prohibition similar to the more restrictive police forces.

"Many jobs — typically those which are safety sensitive — have adopted a zero tolerance policy, but as cannabis becomes more integrated into society, we're going to need to consider policies that are reflective of this change and embrace a better understanding of cannabis and its effects," Valleriani wrote.

"Some jobs require drug and alcohol tests as a condition of employment, but it begs the question of how workplaces can manage the risk of impairment without an accurate measure of it."

The study was published this week in the Journal of Drug and Alcohol Dependence. Macdonald was not involved with it.

During the study, a placebo group was given joints with no THC to smoke, but it turned out that participants were well aware of whether they had consumed THC or not, with over 90 per cent in both groups accurately saying what group they were in.

When they were asked whether they would be comfortable driving a real car, 80 per cent of the placebo group said they would, as opposed to under 30 per cent in the THC group.

"It has the biggest sample size of any sample that I know of," Macdonald said of the study's 91 participants.

CAMH and Health Canada, where the study's lead author works, did not make anybody available for an interview.