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Nelson said other research has shown that not only do many motorists believe marijuana “isn’t that unsafe,” some believe it makes people better drivers.

“It’s the notion that, ‘Oh, people who are high drive slower, and they’re more cautious because they don’t want to get caught,’ which may or may not be true. But it certainly is also true that when you’re impaired by marijuana, your reaction time isn’t what it needs to be and you’re less able to manage complex, or divided attention tasks,” he said. “The safety concern is lost on a majority of people.”

Since 2008, police in Canada have been able to demand roadside sobriety tests on suspected drugged-drivers. If the driver fails, police can demand he or she submit to an exam by a DRE, who can also demand a sample of bodily fluid, typically urine, for testing. But traces of cannabis can remain in urine for weeks.

Beirness worries that setting a legal THC limit could send a message to users that it’s safe, within limits, to drive high. While the effects of booze are “profoundly physical” — slurred speech, lost balance, warped motor co-ordination — the effects of cannabis are cognitively based. “They’re in your head,” he said.

When you’re driving, you have to concentrate on a million different things at once

“And you can see effects on things like concentration. When you’re driving, you have to concentrate on a million different things at once, but you can’t switch your attention as easily” when high on pot.

When governments introduced the 0.08 blood limit for alcohol, “we probably made a mistake,” added Beirness, a senior research associate for the Canadian Centre on Substance Abuse.

“What we did essentially was tell people it was OK to drive after drinking” as long as they didn’t exceed the limit.

Today, decades later, “we’re pondering the same question, and whether (if) we set a limit, any limit, whatever it might be, for cannabis, we’re still saying to people, ‘it’s still OK to go out and smoke cannabis and drive. Just don’t get over that limit.’

“What we need to do is create a culture where driving after smoking cannabis is just not OK.”

• Email: skirkey@postmedia.com | Twitter: sharon_kirkey