There are two kinds of buzzed people who take the wheel of his simulated Chevy in the basement of a CAMH building, Dr. Robert Mann observes: those who drive cautiously and those who let ’er rip.

The scientist with the Centre for Addiction and Mental Health points to the flat-screen TVs that serve as the “windshield” for the extracted cab of a Chevrolet compact — complete with steering wheel, dashboard, gear shift and pedals — and explains the clinical study he’s been working on since 2013.

Mann wants to know how well people can drive when they’re high. He’s inviting volunteers to smoke up and get behind the wheel of the “state of the art” driving simulator housed in the building off College St.

“There’s a fair amount of uncertainty on the impact of cannabis on driving skills,” he said in an interview this week. “There’s still a discussion, still an argument, about what the effects are.”

That’s why Mann and his team at CAMH are trying to determine the relationship between marijuana consumption and driving ability. How high can you be before you’re a bad driver?

It isn’t an easy question to answer. A review published in 1999 of marijuana-driving research concluded that, while cannabis is the second-most frequent substance found in the blood of people involved in car crashes (after alcohol), there was little evidence that it increased the risk of collisions. But similar reviews from 2011 and 2012 found that driving on cannabis “significantly increased” the risk of a collision.

Mann hopes his study can dig deeper, to help determine how high is too high, especially as the federal government has promised to legalize marijuana.

“That’s an issue that jurisdictions are looking at,” he said, pointing to U.S. states where recreational weed is now legal. “The (existing) evidence has pretty much convinced me that if you are driving under the influence of cannabis, you are at an increased risk of getting in a collision… We don’t know what dose relates to that yet.”

The CAMH study focuses on 19- to 25-year-olds who are regular cannabis users, because research suggests members of that age group are more likely to drive high than drunk, Mann said.

Volunteers are carefully screened with psychological questionnaires and blood and urine tests to make sure they aren’t uninitiated to marijuana or dependent on the drug, Mann explained. Once approved, volunteers enter a smoking room at CAMH and light up a joint made from either normal marijuana or pot that’s had the active ingredient — THC — extracted entirely. They have their blood sampled so that researchers can later determine how high they were, and then it’s on to the driving simulator.

The machine shifts and vibrates while computerized highway landscapes roll by on three large screens. The experience is similar to real driving, except that the effect of images swirling on the screens can provoke mild vertigo, Mann said.

After getting the hang of the simulator for a few minutes — and any desire to gun the gas and go off-road is sufficiently indulged — participants drive for 10 minutes through this video-game-like universe, rounding bends and avoiding traffic in a replication of the usual monotony of driving in real life.

Then Mann asks them to drive for another 10 minutes while counting backwards by threes from a random number, such as 797, to see how they handle distractions. The simulator carefully tracks the driver’s movements and speed. And yes, there are sometimes crashes.

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The subjects return 24 hours later to repeat the test, and then come back again a final time 24 hours after that. Mann said that allows the researchers to measure any lingering effects.

So far, they’ve tested 75 people. The goal is to have 114 subjects participate in the study by the time it finishes in the spring. Mann said the study was approved by the Canadian Institutes of Health Research. He plans to publish his results in the coming months.