Some people think they drive better after a joint. The science says otherwise. Here are the facts on driving high

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From the second-story window of the High Life Social Club in Halifax, which is planted discreetly above a café serving all-day breakfast, the owner Chris Henderson points to the bus stop across the street. Until a few weeks ago, it displayed a poster for every departing patron of his cannabis-friendly lounge to see, part of a citywide public-education campaign to prepare for the legalization of marijuana on Oct. 17.

“Drive better on weed?” the poster asked, with a touch of snark, as if you’d have to be on drugs not to know the right answer. As it happens, that’s a question easily answered on a warm Tuesday evening in July, doused in the sweet, skunky perfume of pot, with club members drinking bottled water between hits on a water bong.

Turns out, they think they do. The science, however, begs to differ.

For instance, a 21-year-old High Life regular, and early childhood educator, lounging on a black couch beside a large mural of the caterpillar from Alice in Wonderland, offered an anecdote about a friend who passed her driving test while stoned. (The woman asked not to be identified; her employers, she noted, wouldn’t be so cannabis friendly.) After a day chasing toddlers, she says, smoking a joint helps her focus on the road. Most nights at the lounge, she waits an hour or so, and drives home. Anyway, she insists, if you find yourself too stoned to drive, just pull over and wait 15 minutes to sober up.

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A retired telecom technician in his 60s wanders over and joins the conversation. While he now bikes everywhere, he describes driving stoned regularly, with no problems. (Not so with alcohol, he notes, which resulted in an impaired-driving conviction three decades ago, and one accident.) Compared to drinking, he says to general agreement, pot keeps you more chilled out on the road – and less likely to rage at other drivers.

Later in the evening, a 19-year-old engineering students fields questions about his own habits at the counter where chocolate is for sale. He would never drink and drive, he explains, and wouldn’t have driven when he first started using cannabis. But six months in, he figures he knows his limit – today, he says, he wouldn’t take a road trip without a joint. It’s not dangerous, he claims. Among his evidence: He always stop at stop signs.

Collectively, they are candid and amiable – and working hard to convince a stranger that they’re upstanding, clearly aware that their drug of choice suffers from some bad PR.

But none of these assertions are supported in the growing research on cannabis. The sum of all the science – the driving simulations and on-road experiments, the cognitive testing, even the albeit mixed crash-collision statistics – shows clearly that there are far from “zero problems” with driving high.

Marilyn Huestis, a leading American expert who has been studying cannabis for 30 years, including offering advice on driving issues to Canadian policy makers, says that for non-daily users, it takes six hours for the effects of one joint to completely disappear. That’s even if your blood level of THC, the drug’s psychoactive ingredient, is low – an issue that creates additional complications for police testing suspect drivers for legal limits, and a public trying to abide by them. Needless to say, if you’re “too stoned” behind the wheel, don’t expect a 15-minute pause to make you safe. Occasional users perform more poorly than regular users in studies. But even for the pros, the danger exists – though, as it happens, familiar stop signs may be the least of it.

At the same time, research does suggest that people who are stoned are more aware of their impairment than those who’ve been drinking – a self-safety check, undeniably, in their favour. Traffic collision statistics show that driving high increases the risk of a crash or collision, although by how much is still the subject of debate, and the overall risk is nowhere near as high as alcohol. For instance, a French study of drivers in fatal collisions, found those with cannabis in their blood were twice as likely to be found culpable for the crash than sober driver, compared to nine times for those with alcohol in their system.

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According to a 2016 study by Norwegian researchers, which updated earlier findings and analyzed 13 international crash-collision studies between 1982 and 2015, cannabis use increases the risk of a crash by about 30 per cent, compared with a sober, attentive driver. Drunk drivers over the legal limit, on the other hand, are more than 600 times more likely to crash, according to report by the National Highway Traffic Safety Administration (NHTSA) in the United States.

In 2016, the NHTSA was also behind what is considered one of the most comprehensive – and controversial – experiments to assess the risk of marijuana and driving. Researchers investigated crashes in real time in Virginia, 24 hours a day for 20 months. They compared the crashed drivers with THC to a control group of drivers stopped in the same location, at the same time as the crash, who agreed to be screened for cannabis. Once researchers adjusted for age and gender – particularly, young and male – and the presence of alcohol, they found no increased risk of collisions compared with drug-free drivers. But despite its great design, there were some flaws in the study’s execution, says Mark Asbridge, the MADD Canada professor in impaired-driving research and prevention at Dalhousie University. The location was near a military base, which regularly tested soldiers for the presence of drugs, an external influence on driving behaviour, and the control-study drivers could refuse to participate, reducing the sample of those with drugs in their system. As with most cannabis studies, there was a delay or problem in getting blood samples, especially for more serious crashes. Even the authors of the U.S. study warned not to interpret their findings – one sample of drivers in one location – to be saying it was safe to drive high.

One of the complications of studying cannabis is how often it is combined with alcohol, a particularly dangerous driving cocktail, the research suggests. In the French study on fatal crashes, for example, drivers who had consumed both alcohol and cannabis were 14 times more likely to be found responsible for the crash when compared with sober drivers – much higher than those who had consumed either drug on its own.

Still, these mixed results suggest why, according to a September 2017 survey commissioned by Public Safety Canada, a hearty portion of Canadians believe that driving high isn’t so bad. Twenty-eight per cent of the 2,132 respondents said they’d driven high, and of those, 17 per cent said the influence of cannabis “posed no real risks.” Nearly one in 10 of all respondents believe cannabis makes a person “a better driver.” For Canadians under the age of 24, risk was perceived to be even lower. In a 2017 large-scale Ontario survey, 8 per cent of teenagers admitted to driving an hour after using cannabis – compared with 4 per cent who said they had driven drunk. The good news is that the driving-high figure has fallen slowly since 2011, when the question was first asked. The bad news is, in the Public Safety survey, 43 per cent of Canadian said they didn’t know how long to safely wait after a joint before driving, with young people more likely to predict shorter wait times.

“We used to say the same thing about drinking and driving. 'I can have a few drinks and it relaxes me.’ ‘I can have one for the road,’” says Robert Mann, a researcher at the Centre for Addiction and Mental Health. Hopefully, he says, changing attitudes about marijuana won’t take as long. “The clear message is that using cannabis increases your risk of an accident.”

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Collision stats clump drivers together to see a highway of cars. They aren’t measuring the safety of individuals, on their own route home. Occasional users appear to score worse when acutely stoned than more frequent users who may have developed a tolerance. (Although, Dr. Heustis notes that the cognitive deficits that result from chronic use may also lower driving skills.) New drivers are likely to be more challenged than veterans. An issue with all marijuana research to date, is that it’s usually focused on smoking the drug, typically at lower THC levels than today’s versions. It’s not clear the difference that newer forms of consumption will have on driving, including edibles, which make a user high more slowly, and can lead to people ingesting too much.

Even with those limitations, however, driving simulations in the lab, show that cannabis clearly impairs drivers, if somewhat differently than alcohol. Drunks put their foot on the gas; stoners dance their toes on the brake. They both weave on the road, but the latter does it more often at a safer speed. The notion that being high “makes you a better driver” likely originates here – cannabis users appear to compensate for their impairment with some caution, such as putting more space between the car in front of them.

So on a quiet ride on a familiar route, a driver under the influence of cannabis may be able to get home without incident. But driving, as Dr. Heustis notes, is marked by unexpected events – a kid riding their bike into the street, an elderly man dropping his key on the crosswalk, another swerving car. Being high, she says, makes it harder for drivers to compensate for surprises – in simulations, marijuana slows reaction times, divides a driver’s attention and makes decision-making foggy. For example, she says, in lab tests, cannabis users can add a row of numbers as well as a sober person – but throw in a complicated problem and they falter. With driving, Dr. Heustis says, they may be concentrating so hard at the just-getting-it-done task, that a sudden rush of complex information is overwhelming.

Back at the High Life Social Club, Chris Henderson, the businessman with a smoking angel tattoo dancing up his neck and a devil staring from his back, tries to navigate the views of his patrons, and the fears of a worried public. He has a marijuana prescription to deal with chronic pain – he could consume up to six joints a day, although he usually takes much less. Even a few hours after using, he is “100-per-cent confident” he is safe on the road. He’d have no problem taking a roadside test, he says, but he worries about being charged for having THC in his blood, even when he is not impaired.

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The new law is “better-safe-than-sorry legislation,” by his way of thinking. “But the most important thing is that the roads remains safe,” he says. “If that means people taking a cab because they smoke a joint, that’s probably not a bad thing.”

Cannabis and driving: Breaking down Canada’s new limits

What’s the new legal limit for cannabis while driving? The government has set 2 nanograms of THC per millilitre of blood as a summary conviction, with up to a $1,000 fine. At 5 ng, a driver would be charged with a criminal offence – similar to the blood alcohol limit of .08 – with a maximum penalty of 10 years in prison. And there’s also a more serious penalty if you are caught with a combined blood limit of .05 and 2.5 ng of cannabis. There is no exception for medicinal users.

How many puffs is that? Unlike alcohol, there’s no clear guideline, and the government hasn’t provided any. It depends on how much and how deeply you inhale, the strength of the cannabis you use and how you consume it, your experience using and your individual characteristics. For edibles, which take longer to take effect, it is even trickier.

What the science says: Research shows that a driver’s THC (the psychoactive ingredient in the drug) blood levels is an unreliable indicator of impairment. While alcohol is absorbed and eliminated in the blood at a linear rate, blood levels of THC fall quickly after smoking and then flatten out. This means people may feel the most high even when their THC blood level is at a very low rate. Since THC leaves the blood so quickly – but remains in the brain for a longer period – a higher legal limit may not capture drivers who are still seriously impaired, especially given the added delay in collecting blood samples. In any event, says Marilyn Huestis, a leading American cannabis researcher, there is no one simple measurement of THC at which everyone using cannabis will be impaired.

How Canada’s law compares: While some U.S. states and countries have zero-tolerance limits for cannabis, Canada’s new legal limits are roughly in line with other jurisdictions, such as Colorado and Washington, where cannabis is legal. Britain, however, allows for a medical defence if the driver can demonstrate they were not impaired.

So what if I use marijuana medicinally? This is a real issue, one that experts have said will probably need to be resolved with court cases. People taking cannabis regularly may have a residual dose of THC in their blood even though they haven’t used recently and are not impaired. Since measuring drugs amounts is complicated, New Zealand’s policing approach, for instance, has emphasized evidence of impairment, and not relied so heavily on blood tests.

What if I get stopped? As before legalization, a police officer may give you a three-part sobriety test – walk the line, balance on one foot, track the finger – if they have reason to believe you have been using cannabis, or any drug that is making you a danger on the road. Police will also be able to administer a saliva test once they are approved and distributed. If you fail those tests, a more detailed interview happens back at the station, but, because THC levels fall so quickly, the new law also allows police to skip right to a blood test.

What’s my best strategy? The guideline recommended by the Canadian Centre for Addiction and Mental Heath – and in research by the National Institute for Drug Abuse in the United States – is to wait at least six hours before driving after a smoking a joint. If you use only occasionally, studies suggest, the effect on your driving will be worse than more frequent users. Be extra careful mixing marijuana with alcohol – when combined, research shows, they may impair drivers much more than either drug alone.

From what you know, in what way(s)

does cannabis affect driving?

Reduces reaction time

and ability to concentrate

74%

Makes you a worse driver

57%

Makes you a more careful driver

9%

Does not change ability at all

3%

Don't know

8%

Nearly one in 10 Canadians think cannabis

makes a person drive more carefully. Seniors

were less likely to agree with this statement,

while young people were more likely to see

the risk as low – with 23 per cent saying

that it made a driver more cautious.

Driving under the influence of cannabis is less

dangerous than driving under the influence

of alcohol...

True

25%

False

62%

Don’t know/no response

13%

One quarter of all respondents – and half

of those between the ages of between the ages

of 25 and 34 – believe that driving high is less

risky than driving drunk. This is generally

supported by research, although experts warn

that “safer” does not mean “safe” since the

evidence still points to an increased risk.

People often don't realize that they are impaired

from using cannabis...

Disagree (1-2)

20%

Neither (3)

8%

Agree (4-5)

65%

Don’t know/no response

7%

Nearly two-thirds of the Canadian surveyed

agreed that people who are high often don’t

realize they are impaired – 42 per cent of those

between the ages of 25 and 34 said they

disagreed. The research suggests that people

using cannabis may be aware of being impaired,

but still overestimate how well they can drive.

Note: Only items with 3% or more shown in chart

TRISH McALASTER / THE GLOBE AND MAIL

SOURCE: EKOS RESEARCH ASSOCIATES INC.

DRUG IMPAIRED DRIVING AD CAMPAIGN, 2017

From what you know, in what way(s)

does cannabis affect driving?

Reduces reaction time

and ability to concentrate

74%

Makes you a worse driver

57%

Makes you a more careful driver

9%

Does not change ability at all

3%

Don't know

8%

Nearly one in 10 Canadians think cannabis

makes a person drive more carefully. Seniors

were less likely to agree with this statement,

while young people were more likely to see

the risk as low – with 23 per cent saying

that it made a driver more cautious.

Driving under the influence of cannabis is less

dangerous than driving under the influence

of alcohol...

True

25%

False

62%

Don’t know/no response

13%

One quarter of all respondents – and half

of those between the ages of between the ages

of 25 and 34 – believe that driving high is less

risky than driving drunk. This is generally

supported by research, although experts warn

that “safer” does not mean “safe” since the

evidence still points to an increased risk.

People often don't realize that they are impaired

from using cannabis...

Disagree (1-2)

20%

Neither (3)

8%

Agree (4-5)

65%

Don’t know/no response

7%

Nearly two-thirds of the Canadian surveyed

agreed that people who are high often don’t

realize they are impaired – 42 per cent of those

between the ages of 25 and 34 said they

disagreed. The research suggests that people

using cannabis may be aware of being impaired,

but still overestimate how well they can drive.

Note: Only items with 3% or more shown in chart

TRISH McALASTER / THE GLOBE AND MAIL

SOURCE: EKOS RESEARCH ASSOCIATES INC.

DRUG IMPAIRED DRIVING AD CAMPAIGN, 2017

Driving under the influence

of cannabis is less dangerous

than driving under the

influence of alcohol...

From what you know,

in what way(s) does

cannabis affect driving?

People often don't realize

that they are impaired from

using cannabis...

Reduces reaction time

and ability to concentrate

Disagree (1-2)

True

74%

20%

Makes you a worse driver

25%

Neither (3)

57%

8%

Makes you a more

careful driver

False

Agree (4-5)

62%

9%

65%

Does not change ability at all

Don’t know/no response

Don’t know/no response

3%

Don't know

7%

13%

8%

Nearly one in 10 Canadians

think cannabis makes a

person drive more carefully.

Seniors were less likely

to agree with this

statement, while young

people were more likely

to see the risk as low –

with 23 per cent saying

that it made a driver more

cautious.

Nearly two-thirds of the

Canadian surveyed agreed

that people who are high

often don’t realize they are

impaired – 42 per cent of

those between the ages

of 25 and 34 said they

disagreed. The research

suggests that people using

cannabis may be aware

of being impaired, but still

overestimate how well they

can drive.

One quarter of all

respondents – and half of

those between the ages of

between the ages of 25 and

34 – believe that driving

high is less risky than driv

ing drunk. This is generally

supported by research,

although experts warn that

“safer” does not mean

“safe” since the evidence

still points to an increased

risk.

Note: Only items with 3%

or more shown in chart

TRISH McALASTER / THE GLOBE AND MAIL

SOURCE: EKOS RESEARCH ASSOCIATES INC. DRUG IMPAIRED DRIVING AD CAMPAIGN, 2017