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How high is too high? Researchers look for ways to measure stoned driving.

marijuana bud
Dr. Andrea Furlan and her team want to figure out a way to test if drivers are too stoned to drive.


Michigan residents will vote on the legalization of recreational marijuana November 9. 

The state has strict laws against driving while drunk, and cops can test how intoxicated someone is with a quick breathalyzer test. 

But if weed is legalized, how will law enforcement identify someone who is driving while high? 

Dr. Andrea Furlan has been searching for this answer. 

She is a staff physician at the Toronto Rehabilitation Institute and an associate professor in the Department of Medicine at the University of Toronto. She spoke with Stateside’s Cynthia Canty about her research in impaired driving. 

While individual states in the U.S. push towards legalization, our Canadian neighbors voted to legalize recreational marijuana nationwide beginning on October 17. Therefore, Furlan’s search for these answers is urgent. 

Luckily for her, Furlan and her team have access to Canada’s most advanced driving simulator. Using virtual reality and a 360-degree projection system, participants feel almost like they are on a real road. 

The simulator is located within the hospital building, and therefore is able to recruit hospital patients with cognitive impairments as well as those under the influence of opioids, anti-depressants, sleeping pills, and cannabis. 

Driving drunk vs driving high 

Alcohol and cannabis have very different effects on the human brain. Furlan said alcohol first impacts the judgement part of your brain located in your frontal lobe. Cannabis, on the other hand, does not. 

“When people are under the influence of cannabis, they still know, they are conscious about the impairment. So there is an opportunity for the person using cannabis to self-regulate to determine if this is safe or not to do,” Furlan explained. 

Unfortunately, Furlan said many recreational marijuana users mix cannabis with alcohol. Those who use marijuana for medicinal purposes are instructed to avoid alcohol.

“If they use alcohol and cannabis together, they will be impaired by the alcohol, but also different areas of the brain will be impaired by cannabis, and they will be much more impaired than only one substance.”

When a participant enters the driving stimulator under the influence of alcohol, Furlan said they drive faster, get much closer to the car in front of them, and do more unsafe maneuvers with their car. 

Those under the influence of cannabis typically drove slower. Furlan said this is not because they are driving safer, but because their reaction time is impaired. 

“People under the influence of cannabis, they aren’t only driving slower. They also have a distorted perception of time and distorted perception of space," Furlan said. "The brain is giving them information that is not totally accurate.”

The degree of impairment is also dependent on the level of THC in the body. THC is the chemical compound which creates the feeling of euphoria. Furlan said more THC could result in drivers taking greater risks. 

Policing high driving 

When an individual’s blood alcohol content (BAC) reaches 0.08, they are legally considered to be impaired under U.S. law. 

While officers can pull over a car and test for BAC on site, Furlan said there is no equivalent for cannabis. 

“People develop tolerance to the effects of cannabis — the THC. Like all patients who use THC regularly, daily, for many years, the body develops this tolerance, and the psychotropic effects are not observed any more, but they will still be tested positive in the saliva, blood, or urine.”

Furlan and her team are working to develop behavioral tests which could be used on the roadside with a computer or tablet. She believes in the future all cars will include screens where a driver will complete a set of tests in order to start the car. 

Until then, Furlan suggests those who smoke or use vaporized forms of cannabis wait six hours before driving a vehicle. If the cannabis is consumed through an edible, individuals should wait 10 to 12 hours. 

This post was written by Stateside production assistant Sophie Sherry. 

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