Quirks & Quarks

Roadside THC tests do not test for impairment. How can science help?

Unlike alcohol, THC levels can't accurately predict whether you're too high to drive

Unlike alcohol, THC levels can't accurately predict whether you're too high to drive

(Brian Higgins/CBC)
Listen8:21

When Canada legalized cannabis use last year, there would be serious concerns around regulating pot and driving because the test THC as a proxy for impairment is scientifically murky.

But scientists are working to create new kinds of roadside tests that will help police make accurate judgements about whether driver's who've been using cannabis are too impaired to operate vehicles safely.

The issue with current regimes for roadside testing were illustrated this week as a Nova Scotia woman announced she is launching a legal challenge to the procedures police used to test her for cannabis impairment.

"I have an amazing legal team," said Middle Sackville native Michelle Gray in conversation with As It Happens host Carol Off. "They feel that my case, my situation here, is a perfect test case to bring before the Supreme Court."

Back in January, Gray — a medical cannabis patient — was pulled over for a routine roadside checkpoint.

"I wasn't really worried," said Gray. "I only had one drink of alcohol for dessert with my son that evening. So I was quite confident when approaching that checkpoint."

But then the officer who pulled her over noticed a faint smell of cannabis in Gray's car.

"I said, 'I have a medical license for my multiple sclerosis.' He said, 'When's the last time you consumed it?'" Gray said the last time she consumed cannabis was about seven hours prior to getting pulled over.

Michelle Gray had her licence suspended for seven days after she tested positive for THC in a roadside test. She was later deemed sober by an RCMP drug recognition expert. (Submitted by Michelle Gray)

The officer administered the roadside saliva test and Gray tested positive for THC in her system. She was given the option to take a blood or a sobriety test back at the station. She opted for the sobriety test and passed.

But because she failed the saliva test, her car was impounded and she lost her license for seven days — despite the fact that she wasn't impaired.

This episode outlines the problem we currently have with roadside testing of cannabis. We just don't have a good way yet to quickly tell if someone is too high to drive.

A person may have a blood level of THC [that's] high, but not be impaired — this is because they develop tolerance. This doesn't happen with alcohol.- Prof. Andrea Furlan, University of Toronto, Toronto Rehabilitation Institute

"There is no correlation between saliva test, urine test, and impairment," said Prof. Andrea Furlan, an assistant professor of medicine at the University of Toronto, as well as a senior scientist at the Toronto Rehabilitation Institute, in conversation with Quirks & Quarks host Bob McDonald. 

"A person may have a blood level of THC [that's] high, but not be impaired — this is because they develop tolerance. This doesn't happen with alcohol."

A Draeger Safety Canada employee demonstrates how to use the cassette of the Drager DrugTest 5000. (CBC)

Differences between cannabis and alcohol effects on driving

There's a lot of variability between people in terms of how impaired they become when consuming cannabis — mostly due to this tolerance effect.

But with inexperienced users, the effect of cannabis on a person's impairment is a bit more pronounced, albeit — still very different than the effects of alcohol.

"People who are impaired by alcohol, they tend to be more aggressive. They lose the ability to self-regulate, which is they don't know that they're impaired, so they drive faster. And they are more aggressive [in] taking risks," said Furlan.

"On the opposite side, people who are impaired by cannabis, they tend to drive slower," added Furlan. And they tend to leave more of a space between their themselves and the vehicle in front. This doesn't, however, mean they're driving safely or capable of responding effectively to emergency situations.

Lingering effects of THC on chronic users

A further complication for testing cannabis impairment is that it doesn't necessarily wear off the way alcohol impairment does.  Chronic or medical cannabis users who consume cannabis every day can still have THC in their system long after the high wears off — in some cases for up to 30 days after abstaining.

Scientists in the U.S. conducted a study where chronic users were sequestered in a lab for 30 days after abstaining, who they then repeatedly tested as the THC was leaving their system. In that study, the scientists discovered some of the chronic users psychomotor functions were still impaired for weeks. 

"But those effects, those cognitive impairments, they are not perceptible," said Furlan. "Because this has been done in laboratory studies, in research studies, they are able to detect some small differences." But in real life, she said those differences would not be noticeable.

Pot users in the National Capital Region now have another option. The first ever legal cannabis shop opened in Gatineau at 73 Boulevard de la Gappe. It employs between 15-20 people. (Darryl Dyck/The Canadian Press)

Developing a better roadside test

Furlan is currently recruiting for a study to evaluate a better roadside test that could be scientifically validated for cannabis impairment.

They'll be trying out a computer-tablet based test, which they hope could give police an accurate tool to measure if drivers are too high to be behind the wheel. They'll be studying a range of different ways impairment can be evaluated, with tests for balance, coordination, memory, and attention. They hope to incorporate these tests on a tablet-based app, and compare how it evaluates impairment to how well volunteers perform in a driving simulator.

"We are creating a scenario — it's a 30 minute driving scenario," said Furlan. This scenario would specifically test for cannabis impairment.

"We are going to have like a pedestrian jumping in front of the car. We'll have another car cutting in front of the driver to see how [the drivers] respond to real life situations," added Furlan. "If they can perform the driving the simulator correctly without any impairment and the tablet also says, 'this person is OK to drive,' we'll put this in a correlation and we'll see, 'Well the tablet really knew who was safe and who was not safe to drive.'"

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