Assessing Canada’s cannabis driving limit

When the Canadian government legalized cannabis in October 2018, it set restrictions on the amount you could consume before driving. Now, nearly three years later, there are efforts to assess the effectiveness of Canada’s cannabis driving limit.

According to a recent article in the Canadian Medical Association Journal (CMAJ), the association between the legal use of recreational cannabis and impaired driving is not clear. While cannabis is proven to affect motor and cognitive functions, the effect of cannabis legalization on motor vehicle fatalities is less clear.

One reason for this is the influence of cannabis varies from person to person and depends on other factors such as how it was consumed. For this reason, the effectiveness and validity of current impaired driving legislation are under scrutiny.

What is the Canada cannabis driving limit?

The Criminal Code specifies a per se driving limit on the amount of THC in a person’s blood. It is an offence to drive with 2-5 ng of THC per 1 ml of blood. There are higher penalties for driving at or above 5 ng/ml THC or a combination of 2.5 ng/ml THC or more and a blood alcohol concentration of 0.05 mg per 100ml of blood or higher.

The police in Canada have two methods to test drivers for cannabis impairment at the roadside: standardized field sobriety tests and testing saliva.

What did the CMAJ article say?

The CMAJ article looked at Canadians’ cannabis consumption habits in the wake of legalization. Statistics Canada showed that the proportion who reported consuming cannabis in the past three months increased from 14% to 17% since 2018. Among recent cannabis users who can drive, 13% reported driving within two hours of consuming marijuana.

In order to get a sense of the association between the legalization of recreational cannabis and fatal motor vehicle collisions, the study examined data from the United States. It looked at all fatal motor vehicle collisions between 2007–2018 for 11 jurisdictions that legalized marijuana before 2019.

The CMAJ study found that legalization was associated with a “small but important relative increase” of 4,843 additional motor vehicle fatalities per year in the US, a 16% rise. If extrapolated for Canada, a 16% increase would result in 308 additional driving deaths per year.

Problems with the standardized field sobriety tests

The CMAJ study also looked at the effectiveness of current steps to detect and deter cannabis-impaired driving in Canada. It suggested, “a multidisciplinary response is required for optimal detection and deterrence” while noting “substantial uncertainty” around the current best measures for detecting cannabis-impaired driving.

Standardized field sobriety tests (SFST), such as the one-leg stand, walk-and-turn and horizontal gaze nystagmus, are used to detect drug impairment at the roadside. While alone they are not enough to convict someone, they can lead to further tests with a trained drug recognition expert (DRE).

However, the article states: “[SFSTs] were developed for assessing impairment related to alcohol use, and their validity for detecting impairment due to cannabis is uncertain.” It also pointed out that DRE training is expensive and time-consuming, and as a result, there are few available in Canada. DREs are also not medically trained, and the accuracy with which they can detect impairment and attribute it to a particular drug varies.

According to the article: “The validity of both Standardized Field Sobriety Tests and the drug recognition expert program for cannabis impairment remains uncertain, and other methods to either complement or replace these approaches should be investigated.”

Problems with the Canada cannabis driving limit

The study also looked at roadside saliva testing and the setting of per se limits for THC. It found that the ability of the limits to withstand legal challenges remains unclear because “they are not based on a demonstrated correlation between THC levels and driving impairment.”

The degree to which specific THC levels cause impairment varies from person to person. Factors like an individual’s tolerance, how frequently they consume cannabis, and how it was consumed (smoking, vaping, or edibles) affect THC concentrations in the body.

“Substantial improvement in the accuracy of current biochemical measures to detect cannabis impairment is needed to both ensure public safety and protect the rights of legal cannabis users,” according to the article. However, it also suggested that limits could be effective from a public awareness perspective by reinforcing the idea that cannabis-impaired driving will be detected.