There is no evidence that THC levels currently penalized under cannabis-impaired driving laws increase the risk of causing a crash, according to a new study. Cannabis use as a risk factor for causing motor vehicle crashes: a prospective study examined blood samples from 3,005 drivers non-fatally injured in crashes in British Columbia.
Samples were taken from patients at trauma centres between 2010 and 2016. The patients were people who were moderately injured following collisions. They had blood taken for medical purposes and any remaining blood not used for tests was then analyzed using a broad-spectrum toxicology test looking not just for cannabis, but any substance that might be there including alcohol, recreational drugs such as cocaine and sedating medications.
At the same time, the authors of the study looked at the collision reports to determine the degree to which the driver was responsible for the collision. They used this information to form an odds ratio, which is the likelihood of a driver’s crash responsibility. They adjusted risk estimates depending on existing statistics about a person’s likelihood of getting into a crash such as sex and age. Factors such as road conditions, weather and the actions of other drivers were also taken into account when calculating driver responsibility.
This study is particularly interesting, from our point of view at least, because it specifically looked at the current thresholds for cannabis-impaired driving enforcement. The results were divided into three with THC concentrations 0-2 nanograms (ng) per millilitre (ml) of blood; 2-5ng/ml and 5ng/ml or more.
Changes to the Criminal Code in December introduced new per se THC limits for the offence of drug-impaired driving. Driving with less than 2ng of THC per ml of blood is not an offence but driving with 2-5ng/ml is a summary conviction offence, punishable by way of a fine of up to $1,000. If you have more than 5ng/ml of THC, or 2.5ng/ml of THC combined with a blood alcohol content (BAC) of .05 or more, then the mandatory minimum penalty is a $1,000 fine for a first offence, 30 days imprisonment for a second offence or 120 days for a third or higher offence.
THC and impairment
It has been argued for decades that there is no evidence of a direct correlation between the presence of THC in a person’s blood and whether they are impaired by cannabis. The corresponding author of this study, Dr. Jeffery Brubacher, said: “As evidence-based (per se limits) are effective in preventing drunk driving, many jurisdictions have set per se limits for THC. Unfortunately, given limited evidence, setting evidence-based per se limits for THC is challenging.”
The study seeks to find out if there is a real-world crash risk associated with cannabis use given concerns that there would be more crashes as a result of cannabis impairment after legalization.
Results
Of the 3,005 drivers included in the study, police reports were available for 2,318. At least one potentially impairing substance was found in 886 drivers (38.3%). Alcohol was detected in 14.4% of drivers while 8.3% tested positive for THC. The most common potentially impairing substance was sedating medications such as sleeping pills and antidepressants at 19.8%.
According to the study, there was no evidence of increased crash risk in moderately injured drivers in both the 0-2ng/ml and 2-5ng/ml THC limits prescribed by law. The study found there may be an increased risk of crash responsibility for drivers with more than 5ng/ml THC, however, this was “not statistically significant”.
While there is evidence to show that cannabis impairs psychomotor skills required for safe driving, according to the study, “there is also evidence that cannabis users are aware of their impairment and compensate by driving more slowly, leaving more headway and taking fewer risks”.
Implications for impaired driving law
A lot of the time, impaired driving laws are influenced more by politics than scientific fact. Understanding cannabis impairment is a difficult task. We are still figuring out how to tell for a fact that a person is impaired by cannabis. It is not like alcohol where BAC is a convenient indicator of whether someone is drunk, although even that has its problems.
THC is much less reliable when it comes to gauging if someone is impaired. It can linger in a person’s blood or saliva long after the effects of cannabis have worn off. Until now, THC has been the method of choice for determining if a driver is impaired by cannabis but, at the very least, this study goes to show that the method has some serious flaws.