Many states are adopting legislation to criminalize driving with THC in your blood. California has test to create any new laws that specifically ban cannabis as an illegal substance, however the laws do make it illegal to drive with any drug in your system that impairs your ability to operate a vehicle safely.

The research shows that predicting impairment through levels of THC is not always easy and often very unreliable. Study after study reveals a quantitative result of THC does not correlate to a level of driving impairment.

A recent study in Ontario looking at the effects of edible cannabis on simulated driving and blood THC levels found that driving impairment was not correlated with blood THC. The new study is expected to be a big help for those facing a marijuana related DUI.

The study is the first of its kind to look at the impact of cannabis edibles on simulated driving, with researchers using an average dose of around 7.3 mg THC to provide real-world context for the impact legally available cannabis can have on driving. 

The 22 participants (sixteen male and six female) were required to have a valid Ontario driver’s licence, to have used cannabis edibles at least once in the past six months, and to drive at least once a month. Participants could be from 19-79 years of age.

Participants were asked to not use cannabis for 72 hours before the test, or any other drugs or alcohol for 12 hours. Researchers gave them three independent, pre-programmed scenarios, including a two-lane rural highway and a “potentially frustrating event” to test the drivers’ speed, and a lateral control test on a four-lane highway to rate drivers’ reaction time.

On average, participants chose to consume about 7.3 mg of THC with 2.14 mg CBD. Eleven chose the maximum of 10 mg THC, while ten chose edibles with 5 mg THC or less. A blood sample was then collected two hours after consuming the cannabis edible or a control candy. The focus of the blood analysis was to look for active THC.

The mean speed of the drivers who consumed a cannabis edible was found to decrease at the two-hour mark, but not at the four or six-hour mark. Some participants noted effects up to six hours after ingestion, with some reporting being less able or willing to drive up to six hours after consuming a cannabis edible. 

While past studies have found evidence of increased swerving (“standard deviation of lateral position”) and decreased reaction time after smoking or vaping cannabis, these effects were not observed in this study. 

It is noteworthy that as a defense attorney for 30 years, I have observed most police officers will try and find impairment and erratic driving in cannabis related cases, even though none actually existed. For example, in one Palos Verdes DUI case I fought, the cop exaggerated the level of impairment and was thoroughly cross examined on that issue.

The researchers speculate that this may be due to the relatively low amount of THC consumed, or the inability of the driving simulator to detect small changes in performance.

After two hours, blood THC was relatively low at about 2.8 ng/mL. Blood THC was significantly increased after consuming the cannabis edible, but the mean increases in blood THC were lower than those reported for smoked cannabis. Researchers also found no direct relationship between blood THC and driving impairment, speculating that “the present study suggests that blood THC may not be as useful for detection of impaired driving after edibles as it may be for the smoked route.”

“Analysis of the relationship of blood THC to SDLP (standard deviation of lateral position) or MS (mean speed) revealed no correlation with blood THC, which fits with emerging evidence from studies of smoked cannabis that there is no linear relationship between blood THC and driving impairment.”

The paper also speculates that it’s possible the participants had a high THC tolerance that allowed them to manage the effects of cannabis and marijuana products more effectively. 

Twelve of the participants reported using cannabis at least once a day, while another six reported consuming it more than once a week. 

The study was approved by the Centre for Addiction and Mental Health (CAMH) Research Ethics Board and the Health Canada Research Ethics Board, and was conducted at CAMH in Toronto, Canada.

Matthew J Ruff, Attorney at Law is a DUI Lawyer and criminal defense attorney with over 25 years fighting for the rights of his clients.