How Does Cannabis Affect Your Driving?

Before kicking off any discussion about cannabis and driving, let’s get straight in there and clarify that taking the wheel while stoned is totally unacceptable. Having said that, a look at some of the latest studies indicates that testing positive for THC may not increase a driver’s likelihood to crash.

Cannabis Impairs Driving

A raft of recent research has produced some interesting findings regarding the impact of cannabis on a person’s driving ability. For instance, a study that appeared last month in the journal JAMA Psychiatry indicated that getting high may impair driving skills for up to four and a half hours[i].

Half an hour after smoking pot, participants in the placebo-controlled clinical trial scored worse on a driving simulator programme than those who didn’t smoke. After an hour and a half, many of those who had consumed cannabis said they no longer felt stoned and would feel safe driving a car. However, their performance on the simulator told a different story as they continued to swerve out of their lane.

At three and a half hours, the difference in driving ability between the cannabis and placebo groups was “borderline”, while all impairment had worn off an hour later. These findings highlight the danger of getting behind the wheel while under the effects of weed. However, they don’t provide any evidence to suggest that the presence of THC in the blood contributes to reduced driving capabilities.

On the contrary, the study authors found no correlation between blood THC levels and driving performance at any time point. In a message to policymakers who may be thinking of criminalising all drivers who test positive for the substance, the researchers, therefore, insist that “per se laws based on blood THC concentrations are not supported.”

Cannabis, Driving And Car Crashes

A separate study published in December attempted to discern if people who test positive for THC are more likely to be involved in car crashes. To do so, they conducted a review of the existing literature on this topic, concluding that recent cannabis use is unlikely to have a major detrimental effect on a person’s driving unless they are actually intoxicated[ii].

The authors identified 13 separate studies linking THC blood concentrations with car crashes. Collectively, the data suggested that people who test positive are 37 percent more likely to be involved in an accident. However, the researchers note that all of these studies were blighted by biases.

For instance, some failed to include a control group, while others neglected to consider certain key variables that may have contributed to drivers’ involvement in a crash. The authors, therefore, developed a tool to identify and filter out these biases.

After re-analysing the data using this tool, they found that recent cannabis use has no impact on a person’s driving and that those who test positive for THC are no more likely than non-users to cause a crash. In contrast, a blood alcohol level of 0.05 percent doubles the risk of being at fault for an accident.

Admittedly, this analysis did not examine the chances of causing a crash when driving under the acute effects of cannabis, and it’s likely that operating a vehicle while baked would increase a person’s likelihood of causing an accident. Nonetheless, the authors advise policymakers that “the removal of cannabis-presence driving offences should be considered (while impairment-based offences would remain).”

[i] Marcotte TD, Umlauf A, Grelotti DJ, Sones EG, Sobolesky PM, Smith BE, Hoffman MA, Hubbard JA, Severson J, Huestis MA, Grant I. Driving Performance and Cannabis Users’ Perception of Safety: A Randomised Clinical Trial. JAMA psychiatry. – https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2788264

[ii] White MA, Burns NR. The risk of being culpable for or involved in a road crash after using cannabis: A systematic review and meta-analyses. Drug Science, Policy and Law. 2021 Dec;7:20503245211055381. – https://journals.sagepub.com/doi/full/10.1177/20503245211055381

This post is also available in: