A new scientific review of available evidence on the relationship between cannabis and driving finds that most research "reported no significant linear correlations between blood THC and measures of driving," although there was an observed relationship between levels of the cannabinoid and reduced performance in some more complex driving situations.

The report, which has not been peer reviewed, was posted late last week on Preprints with The Lancet by an eight-author team representing Canada's Centre for Addiction and Mental Health, Health Canada and Thomas Jefferson University in Philadelphia. It identified and assessed a dozen peer-reviewed studies measuring "the strength of the linear relationship between driving outcomes and blood THC" published through September 2023.

"The consensus is that there is no linear relationship of blood THC to driving," the paper concludes. "This is surprising given that blood THC is used to detect cannabis-impaired driving."

Most states where cannabis is legal measure THC intoxication by whether or not someone's blood THC levels are below a certain cutoff. The study's findings suggest that relying on blood levels alone may not accurately reflect whether someone's driving is impaired.

"Of the 12 papers included in the present review," authors wrote, "ten found no correlation between blood THC and any measure of driving, including [standard deviation of lateral position (SDLP)], speed, car following, reaction time, or overall driving performance. The two papers that did find a significant association were from the same study and found significant relationship with blood THC and SDLP, speed and following distance."

"Driving after the use of cannabis may be difficult to detect through blood THC, except in situations where there is a high task complexity."

Authors certainly aren't suggesting that there's no relationship between THC consumption and impaired driving -- only that the relationship more appears nuanced and complex than simply the amount of the cannabinoid in someone's blood. For example, they noted that some research showed that "when a median split was conducted based on blood levels of THC, there were more changes in driving in those who were above the legal limits" even though those same studies did not find a clear correlation between blood THC and driving.

Authors said that detail suggests there indeed "may be limits above which driving is impaired, which may explain why the one study with high doses found significant correlations between blood driving and THC."

Complexity of tasks behind the wheel may also seemed to matter. The one study that found correlations, the review's authors point out, "used complex driving situations that consisted of a combination of rural, urban and interstate roads" that "involved distractors such as deer emerging in rural areas, car doors opening into traffic and kids on bicycles." Drivers in that study were also told to do a second task while driving, for example watching lights in a rear-view mirror or making a selection on the car stereo.

"Thus, scenario and task complexity may be an important variable in revealing an association between blood THC and driving," the new paper says, encouraging future research to "vary the task demands of the drive to unravel the complex relationship of blood THC to driving."

Despite the new paper's findings that there's little evidence of a linear relationship between blood THC levels and driving performance, authors say their conclusions have "important implications for road safety," pointing to roadway situations they describe as challenging and complex as well as "increases in potency of cannabis over the past years."

"Current methods of detection of impairment may be suited to some types of situations," they opine, "but more large-scale studies on the relationship of blood THC and driving are needed that systematically vary driving complexity and cannabis potency."

The new review is by no means the first research to challenge the popular view that THC blood levels are a suitable proxy for driving impairment. In 2015, for example, a U.S. National Highway Traffic Safety Administration (NHTSA) concluded that it's "difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects," adding that "it is inadvisable to try and predict effects based on blood THC concentrations alone."

In a separate report earlier this year, NHTSA said there's "relatively little research" backing the idea that THC concentration in the blood can be used to determine impairment, again calling into question laws in several states that set "per se" limits for cannabinoid metabolites.

"Several states have determined legal per se definitions of cannabis impairment, but relatively little research supports their relationship to crash risk," that report says. "Unlike the research consensus that establishes a clear correlation between [blood alcohol content] and crash risk, drug concentration in blood does not correlate to driving impairment."

Similarly, a Department of Justice (DOJ) researcher said in February that states may need to "get away from that idea" that marijuana impairment can be tested based on the concentration of THC in a person's system.

"If you have chronic users versus infrequent users, they have very different concentrations correlated to different effects," Frances Scott, a physical scientist at the National Institute of Justice (NIJ) Office of Investigative and Forensic Sciences under DOJ, said.

That issue was also examined in a recent federally funded study that identified two different methods of more accurately testing for recent THC use that accounts for the fact that metabolites of the cannabinoid can stay present in a person's system for weeks or months after consumption.

Back in 2022, Sen. John Hickenlooper (D-CO) sent a letter to the Department of Transportation (DOT) and NHTSA seeking an update on the status of a federal report into testing THC-impaired drivers. The department was required to complete the report under a large-scale infrastructure bill that President Joe Biden (D) signed, but it missed that deadline and is unclear how much longer it will take.

Last summer, a congressional report for a Transportation, Housing and Urban Development, and Related Agencies (THUD) bill said that the House Appropriations Committee "continues to support the development of an objective standard to measure marijuana impairment and a related field sobriety test to ensure highway safety."

A study published in 2019 concluded that those who drive at the legal THC limit -- which is typically between two to five nanograms of THC per milliliter of blood -- were not statistically more likely to be involved in an accident compared to people who haven't used marijuana.

Separately, the Congressional Research Service in 2019 determined that while "marijuana consumption can affect a person's response times and motor performance ... studies of the impact of marijuana consumption on a driver's risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage."

Another study from 2022 found that smoking CBD-rich marijuana had "no significant impact" on driving ability, despite the fact that all study participants exceeded the per se limit for THC in their blood.