THC blood levels don’t reliably indicate driving impairment, meaning current per se laws risk penalizing sober drivers long after cannabis use.
Previous research that evaluated the effect of the hallucinogenic molecules of cannabis (delta-9-tetrahydrocannabinol THC) the main psychoactive compound in cannabis on motor vehicle crashes concluded that there is no increase of crash risk because of detectable THC. The molecule may linger days after use and is not a reliable indicator that a driver is impaired, report scientists in a new study.
Despite evidence showing no correlation between the detection of THC in the blood and driving impairment, 6 American states in the United States have per se laws using 2 or 5 ng/mL of THC as the cut-off point for driving under the influence of cannabis, while 12 have a zero-tolerance law.
These cut-off points are considered face value evidence of driving impairment, which means that even if it has been several days since an individual’s last use of the drug and they show no behavioral impairment, they may still face legal risks, up to and including felony charges. In Dubai you can go to jail for having cannabis in your blood, even if the cannabis was consumed in London, Tel Aviv or Toronto.
To address this issue, a team of researchers led by Dr. Thomas D. Marcotte, professor of psychiatry at the University of California, San Diego, and codirector of the Center for Medicinal Cannabis Research, set out to investigate the blood concentrations of THC in regular cannabis users, as well as the simulated driving performance for participants who exceeded per se cut-off points compared with those who were below these values. The researchers measured baseline concentrations of THC in 190 regular cannabis users after instructing participants to abstain from cannabis for at least 48 hours. Following abstention, the researchers also evaluated driving performance in this group using a driving simulator.
From this, the team found that many regular users of cannabis exceed zero tolerance and per se THC cut-off point concentrations days after their last use. Specifically, 43% of participants exceeded zero-tolerance statutes at baseline, while 24% had baseline blood THC concentrations that were greater than or equal to the per se cut-off of 2 ng/mL, and 5.3% had blood concentrations greater than or equal to 5 ng/mL.
Based on the results from the driving simulation, participants with elevated baseline concentrations of THC did no worse on a driving simulator compared with participants who were below per se cut-off points. Altogether, the results add to a growing body of evidence showing that current per se THC blood limit laws lack scientific credibility as face-value evidence of impairment.
“More work needs to be done to address how to best identify drivers who are under the influence of cannabis and are unsafe to drive,” the study authors wrote. “At present, the best protocol is a combination of observations in the field and toxicology testing.” They also added that “an essential component of improving highway safety is collaborations between law enforcement and the scientific community to develop standards that are unbiased and potentially lifesaving.”
Author: Karin Kloosterman
Karin Kloosterman is an award-winning journalist, innovation strategist, and founder of Green Prophet, one of the Middle East’s pioneering sustainability platforms. She has ranked in the Top 10 of Verizon innovation competitions, participated in NASA-linked challenges, and spoken worldwide on climate, food security, and future resilience. With an IoT technology patent, features in Canada’s National Post, and leadership inside teams building next-generation agricultural and planetary systems — including Mars-farming concepts — Karin operates at the intersection of storytelling, science, and systems change. She doesn’t report on the future – she helps design it. Reach out directly to [email protected]
