The detection of THC in blood is not correlated with changes in simulated driving performance, according to data published in the journal JAMA Psychiatry

A team of investigators affiliated with the University of California, San Diego assessed subjects’ simulated driving performance after inhaling either low-potency (six percent), moderate-potency (13 percent), or placebo cannabis. 

Compared to their baseline performance, those subjects who smoked either low-potency or moderate-potency cannabis exhibited significant changes for a period of 3.5 hours. These changes were most acute for the first 90 minutes. Cannabis inhalation was not associated with any uptick in crash risk. Researchers identified no correlation between subjects’ blood/THC levels and driving performance at any point during the study. Consistent with prior studies, authors reported, “The current results reinforce that per se laws based on blood THC concentrations are not supported [by evidence].”

Per se traffic laws make it a crime for a driver to operate a motor vehicle with trace levels of either THC or its metabolites in their blood or urine, regardless of whether there exists any demonstrable evidence that the driver is under the influence. 

Most of the study’s subjects perceived themselves to be safe to drive after 90 minutes despite continuing to exhibit lingering impairment for several hours thereafter. Authors reported that drivers in the THC groups exhibited no differences in performance compared to controls after 4.5 hours. 

Researchers concluded: “In a placebo-controlled parallel study of regular cannabis users smoking cannabis with different THC content ad libitum, there was statistically significant worsening on driving simulator performance in the THC group compared with the placebo group. … A lack of insight regarding driving impairments, particularly at 90 minutes, is of concern, given that users will likely self-evaluate when they feel safe to drive. … The lack of relationship between blood THC concentration and driving performance raises questions about the validity of per se laws. … Future research should address factors such as individual biologic differences, personal experience with cannabis, and cannabis administration methods in relation to driving impairment.”

Prior studies have shown that THC inhalation is associated with changes in driving behavior, such as an increased likelihood of weaving and a decrease a drivers’ average speed. These and other changes are typically less pronounced in subjects who are more habitual cannabis consumers, but they may be exacerbated when alcohol and marijuana are ingested in combination with one another.

By contrast, the ingestion of CBD-dominant cannabis strains has not been associated with similar changes in driving performance.

Full text of the study, “Driving performance and cannabis users’ perception of safety: A randomized clinical trial,” appears in JAMA Psychiatry. Additional information is available from the NORML fact sheet, ‘Marijuana and Psychomotor Performance.’

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