A recent scientific review of existing research has found little evidence supporting a direct correlation between blood THC levels and driving performance.
The study, published last week on Preprints with The Lancet, was conducted by a team from Canada’s Centre for Addiction and Mental Health, Health Canada and Thomas Jefferson University. It assessed twelve peer-reviewed studies up to September 2023.
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“The consensus is that there is no linear relationship of blood THC to driving,” the authors concluded. This finding challenges the common practice in many states of using blood THC levels to determine cannabis-related driving impairment.
Complex Driving Scenarios Matter
The review found most studies showed no significant correlation between blood THC and driving measures like speed and reaction time. However, it did identify some nuances in the findings.
“Driving after the use of cannabis may be difficult to detect through blood THC, except in situations where there is a high task complexity,” the authors noted.
Out of the twelve studies analyzed, ten found no relationship between blood THC levels and driving outcomes. However, two papers did find significant associations, though both originated from the same study that evaluated more complex driving situations. In those scenarios, participants faced varied conditions, including rural and urban environments and tasks that required divided attention.
“Thus, scenario and task complexity may be an important variable in revealing an association between blood THC and driving,” the report states.
Implications For Road Safety
The review suggests that current methods for detecting impairment may not adequately reflect the complexities of real-world driving scenarios.
“Current methods of detection of impairment may be suited to some types of situations,” the authors stated, “but more large-scale studies on the relationship of blood THC and driving are needed that systematically vary driving complexity and cannabis potency.”
The study aligns with previous findings from the U.S. National Highway Traffic Safety Administration (NHTSA), which has noted challenges in establishing a clear relationship between blood THC concentrations and driving performance. In a 2015 report, NHTSA cautioned against relying solely on blood THC levels to predict impairment.
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