As the wave of cannabis legalization continues to sweep across the United States, a recent study from the University of California, San Diego, has raised critical questions about the effectiveness of field sobriety tests (FSTs) in identifying drivers impaired by cannabis.
Study, led by Dr. Thomas D. Marcotte and his team at the Center for Medicinal Cannabis Research, is the first large-scale investigation to assess how well FSTs can distinguish between individuals who have consumed cannabis and those who have not, and whether these tests correlate with actual driving impairment.
The Context
With more states legalizing cannabis for both medicinal and recreational use, law enforcement agencies are increasingly relying on FSTs to assess drivers' sobriety. These tests, originally developed and validated for alcohol impairment, include tasks like the Walk and Turn, One Leg Stand, and Finger to Nose tests, which assess balance, coordination, and divided attention. However, their accuracy in detecting impairment from cannabis, particularly Δ9-tetrahydrocannabinol (THC)—the primary psychoactive compound in cannabis—has been a topic of debate.Study Design and Findings
Conducted between February 2017 and June 2019, the double-blind, placebo-controlled randomized clinical trial involved 184 participants aged 21 to 55 who had used cannabis in the past month. Participants were randomly assigned to smoke either a placebo, low-dose THC (5.9%), or high-dose THC (13.4%) cannabis cigarette and then underwent FSTs at multiple time points.The results were striking. Law enforcement officers classified 81% of participants who had smoked THC as impaired based on FSTs, compared to 49.2% of those who received the placebo. While this indicates that FSTs are somewhat effective at identifying cannabis use, the high rate of false positives among the placebo group raises concerns. Notably, 99.2% of participants classified as impaired by FSTs were suspected of having consumed THC, regardless of their actual exposure.
Implications for Road Safety
The study also linked poor performance on certain FSTs to worse outcomes in driving simulators, reinforcing the idea that THC can impair driving ability. However, the overlap in FST impairment between THC and placebo groups suggests that FSTs, on their own, may not be reliable indicators of THC-specific impairment.The Future of Cannabis Impairment Detection
The findings underscore the need for developing more accurate, objective tools for assessing cannabis impairment. As cannabis legalization expands, so does the urgency for effective methods that can distinguish between recent cannabis use and actual impairment.Conclusion
While FSTs remain a cornerstone of roadside sobriety assessments, this study suggests that they may not be sufficient to accurately identify cannabis-related impairment. As the legal landscape around cannabis continues to evolve, so too must the tools and techniques used to enforce driving laws.For more details, you can access the full study here (clearnet).
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