Cannabis Impairment & Driving: Are Field Sobriety Tests Accurate?

Numerous studies have shown the effects of cannabis on driving performance, and the overwhelming consensus is that it impairs driving ability. Research suggests that drivers under the influence of cannabis demonstrate a decreased ability to maintain a steady speed, maintain proper lane positions and react to unpredictable situations on the road.

According to research published in the Journal of Studies on Alcohol and Drugs last year, the states with legal adult-use marijuana programs experienced increased traffic crashes and fatalities.

To tackle the growing issue, law enforcement is utilizing different tactics. However, impairment by cannabis, unlike alcohol, is still hard to objectively measure by any scientifically proven methodology.

Instead, law enforcement officers utilize behavioral or field sobriety tests to assess a driver's level of impairment. Considering that these kinds of tests have been usually used to measure impairment by alcohol, researchers at the University of California San Diego Center for Medicinal Cannabis Research opted to examine just how accurate FSTs are in detecting impairment by Delta-9-tetrahydrocannabinol (THC).

The findings, published on Wednesday in JAMA Psychiatry, showed that the tests might not be accurate enough to indicate impairment caused by THC, even though the tests conducted by law enforcement officers were able to distinguish between individuals who had consumed THC and those who hadn't at specific points in time.

Why?

"While cannabis can be impairing, the effects vary for each individual," Thomas Marcotte, Ph.D., professor of psychiatry at UC San Diego School of Medicine and co-director of the Center for Medicinal Cannabis Research at UC San Diego, said." Driving is a complex task that requires intact attention and motor skills to stay safe."

The Study

The research sample included 184 individuals aged 21 to 55 who reported regular cannabis use. During the experiment, a group of 63 individuals used a placebo cannabis cigarette, while another group of 121 individuals consumed a cannabis cigarette containing THC. Those who ingested THC experienced a median state of intoxication at 64 on a scale of 0 to 100.

The law enforcement officials subsequently conducted tests to assess the participants' various skills, including balance, coordination, multitasking, and eye movement. They used the tests the Walk and Turn,  One Leg Stand, Finger to Nose, Lack of Convergence and Modified Romberg.

The findings revealed that officers labeled a significantly more significant percentage of individuals in the THC group as impaired when assessing them through field sobriety tests, as opposed to the placebo group. This difference was observed in three out of the four-time points assessed.

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Photo: Courtesy of Samuele Errico Piccarini on Unsplash