Study: Police Unable to Accurately Determine THC-Induced Impairment via Field Sobriety Tests

San Diego, CA: Trained police officers are frequently unable to discriminate between those who are under the influence of THC and those who are not based upon subjects’ performance on field sobriety tests, according to data published in the journal JAMA Psychiatry

Researchers affiliated with the University of California at San Diego performed a double-blind, placebo-controlled randomized clinical trial to evaluate whether the use of field sobriety tests (FSTs) are valid measurements for determining if drivers are under the influence of THC.

Consistent with prior analyses, police officers frequently misinterpreted subjects’ FST performance so that they deemed unimpaired participants (those who had smoked placebo cigarettes) to be under the influence. Overall, officers incorrectly classified 49.2 percent of the placebo group as impaired based upon their FST performance.

Investigators concluded, “The findings of this study suggest that (1) FSTs are useful adjuncts but do not provide strong objective evidence of THC-specific impairment and (2) additional efforts to validate existing methods and provide law enforcement with new, effective tools for identifying impairment are needed.”

Authors of an accompanying editorial in the journal added: “Field sobriety tests as administered by highly trained police officers are insufficient to detect cannabis-induced impairment. … The legal implication of these findings can be major given that FSTs are currently part of the evaluation protocol in North America to detect drivers who are cannabis impaired.”

Some of the study’s findings had initially been published in May in the journal Clinical Chemistry
The results of a 2021 study by investigators with John Hopkins University similarly reported that subjects’ performance on key elements of the field sobriety test, such as the ‘walk-and-turn’ test and the one leg stand, “showed little sensitivity to cannabis-induced impairment.” By contrast, that study’s authors acknowledged that the use of the mobile device performance application, DRUID, was adequately sensitive to cannabis-induced changes in subjects’ performance. NORML has frequently opined in favor of the use of performance testing technology as a more reliable indicator of cannabis-induced impairment. 

In 2017, justices on the Massachusetts Supreme Judicial Court ruled in Commonwealth v. Gerhardt that standard roadside field sobriety tests cannot “be treated as scientific tests establishing impairment as a result of marijuana consumption.” They added, “Likewise, an officer may not testify that a defendant ‘passed’ or ‘failed’ any FST, as this language improperly implies that the FST is a definitive test of marijuana use or impairment.”

Full text of the study, “Evaluation of Field Sobriety Tests for identifying drivers under the influence of cannabis: A randomized clinical trial,” appears in JAMA Psychiatry. Additional information on cannabis and driving performance is available in the NORML Fact Sheet, “Marijuana and Psychomotor Performance.