Study: No Correlation Between THC Detection and Driving Impairment

Neither the detection of THC nor its metabolites in subjects’ blood, breath, or oral fluid is correlated with psychomotor performance, according to the results of a state-funded study conducted by researchers affiliated with the University of California, San Diego.

Investigators assessed participants’ driving simulator performance at multiple points in time following their inhalation of cigarettes containing either moderate levels of THC (six percent), high levels of THC (13 percent), or no THC. Blood, oral fluid, and breath samples were collected from study subjects. Trained law enforcement officers also assessed subjects’ performance on Standard Field Sobriety Tests at various times following participants use of either cannabis or placebo.

Authors concluded: “In the largest trial to date involving experienced users smoking cannabis, there was no correlation between THC (and related metabolites/cannabinoids) in blood, OF [oral fluid], or breath and driving performance. … The complete lack of a relationship between the concentration of the centrally active component of cannabis in blood, OF, and breath is strong evidence against the use of per se laws for cannabis.”

The findings are consistent with numerous other studies finding that the detection of THC is not predictive of driving impairment.

NORML has long opposed the imposition of THC per se thresholds for cannabinoids in traffic safety legislation, opining: “The sole presence of THC and/or its metabolites in blood, particularly at low levels, is an inconsistent and largely inappropriate indicator of psychomotor impairment in cannabis consuming subjects. … Lawmakers would be advised to consider alternative legislative approaches to address concerns over DUI cannabis behavior that do not rely solely on the presence of THC or its metabolites in blood or urine as determinants of guilt in a court of law. Otherwise, the imposition of traffic safety laws may inadvertently become a criminal mechanism for law enforcement and prosecutors to punish those who have engaged in legally protected behavior and who have not posed any actionable traffic safety threat.”

Currently, five states – Illinois, Montana, Ohio, Pennsylvania, and Washington – impose various per se limits for the detection of specific amounts of THC in blood while ten states (Arizona, Delaware, Georgia, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, Utah, and Wisconsin) impose zero tolerant per se standards. In these states, it is a criminal violation of the traffic safety laws to operate a motor vehicle with detectable levels of THC in blood – even absent any demonstrable evidence of psychomotor impairment. 

The UCSD study also reported that law enforcement officers frequently interpreted subjects’ FST performance in such a manner that they deemed unimpaired participants (those who had smoked placebo cigarettes) to be under the influence. Authors acknowledged, FSTs classified 49.2 percent of the placebo group as impaired.”

NORML has long emphasized that although SFTs have been validated for detecting subjects under the influence of alcohol, they have never been validated for identifying subjects impaired by other controlled substances.

In 2017, the Massachusetts Supreme Judicial Court ruled that standard roadside field sobriety tests are not reliable indicators of marijuana-induced impairment.

According to a 2021 Johns Hopkins University School of Medicine study, subjects’ performance on the tasks required by conventional field sobriety tests, like the ‘walk-and-turn’ test and the one leg stand, “show little sensitivity to cannabis-induced impairment.”

By contrast, authors of the John Hopkin’s study 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. 

The full text of the UCSD study, “Driving Under the Influence of Cannabis: Impact of Combining Toxicology Testing with Field Sobriety Tests,” appears online in the journal Clinical Chemistry. Additional information on cannabis and psychomotor performance is available from NORML’s fact-sheet.