CRS Report: THC Levels Not Correlated With Driver Impairment

Driving performance

Washington, DC: The presence of THC in blood is not a consistent predictor of either driver performance or impairment, according to the conclusions of a new Congressional Research Service report assessing cannabis and psychomotor performance.

The report acknowledges: "Research studies have been unable to consistently correlate levels of marijuana consumption, or THC in a person’s body, and levels of impairment. Thus, some researchers, and the National Highway Traffic Safety Administration, have observed that using a measure of THC as evidence of a driver’s impairment is not supported by scientific evidence to date."

It further reports that data is "conflicting" with regard to whether marijuana usage plays a substantial role in traffic accidents, noting, "Levels of impairment that can be identified in laboratory settings may not have a significant impact in real world settings, where many variables affect the likelihood of a crash occurring."

It concludes: "There is as yet no scientifically demonstrated correlation between levels of THC and degrees of impairment of driver performance, and epidemiological studies disagree as to whether marijuana use by a driver results in increased crash risk. … Based on current knowledge and enforcement capabilities, it is not possible to articulate a similarly simple level or rate of marijuana consumption and a corresponding effect on driving ability."

The findings are consistent with prior studies by the National Highway Traffic Safety Administration and others reporting that the presence of THC in blood, particularly at low levels, is not consistently correlated with either psychomotor impairment or crash culpability.

Six states – Illinois, Montana, Nevada, Ohio, Pennsylvania, and Washington – impose various per se limits for the presence of specific amounts of THC in blood while twelve states (Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, South Dakota Utah, and Wisconsin) impose zero tolerant per se standards. In those states, it is a criminal violation of the traffic safety laws to operate a motor vehicle with any detectable levels of THC in blood. Colorado law infers driver impairment in instances where THC is detected in blood at levels of 5ng/ml or higher.

For more information, contact Paul Armentano, NORML Deputy Director. Full text of the CRS report, "Marijuana Use and Highway Safety" is available here. Additional information is available from the NORML fact-sheet, "Marijuana and Psychomotor Performance."