The presence of THC in blood is not correlated with driving performance and is not a reliable indicator of psychomotor impairment, according to recommendations made by a state-appointed traffic safety task force.
A report issued by the Michigan Impaired Driving Safety Commission finds that peak THC blood levels are not associated with maximal behavioral impairment and further finds that the compound’s influence upon driving performance varies significantly among individual consumers. As a result, “The Commission recommends against the establishment of a threshold of delta-9-THC bodily content for determining driving impairment and instead recommends the use of roadside sobriety tests to determine whether a driver is impaired.”
The Commission’s recommendations are similar to those previously issued by the American Automobile Association, the National Highway Traffic Safety Administration, and other traffic safety experts who have similarly opined against the imposition of per se thresholds for the presence of THC. NORML similarly argues that the identification of THC in blood is a poor predictor of either recent cannabis exposure or impaired performance.
The Commission’s report further opines that subjects influenced by cannabis “typically drive slower, keep greater following distances, and take fewer risks than when sober.” They add, “While there is some uncertainty as to the crash risk associated with cannabis impairment alone, the research is clear that the risk is lower than that of alcohol impairment.”
Five states — Montana, Nevada, Ohio, Pennsylvania, and Washington — impose various per se limits for the detection of specific amounts of THC in blood while eleven states (Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, 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 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.