As the coauthor of a study published by the AAA Foundation for Traffic Safety, toxicologist Dr. Barry Logan analyzed the scientific validity of laws governing THC-impaired driving. Dr. Barry Logan, who serves as the chief scientist and senior vice president of forensic science initiatives for NMS Labs, has shared his findings on the topic as a lecturer and as a media consultant.
A total of six U.S. states have passed laws that set a maximum legal limit of THC, the active ingredient in cannabis, that drivers may have in their blood. In 2016, the AAA Foundation for Traffic Safety asked a group of researchers to evaluate these laws.
The researchers assessed sobriety tests and toxicology reports of drivers who had been arrested for suspected driving while impaired by marijuana. They found that 70 percent of drivers, whose behavior indicated potential impairment, showed blood THC levels under the set maximum.
Data also showed that the performance of drivers who had high concentrations of THC in their blood did not differ significantly from those who had concentrations below the limit. However, the performance of individuals in both groups was poorer than those who had no THC in their blood. This means that a low-concentration user may drive with the same impairment as a high-concentration user but may not be held liable under the law.
In contrast, chronic THC use can lead to continuing high levels of the drug in the bloodstream. A blood test of a driver who hadn’t ingested THC in several days might show THC levels similar to those of a more recent user. Nevertheless, because much of ingested THC leaves the body quickly, the time between impairment and blood testing may lead to inaccurate measurements.
These findings led researchers to conclude that blood testing is not yet a reliable indicator of a driver's impairment. The team agreed with the AAA's suggestion that impairment evaluation should depend primarily on the observation of a trained officer, who has assessed the individual's balance, coordination, speech, and performance on specific tasks, as well as measurable metrics such as blood pressure and pulse.
