Driving Simulator Study: THC Per Se Limits Are Unreliable Predictors of Psychomotor Performance


Sydney, Australia: The presence of THC concentrations in either blood or oral fluid are unreliable indicators of driving impairment, according to data published in the journal Traffic Injury Prevention

Australian researchers assessed the relationship between THC levels and driving performance in 14 volunteers. Participants vaporized cannabis samples of varying potencies (high THC/low CBD, equal ratios THC and CBD, and nominal THC/placebo). Volunteers performed on a driving simulator. Blood and oral fluid samples were collected 30 minutes following inhalation and again 3.5 hours later.

Researchers reported that neither the presence of THC in blood nor in oral fluid was a reliable measurement of driving performance. They acknowledged that nearly half of the study’s participants failed to show driving impairment 30 minutes following cannabis inhalation, despite possessing THC levels above commonly imposed per se limits (e.g., 5ng/ml in blood or oral fluid). Conversely, several participants did show impairment 3.5 hours following vaporization, at a time when their THC levels were below per se limits.

“The blood and oral fluid per se limits examined often failed to discriminate between impaired and unimpaired drivers,” authors reported. “Moreover, blood and oral fluid THC concentrations were poorly correlated with driving impairment. … It is almost impossible to infer how much cannabis was consumed, or when it was consumed, based solely on a given concentration of THC in any biological matrix.”

They concluded: “Due to erratic and route-dependent differences in THC pharmacokinetics as well as significant inter- and intra-individual variability, blood and oral fluid THC concentrations, unlike BAC [blood alcohol concentrations] for alcohol, provide little information as to the amount of cannabis consumed or the extent to which an individual may be intoxicated. Collectively, these results suggest that the per se limits examined here do not reliably represent thresholds for impaired driving.”

The findings are consistent with those of several other studies – such as those herehere, and here – reporting that the presence of THC is an unreliable predictor of either recent cannabis exposure or impairment of performance. A 2019 report issued by the Congressional Research Service similarly concluded: “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.”

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.”

Full text of the study, “The failings of per se limits to detect cannabis-induced driving impairment: Results from a simulated driving study,” appears in Traffic Injury Prevention. Further information is available from the NORML fact sheet, “Marijuana and Psychomotor Performance.

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