Aurora, CO: Those with a history of frequent cannabis use exhibit only minor changes in driving performance shortly following marijuana smoking, according to driving simulator data published in the journal Traffic Injury Prevention.
Investigators with the University of Colorado, Anschutz Medical Campus and the University of Iowa assessed driving simulated performance in a cohort of frequent and infrequent cannabis consumers. Participants provided their own cannabis, which contained between 15 and 30 percent THC. Following cannabis smoking ad-libitum, subjects completed a series of distracted driving scenarios.
Authors reported: “Those with a pattern of occasional use were significantly more likely to experience a lane departure during distraction periods after acute cannabis use relative to baseline, while those with daily use did not exhibit a similar increase.” Consistent with other studies, researchers further acknowledged, “Participants with a pattern of daily use decreased their speed, which may be interpreted as a drug effect or as a compensatory strategy.”
Separate studies have previously reported that repeated cannabis exposure is associated with either partial or even full tolerance in particular domains, including cognitive and psychomotor performance.
The study’s authors concluded: “The results provide evidence that a pattern of occasional use was associated with performing worse after acute cannabis smoking as it relates to lane departures. Those with a pattern of occasional use also behave differently with respect accelerator position, and there was a trend that those with a pattern of daily use decreased the speed. This would be consistent with the hypothesis of tolerance, with individuals with daily use being somewhat less affected by or better able to mitigate the effects of acute cannabis smoking. This may indicate that those who use daily may perceive a potential adverse impact of acute cannabis use on driving performance and may attempt to compensate by slowing down to have more time to react to changes in the roadway. Further research is needed to understand the effects during longer and more complex secondary tasks.”
Though not a primary focus of the study, investigators did assess subjects’ baseline THC/blood levels upon their admission to the lab. Consistent with prior research, those subjects who reported daily cannabis use tested positive for THC in their blood (mean THC blood level: 5ng/ml) despite having abstained from marijuana for at least the past 12 hours.
NORML has long advocated against the imposition of THC blood thresholds as predictors of impairment, and per se traffic safety limits in particular, because they are not consistently correlated with changes in subjects’ performance and because residual THC levels may linger in blood for several hours or even days post-abstinence. Alternatively, NORML has called for the expanded use of performance-based tests, like DRUID.
Full text of the study, “Influence of cannabis use history on the impact of cannabis smoking on simulated driving performance during a distraction task,” appears in Traffic Injury Prevention. Additional information is available from the NORML fact sheet, ‘Marijuana and Psychomotor Performance.’