Daily consumers exhibit tolerance to the acute psychomotor-influencing effects of cannabis, according to driving simulator data published in the journal Traffic Injury Prevention.

Researchers affiliated with the University of Colorado Anschutz. Medical Campus assessed simulated driving performance in a cohort of daily cannabis consumers, occasional consumers, and controls (non-users). Daily users consumed either high-potency cannabis flower or concentrates containing, on average, 78 percent THC. Occasional consumers only inhaled cannabis flower. All consumers used cannabis ad libitum for up to 15 minutes. Study participants drove on a computer simulated course 20 minutes following cannabis consumption and once again 80 minutes later.

Consistent with prior studies, daily consumers exhibited few changes in psychomotor performance compared to controls. Specifically, daily consumers demonstrated improvements in SDLP (standard deviation in lateral positioning) following cannabis ingestion. Both daily and occasional cannabis consumers reduced their speed following cannabis use, whereas those in the control group typically increased their speed.

Unlike daily users, occasional cannabis consumers exhibited minor detriments in SDLP performance following cannabis inhalation. However, these changes were not statistically significant compared to controls (whose follow up SDLP performance also deviated from their baseline).

“The relative absence of significant differences in driving performance after cannabis across participants groups was somewhat surprising, given the high THC concentration of product used, and the relatively high level of self-reported drug effect,” researchers reported. “It was notable that the daily use group who inhaled concentrates showed the least number of significant differences as compared to the control group, having little to no change in the average SDLP and speed across the three drives. The absence of decrements in driving performance (assessed by lane departures or SDLP) among the daily-concentrate group is consistent with tolerance to acute impairing effects of cannabis.”

Researchers also failed to identify any correlations between THC/blood concentrations and impaired driving performance – a finding that is also consistent with other studies. “These findings reaffirm that the presence of THC in blood is an inconsistent and largely inappropriate indicator of psychomotor impairment in cannabis consuming subjects,” NORML’s Deputy Director Paul Armentano said. Accordingly, NORML has long opposed the imposition of per se THC limits for motorists and has alternatively called for the expanded use of mobile performance technology like DRUID.

The study’s authors concluded: “Taken as a whole, these findings indicate that acute cannabis use impaired driving performance more among the participants with a pattern of non-daily use (less than 4 times per week). … The absence of decrements in driving performance in the daily use groups support a role of tolerance in mitigating acute impairment. When changes in driving performance were observed, the effect size was notably small. These findings underscore the challenges of developing standardized impairment thresholds in the presence of large inter-individual variability in driving performance, and tolerance to cannabis with daily use.”

An abstract of the study, “Impact of cannabis use on lateral control and speed: A driving simulator study,” appears online. Additional information is available in the NORML fact-sheet, ‘Marijuana and Psychomotor Performance.’