Daily Marijuana Consumers Exhibit Insignificant Changes in Simulated Driving Performance After Smoking

Aurora, CO: Daily cannabis consumers display insignificant changes in simulated driving performance compared to non-users following marijuana inhalation, according to data published in the journal Accident Analysis and Prevention.

A team of researchers affiliated with the University of Colorado assessed driving simulator performance among subjects with a history of daily cannabis use, occasional cannabis use, and no history of current use. Consumers inhaled cannabis ad libitum over a 15-minute period and then engaged in simulated driving 30-minutes later. Investigators explicitly assessed consumers’ abilities to maintain lateral positioning and to maintain a specific rate of safe speed as compared to non-users. 

Those with a history of occasional cannabis use exhibited a significant increase in SDLP (standard deviation in lateral positioning) following cannabis inhalation. Occasional users also drove faster than non-users, but not to a degree that reached statistical significance. By contrast, daily users displayed insignificant changes in SDLP following cannabis inhalation and drove at slightly slower speeds. 

Authors reported: “In this study of the acute effects of cannabis use on driving performance among participants with a history of using cannabis daily or occasionally, we found evidence for decrements of driving performance in both groups relative to baseline for SDLP, that was of moderate size and statistical significance only in the occasional users. Small, statistically significant decreases in speed were observed in the daily use group.”

The findings are consistent with prior driving performance studies showing that acute cannabis exposure may influence SDLP performance and that more habitual consumers become tolerant to marijuana’s effects on psychomotor skills. By contrast, other studies have shown that the use of cannabis in combination with alcohol can greatly influence psychomotor performance, even among more experienced marijuana consumers.

The study’s findings also reaffirmed that elevated THC/blood levels are not necessarily predictive of either increased adverse driving performance or outcomes. Among daily users, mean THC/blood levels were six-times higher than they were for occasional users, despite only nominal differences in the two groups’ driving performance.

Full text of the study, “Simulated driving performance among daily and occasional cannabis users,” appears in Accident Analysis and Prevention. Additional information on cannabis’ influence on driving is available in the NORML fact sheet, ‘Marijuana and Psychomotor Performance.’