Influence of consumption history on simulated driving performance
People with a history of frequent cannabis use show only minor changes in their driving performance shortly after smoking marijuana, according to driving simulation data published in the journal Traffic Injury Prevention.
Researchers from the University of Colorado, Anschutz Medical Campus, and the University of Iowa evaluated the driving simulation performance of a cohort of frequent and infrequent cannabis users. Participants provided their own cannabis, which contained between 15 and 30% THC. After smoking cannabis ad-libitum, subjects performed a series of distracted driving scenarios.
The authors reported, “Those with an occasional drinking habit were significantly more likely to experience lane switching during periods of distraction after acute cannabis use compared to baseline, while those with daily did not show a similar increase. Consistent with other studies, the researchers also acknowledged that "participants with a daily drinking habit had decreased speed, which can be interpreted as a drug effect or as a compensatory strategy." »
Separate studies have previously reported that repeated exposure to cannabis is associated with partial or even complete tolerance in particular areas, including cognitive and psychomotor performance.
The study authors concluded, “The results provide evidence that an occasional smoking habit was associated with poorer performance after acute cannabis use with respect to lane departures. People with an occasional drinking habit also behave differently with respect to throttle position, and there is a tendency for speed to decrease in people with a daily drinking habit. This would fit with the tolerance hypothesis, with daily users being somewhat less affected by the effects of acute cannabis use or better able to mitigate them. This could indicate that those who use daily may perceive a potential negative impact of acute cannabis use on driving performance and attempt to compensate by slowing down to have more time to react to changes in the road. Further research is needed to understand the effects during longer and more complex side tasks. »
Although not the primary focus of the study, investigators assessed baseline levels of THC in the subjects' blood upon admission to the laboratory. Consistent with previous research, subjects who reported daily cannabis use tested positive for THC in their blood (mean THC blood level: 5ng/ml) despite abstaining from marijuana use for at least last 12 hours.
These results demonstrated that the occasional use model displayed more road trips after acute cannabis use compared to the daily use model. On the other hand, it was found that users in the daily use group showed a decrease in their speed, which can be interpreted as compensation for the effects of the drug. Further research is needed to understand the effects of the drug on longer and more complex secondary tasks.
NORML has long argued against imposing blood THC thresholds as predictors of impairment, and in itself , because they are not consistently correlated with changes in subject performance and because residual THC levels may persist in the blood for several hourss or days even after abstinence. Alternatively, NORML called for the expanded use of performance-based testing, such as DRUID .