A study involving the use of a driving simulator to assess the ability of those aged over 60 to drive after using cannabis has shown no direct link between driving performance and THC blood levels.
Researchers from the University of Toronto in Canada published the results in the Journal JAMA. The participants were aged between 60 and 79, and all were regular users of cannabis. They were asked to complete various carefully designed driving tasks on the simulator to asses their reaction times, lane positioning and speed, as well as self-reporting their perceived ability to drive. Participants did this twice, once without THC in their blood, and a second time after they had used cannabis.
“Participants were asked to bring their own legally purchased cannabis to the laboratory. They were asked to smoke the cannabis as a joint with no tobacco. They were given the choice of rolling their own joint or bringing a preroll that was purchased from a retail outlet. Participants were allowed to smoke ad libitum,” the authors wrote. “Since this was an ecologically valid design, participants were provided with only a few instructions: they were told to smoke their usual amount to achieve their desired effect and to stop if they felt strange or unwell. They were told that they were not required to smoke the entire cannabis cigarette.”
Although the results did show a small negative variation in lane positioning, or weaving, after smoking cannabis when compared to baseline results, this was not correlated with THC blood levels. Levels of weaving were described by researchers as being “less pronounced” than from people who had a blood alcohol level of 0.05.
“Significant differences” were also noted in the average speed participants drove during the simulator tasks, with slightly lower speeds being linked to having elevated THC blood levels.
Reaction times were not affected at all, they remained the same for drivers despite THC blood levels. Despite only small, or no differences being found in driving ability while under the influence of cannabis when compared to not having THC in their blood, participants did self-report a low sense of impairment and a lower willingness to drive.
“Blood THC levels were significantly increased at 30 minutes but not 180 minutes. Blood THC was not correlated with SDLP [standard deviation in lateral positioning] or MS [mean speed] at 30 minutes, and SDLP was not correlated with MS. Subjective ratings remained elevated for 5 hours and participants reported that they were less willing to drive at 3 hours after smoking.”
However, the study emphasises that those over 60 years of age who drive under the influence of cannabis must exercise caution or refrain from driving. “The purpose of the present study was to investigate the association between cannabis and driving and blood THC levels in older adults. There was no correlation between blood THC concentration and SDLP or MS. The lack of correlation between driving and blood THC fits within emerging evidence that there is not a linear relationship between the two.”
“The present study provides an ecologically valid demonstration that cannabis can impair driving in older adults when they smoke their usual product. Consistent with emerging data, blood THC level was not correlated with driving behaviour. Older drivers should refrain from using cannabis when contemplating the operation of a motor vehicle.”