Study Shows Weed May Impair Performance of Older Drivers


Using weed may impair the driving performance of older cannabis users who get behind the wheel, according to the results of a recently released study. 

The study by Canadian researchers found that older drivers who used cannabis before driving showed an increase in lane weaving and a reduction in speed compared to a control driving session. The results of the study were published last week by the journal JAMA Open Network.

“Epidemiological studies have established that cannabis increases the risk of a motor vehicle collision,” the study’s authors wrote in their report on the research. “Laboratory studies have demonstrated that this impairment results in increased weaving, slowed reaction time, and compensatory changes in speed and following distance.”

Noting that cannabis use is rising among older adults, the researchers wrote that the effect weed has on driving performance could be amplified by factors associated with the driver’s age. They added, however, that long-term cannabis users might develop a tolerance that mutes the effects that weed can have on driving performance.

“Older adults may be particularly affected by cannabis, given age-related changes in cognition, metabolic changes that may prolong or enhance the effects of cannabis, and the concomitant use of medications,” the researchers wrote. “Conversely, older users of cannabis may have been using cannabis for many years and cannabis may have a diminished impact in this population due to development of tolerance.”

To test these hypotheses, the researchers used advertisements on public transit and social media to recruit a group of participants aged 65 to 79. After providing consent and meeting the qualifying criteria, participants completed a practice session that included a driving simulator. 

Participants then underwent two test sessions, each lasting about seven hours, with a minimum of 72 hours between sessions. Participants were instructed to abstain from cannabis, alcohol and other recreational drugs for at least 12 hours before each session.

The study participants were tested for drug and alcohol abstinence before each session using a breathalyzer, saliva testing and a 14-panel urine screening. Participants also completed a Marijuana Withdrawal Checklist to assess cannabis withdrawal levels. 

Before one session, participants smoked cannabis in a negative pressure room, using weed they had legally obtained themselves. Participants were permitted to smoke as much cannabis as they desired to reach their own comfort level. The average potency of the cannabis participants smoked was 18.74% THC. Before the other session, which was conducted as a control, the participants rested in the same room without smoking cannabis.

The researchers then took blood tests for THC and metabolite measurement before the participants operated the driving simulator at 30 minutes and 180 minutes after using cannabis or resting. Researchers took additional blood samples before each session with the driving simulator. After each driving test, participants underwent cognitive and subjective tests and completed visual analog scales to measure their willingness to drive and perceived impairment.

High Drivers Performed Worse in Driving Simulator Tests

The results of the driving simulator test showed that 30 minutes after smoking cannabis, participants showed a significant increase in Standard Deviation of Lateral Position (SDLP), which measures how much a driver is weaving in the traffic lane. Participants also showed a decrease in mean speed compared to the control session. The effects on driving performance were not observed 180 minutes after cannabis use, a pattern that was consistent in both single-task and dual-task scenarios.

Blood THC levels increased 30 minutes after cannabis use, although the increase did not correlate with the changes in SDLP or mean speed. The increase in SDLP and decrease in mean speed were consistent with previous research on the effects cannabis can have on driving.

The change in SDLP was similar to the effect in studies of alcohol-impaired driving, where a blood alcohol concentration (BAC) of 0.05% resulted in comparable increases in SDLP. The change was not as high, however, in the dual-task task condition compared with a BAC of 0.05%.

The researchers noted that the decrease in mean speed might be a change made by participants in response to their awareness of their intoxication. Overall, the study found that cannabis use did adversely affect the driving performance of older adults. 

“The present study provides an ecologically valid demonstration that cannabis can impair driving in older adults when they smoke their usual product,” the researchers wrote in their conclusion. “Consistent with emerging data, blood THC level was not correlated with driving behavior. Older drivers should refrain from using cannabis when contemplating operation of a motor vehicle.”

The researchers noted several limitations of the study, including the fact that most participants were white and more than half were male. However, the findings illustrate the importance of understanding the effects that cannabis can have on older drivers.



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