Kelly Hughes

Tags: cannabis; driving performance; simulator; young adult drivers; sober

 

The problem: Cannabis use has increased in young adults over the last decade and as states move to legalize medical and recreational use, the potential impact on driving safety should be taken into consideration.

 

Why do this study?  The acute effects of cannabis on driving are well known.  However, much less is known about the effects of long-term cannabis use on driving ability.  Cannabis is known to impair mental faculties in several ways that could directly translate into difficulty controlling a vehicle.

 

The study:  The data for this analysis was drawn from 4 previous studies that occurred at the National Advanced Driving Simulator at the University of Iowa. The data from these studies was sorted by subject age and cannabis use; identifying 30, out of 72, subjects who used cannabis and fit in the target age group (18-23 years old). All cannabis users were sober at the time of testing. Subjects were evaluated on their vehicle control in simulated urban, rural and interstate night environments. Driving ability was evaluated in two dimensions: laterally, where side-to-side movements were scored based on lane position and frequency of steering wheel adjustments, and longitudinally, where front-to-back movements were measured by average speed, relative to the speed limit, and frequency of using the accelerator pedal.

 

For lateral control, cannabis users steered their vehicles more slowly and less frequently than non-users did, but overall there was no significant difference in their ability to stay in the lane. In comparing longitudinal control, cannabis users drove more slowly and used the accelerator pedal less frequently than non-users.

 

Conclusions:  Young adult drivers who use cannabis drove more slowly and produced significantly less frequent steering and accelerator pedal activity than those who did not use cannabis. This suggests cannabis users are more passively engaged in controlling the vehicle. Further studies are necessary to determine if these effects persist after long-term abstinence, and whether other age demographics are affected similarly.

 

What does this study add?  This suggests cannabis use can have a lasting detrimental effect on driving ability, even if the driver is sober while operating the vehicle. These results highlight the need to study impairments in cannabis users not just during intoxication, but also while they are sober.

 

Funder:  National Institutes of Drug Abuse funded the collection of this data under contract numbers HHSN271201100009C andHHSN271201800015C.

 

Author conflicts:  None Reported

 

Commentary: (Kelly Hughes)

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This study is interesting because it looks at the prolonged effects of cannabis use, where most previous studies have focused on the acute effects during intoxication. The results of this study are important because they imply that drivers who use cannabis may still be a public health concern even while they are sober, which could have significant bearing on the specifics of legislation involving the legalization of cannabis. However, it should be noted that this study analyzed data drawn from several pre-existing data sets, rather than from a study specifically designed to address the driving ability of sober cannabis users. Therefore, there are several important questions that this study does not address. Most importantly, we don’t know whether these differences are significant enough to increase the risk of accidents.  That is, these may be scientifically real, but unimportant from a public health perspective.  Also, the study does not offer any conclusions about whether cannabis has these same effects on drivers of older or younger age, or whether there is any correlation between the frequency/severity of use and the level of impairment. It also does not tell us whether these residual effects diminish over time, and if so, what that timeline might be. These are important questions that would need to be considered when applying such findings to policy making, and thus further studies should be pursued to answer them.