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Drug-Impaired Driving

How Marijuana Affects Driving

In recent years, State actions to legalize the use of marijuana for medical and recreational use have increased concern over potential risks of driving impaired by marijuana. Other than alcohol, it is the drug that is most frequently detected in drivers’ systems after a vehicle crash, as well as the general driving population (Compton & Berning, 2017; Kelley-Baker et al., 2017; Lacey et al., 2009; Walsh et al., 2005).

NHTSA’s Crash Risk Study, the largest of its kind ever conducted, assessed whether marijuana use by drivers is associated with greater risk of crashes. The survey found that marijuana users are more likely to be involved in crashes, but that the increased risk may be due in part because marijuana users are more likely to be in groups at higher risk of crashes. In particular, marijuana users are more likely to be young men – a group already at high risk. NHTSA is conducting another crash risk study, however; this time, only serious injury and fatal crashes will be investigated.

We know that marijuana can be dangerous when combined with driving. Studies show that marijuana impairs psychomotor skills, lane tracking, and cognitive functions (Robbe et al., 1993; Moskowitz, 1995; Hartman & Huestis, 2013), but it is still unclear the extent to which it contributes to the occurrence of vehicle crashes. Some studies have attempted to estimate the risk of driving after marijuana use (Li et al., 2012; Asbridge et al., 2012), but these remain inconclusive in terms of predicting real-world crash risk.

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