Weed Use and Driving Research

Whenever you get behind the wheel, you’re activating multiple brain networks that help you to coordinate the sensory, motor, and cognitive neural circuits that you need to get safely from one place to the next.

While driving may seem totally effortless you, it’s actually an incredibly complex task! Acute marijuana intoxication tampers with these brain circuits in a way that makes safe driving all the more difficult.

Complex decision making, fast reaction time, and coordination are skills that are crucial to safe driving. If you find yourself in a situation where you anticipate needing to drive, think twice before you decide to use weed!

Does Marijuana Impair Driving Ability?

Several studies have demonstrated a direct relationship between the concentration of THC in people’s blood and their degree of driving impairment (Lenné, Accid Anal Prev, 2010; Hartman, Clin Chem, 2013).

A 2021 meta-analysis examining 80 publications assessed several measures of driving performance and driving-related cognitive skills also showed a THC-driving impairment link. The authors noted that regular marijuana users experienced less impairment than occasional marijuana users (McCartney, Neurosci Biobehav Rev, 2021).

Combining Weed and Alcohol While Driving

A 2015 study examined the effects of inhaled marijuana on simulated driving performance: researchers found that a 5 mcg/L blood THC concentration combined with a 0.05 g/210L breath alcohol concentration (which is what a breathalyzer measures) produced the same impairment as a 0.08 g/210L breath alcohol concentration (Hartman, Drug Alcohol Depend, 2015).

This is most likely because marijuana’s depressive impact on your central nervous system is synergistic with alcohol’s depressive effects, leading to an even greater risk of driving impairment when the two are combined.

Does Weed Increase Your Car Accident Risk?

While the impact that marijuana has on your ability to drive is hard to dispute, its actual impact on rates of car accidents is less straightforward. A 2008 study found that drivers with THC in their blood were about twice as likely to have been in a fatal crash than drivers who had not used drugs or alcohol, although it’s difficult to say what exact role marijuana played in those accidents (Biecheler, Traffic Inj Prev, 2008).

This is because THC can often be detected in the blood long after use, and concurrent use of marijuana and alcohol is common.

Several large analyses looking at multiple studies found that the risk of being involved in a car accident increased significantly after using marijuana, showing that marijuana may double your risk of being in an accident (Elvik, Accid Annal Prev, 2013).

However, a different study conducted by the National Highway Traffic Safety Administration found that there was no association between marijuana use and car accidents after controlling for factors like drivers’ ages, genders, and levels of alcohol intoxication (Compton, Drug and Alcohol Crash Risk, 2015).

In summary, the jury is still out on the exact impact marijuana use has on rates of car accidents. We do know, though, that marijuana impacts your ability to drive.