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! Just like drinking and driving, driving while high is driving under the influence.
Several studies have demonstrated a direct relationship between the concentration of THC, marijuana’s active cannabinoid substance, in people’s blood and their degree of driving impairment (Lenné, Accid Anal Prev, 2010; Hartman, Clin Chem, 2013). One study examined the effects of inhaled marijuana on simulated driving performance: they 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.
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. One 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. If you still need convincing, in our next section, we’ll dive into the possible neurobiological mechanisms for why this makes sense.