How Much Does Marijuana Affect Your Driving?

We all know not to drink and drive. Research has documented the effects of alcohol on our bodies, minds, and ability to make sound judgments. Less documented is the effect of marijuana on our ability to operate motor vehicles, though the recent passing of recreational and medical marijuana initiatives across the country raises interesting legal questions. Learn how marijuana affects your driving and how officers enforce operating under the influence of this drug.


According to a recent Gallup poll , 70% of Americans reported thinking that driving under the influence of marijuana was “not much of a problem,” or a “somewhat serious problem.” By contrast, 79% of those surveyed thought driving under the influence of alcohol was a “very serious problem.” Only 29% thought driving after smoking marijuana was a “very serious problem.”

While the Centers for Disease Control and Prevention estimate that a third of all fatal crashes are due to alcohol impairment , data on marijuana impairment are less clear. Here’s one thing we do know: it’s safer to drive while not under the influence of any substance, though driving under the influence of marijuana may be safer than alcohol.

Marijuana intoxication does not have a robust body of research, since it has only recently been legalized in certain municipalities and states. A review of 60 studies presented at the International Conference on Alcohol, Drugs, and Traffic Safety in 1995 found marijuana does impair all of the cognitive abilities we need to drive safely, including tracking, visual function, divided attention, and motor coordination.

A 2009 study in the American Journal on Addictions found cognitive impairments from marijuana use led to a modest reduction in overall driving performance. A more recent study in the journal Drug and Alcohol Dependence found drivers who vaporized marijuana were more likely to weave in their own lane than the control group, but they were not more likely to swerve out of their lane or speed. By contrast, those under the influence of alcohol were more likely to perform all three.


In light of states like Colorado and Oregon passing recreational marijuana use laws, the National Institute on Drug Abuse, a division of the National Institutes of Health, began researching the link between marijuana use , cognitive impairment, and traffic accidents. They recruited 18 occasional marijuana smokers between the ages of 21 and 37, and then put them in a driving simulator at the University of Iowa. Each participant imbibed a different concentration of THC, alcohol, or placebo. Researchers tested over 250 parameters of driving ability.

The study revealed participants who had concentration of 13.1 ug/L showed impairments comparable to a driver who had an alcohol level of .08%, the legal limit. The current legal limit for THC concentration is 5 ug/L in Washington and Colorado.

The concern increases when alcohol and cannabis are imbibed together, which research shows is common. Drivers who used both weaved within their lane even when they were within the legal limit for both.


In Savannah and all over Georgia, it is unlawful to imbibe any illicit substance and get behind the wheel of a vehicle. The current .08% blood alcohol content standard is the result of years of evidence-based research – and some wonder if marijuana will follow suit.

For now, marijuana use remains illegal in Georgia. Anyone found possessing, smoking, or operating a motor vehicle while under the influence of marijuana may face fines and even jail time. Regardless, there is no safe established legal limit for cannabis. If you’ve recently smoked marijuana, don’t get behind the wheel of a car.