While some young people believe marijuana use has little-to-no effect on driving ability, a new study found that marijuana use impairs driving similar to alcohol use. The study, funded by the National Institute on Drug Abuse (NIDA), the Office on National Drug Control Policy (ONDCP), and the National Highway Traffic Safety Administration (NHTSA), was released this week and published in the journal Drug and Alcohol Dependence.

Using a driving simulator, researchers found that marijuana use impairs one measure of driving performance. People driving with blood concentrations of 13.1 µg/L THC – the main psychoactive ingredient in marijuana – showed increased weaving within the lane, similar to those with 0.08 breath alcohol, the threshold for impaired driving in many states.

The study found that drinking alcohol and smoking marijuana had an additive effect, so that drivers using both substances weaved within lanes even if their blood THC and alcohol concentrations were below the impairment thresholds for each substance alone.