There has been great concern that with increases in the number of places that allow legal marijuana use, incidents of people driving while under the influence of marijuana would see a corresponding increase. A study in New Zealand found that people who frequently used marijuana, especially before driving, were more likely to be injured in car accidents than those who did not. Another study in France showed that, even controlling for accidents in which alcohol played a part, marijuana users were twice as likely to cause fatal accidents than non-users.
A newer study, done by the National Highway Traffic Safety Administration and released in February 2015, concluded that there is no definitive link between using marijuana and an increased chance of being in an auto accident. When the study controlled for demographic variables, its results showed that marijuana users had a probability of being in an accident equal to that of drivers who had not used alcohol or other drugs. Regardless, the study still concluded that marijuana users have an overall 25 percent higher likelihood of being in auto accidents than non-users.
Guidelines regarding what level of THC is considered a DUI vary by state, and traces of THC may stay in the body for days or weeks after use (especially for frequent users). So avoiding driving soon after marijuana use is still a good idea.