The US Department of Transportation National Highway Traffic Safety Administration stated the following in its July 2017 report "Marijuana-Impaired Driving: A Report to Congress," available at

“Smoking marijuana has been shown to affect a number of driving-related skills. Laboratory, simulator and instrumented vehicle studies have shown that marijuana can impair critical abilities necessary for safe driving… An interesting finding from this research is that after smoking marijuana, subjects in most of the simulator and instrumented vehicle studies on marijuana and driving typically drive slower, follow other cars at greater distances, and take fewer risks than when sober.”

July 2017 - NHTSA Marijuana-Impaired Driving Report to Congress