“Drugs that can help reduce the symptoms of a disease can also affect a person’s ability to drive safely. That is why some prescription drugs come with warnings not to drive for a certain amount of time after taking them.
Cannabis impairs driving skills most severely during what is known as the acute phase, which typically lasts for up to 60 minutes after smoking.
That is followed by post-acute (the phase after the acute one) and residual phases. The residual phase is 150 minutes or more after smoking [marijuana], during which impairment subsides rapidly.
The degree of impairment during the residual phase depends on the amount of THC consumed. After smoking a so-called typical dose (about 20 mg) of THC, the residual phase lasts 2-3 hours.”Nov. 21, 2005