After their own independent intensive studies, the US government in 1993 and the British government in 2000 both concluded that marijuana intoxication does not seriously impair driving ability, primarily because marijuana does not impair vision and judgement the way that alcohol does. Judgement is the key issue, as the marijuana user is consciously aware of his impairment (unlike the alcohol user) and thus generally compensates by driving slower and more cautiously. It should be noted, however, that all DUI/DWI laws in the US do include marijuana and many prescription drugs as well as alcohol, and as such driving under the influence is a crime.
From the USDOT study:
U.S. Department of Transportation,
National Highway Traffic Safety Administration
(DOT HS 808 078), Final Report, November 1993:
"This program of research has shown that marijuana, when taken alone, produces a moderate degree of
driving impairment which is related to the consumed THC dose. The impairment manifests itself mainly in the
ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison
with changes produced by many medicinal drugs and alcohol. Drivers under the influence of marijuana retain
insight in their performance and will compensate, where they can, for example, by slowing down or increasing
effort. As a consequence, THC's adverse effects on driving performance appear relatively small."