Public officials and safety advocates worry there will be more drivers high on pot and a big increase in traffic deaths as states liberalize marijuana laws.
It's not clear, though, whether those concerns are merited. Researchers are divided on the question.
Studies of marijuana's effects show the drug can slow decision-making, decrease peripheral vision and impede multitasking. But drivers high on pot tend to be aware that they are impaired and try to compensate by driving slowly, avoiding risky actions and allowing extra room between vehicles.
On the other hand, combining marijuana with alcohol appears to eliminate the pot smoker's exaggerated caution and to increase driving impairment beyond the effects of either substance alone.
It is illegal in all states to drive while impaired by marijuana.