Safety advocates worry that more drivers high on pot will lead to a big increase in traffic deaths as states liberalize their marijuana laws.
Researchers who have studied the issue are divided.
Marijuana can slow decision-making, decrease peripheral vision and impede multitasking -- all those are important driving skills.
But unlike with alcohol, stoned drivers tend to be aware they're impaired. And they try to compensate by driving slowly, avoiding risky actions such as passing other cars, and allowing extra room between vehicles.
Combining marijuana with alcohol appears to eliminate that caution and increase driving impairment beyond the effects of either substance alone.
Jonathan Adkins of the Governors Highway Safety Association says legalization of marijuana for recreational use in Colorado and Washington is a wake-up call for safety advocates.