Who's stoned? It could be a serious question for authorities in Washington. | Health

How do you tell if someone is too stoned to drive? It's a serious question that Washington law enforcement is going to have to find an answer to if voters approve legalizing the drug for recreational use this Fall.

States that allow medical marijuana have struggled with determining impairment levels for years.

Authorities envision a legal threshold for pot that would be comparable to the blood-alcohol standard used to determine drunken driving.

The only problem: Unlike alcohol, marijuana stays in the blood for hours, long after the high wears off. There is no quick test to determine someone's level of impairment, but scientists are working on it.