Officials are grappling with a problem no one's sure how to solve: How do you tell if someone is too stoned to drive?

States that allow medical marijuana have struggled with determining impairment levels for years. And voters in Colorado and Washington state will decide this fall whether to legalize the drug for recreational use, bringing a new urgency to the issue.

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

But unlike alcohol, marijuana stays in the blood long after the high wears off a few hours after use, and there is no quick test to determine someone's level of impairment - not that scientists haven't been working on it.