Sean McAllister, a Denver criminal defense attorney, says he’s getting five calls a week from clients with pot DUI citations.

“If you were talking about this concept with alcohol and told people, ‘We got a test that can say if you drank in the last 24 hours and if you fail it we’re going to arrest you for DUI,’ we would be occupying the Capitol right now,” McAllister says.

Users don’t have guideposts for marijuana impairment, he says. Is half a joint too much? What about two or five bong rips? No one is sure.

It doesn’t help that marijuana doesn’t metabolize predictably like alcohol, says John Lacey, a traffic safety expert based in Maryland.