I could foresee a day when marijuana would be legal, and there would be limits associated with it, much like alcohol. I don't know if they have a cost effective test that will tell them if say you've smoked in the last 12 hours or if it's currently affecting you. I imagine the test would have to come out before anything like what I described occurring.
Pretty easy. They would open their trunk and present you with a case of mountain dew and three bags of cheese puffs.
If you moved to sample any, BOOM, you're stoned and off to jail!
Damn cost effective test (($6.00 case of dew, 3 @ $0.99 cheese puffs)