Marijuana can stay in your bloodstream for two weeks, one month, even longer. (It's why your stoner friends tell you to give it a break for 2-4 weeks before a job interview).
So, let's say you're a perfectly legal medical marijuana user and you have a doctor's appointment two days from now. You put the pipe down and, on that morning, you drive. You're pulled over, your blood or urine is tested, and you're busted for DUI.
But you're not at all high.
http://blogs.laweekly.com/informer/2012/03/marijuana_dui_mandatory_blood_law_cal...
