Here's an interesting article on DUI based on marijuana:Opposing Views: Obama Drug Policy: Smoke Pot Months Ago, Get a DUI
In Illinois, even the presence of a metabolite of marijuana in the blood or urine of a motorist is a criminal offense. A metabolite is an inert substance (meaning a substance that has no effect) that is left over in the blood or urine even after the active substance which might cause impairment is no longer present. This is an absurdity in Illinois DUI law which means that even if a driver smoked marijuana a day earlier and is not currently impaired or affected in any way while driving, he or she will still be found guilty of committing the offense of DUI.
Such a law may have made sense decades ago when more specific methods of testing for the presence of intoxicating substances like marijuana were not readily available, but nowadays, this law makes the "stupid" pile.