When I served on a NYS Grand Jury a while back, we reviewed 51 cases in our month of duty. Most cases were drug and DWI/DUI related. Often times the defendant was charged with possession, but not DUI even if they were smoking pot when they were pulled over and arrested. The DA's explained that unlike alcohol there's no set standard on drug intoxication, and that in the field especially, drugs can't accurately be measured in the blood stream with respect to level and possible effects like alcohol can. Alcohol is well established but there's just way too many drugs and they all have their different cause and effect. Just saying that somone had been smoking whatever wasn't enough for them. So they basically never charged DUI in drug cases because they said it just "complicated" the matter. Oh, but when they could they WOULD toss in a few traffic charges such as speeding or failure to signal a left turn! Go figure. Now in situations in which the drug in question isn't illegal but you've admitted to driving after taking a few of whatever I'd have to believe it's a whole different ballgame.