Disclaimer: I am not in any way saying that people should drink and drive. I have lost friends to drunk drivers and seen the toll a dui charge can have on a family, though never in my own family thankfully. I am saying that the law is subjective and the arrests are subjective and law enforcement could easily violate our rights if there are no checks and balances.
I have to share this with you. I served as a juror on a dui trial. The state's job is to prove beyond reasonable doubt that 1) the defendant was driving the vehicle and 2) while driving the vehicle the defendant was intoxicated to the point that he/she was not in possession of their normal facilities (the ability to walk, talk, drive, make clear decisions).
So, if you are sleeping in your car and an officer comes up, knocks on the window and proceeds to arrest you for DUI, they will have to prove BEYOND A REASONABLE DOUBT that you were driving while in that condition. If you weren't driving then take your chances on the justice system. I have learned in the past couple of years though that "the letter of the law" can be fickle, open to some interpretation, and that half of what is said can never be considered as evidence.
Personally, I think it is a crap shoot. Is the jury going to listen to the instructions and judge the case based SOLEY ON THE EVIDENCE AND TESTAMONY PROVIDED IN THE TRIAL ? It could go either way. Just because someone did ok on a field sobriety test does not mean that they were definitely not impared beyond their normal capacity. I've talked to someone who passed with flying colors though he knew he should not have been driving. On the flip side, just because a person has had a drink (could have been a glass of wine that has not impacted their normal facilities) or because a person smells of alcohol (could have been spilled on them), or doesn't speak clearly is not an automatic sign of guilt either. Smelling of alcohol does not impair your facilities, right?
In our case, the jury of his peers had no choice but to return a verdict of not guilty.