Sometimes there are significant defenses which can result in the entire case being dismissed. More often there are smaller defenses which can put the defense in a better bargaining position. And of course, there are some cases where the prosecutor's side of the case is very good. Oftentimes this is a simple question to answer. In some cases, however, it is not so clear.
Some cases involve people who are sleeping in a vehicle.
Others involve people who are changing a flat tire or trying to get a vehicle out of a snowbank. In these cases the police will generally presume that the person was driving, or is in physical control of the vehicle, and will proceed with the arrest procedure. It is not legally required that the police officer actually witness a person operating the vehicle. If the police officer observes enough to reach a reasonable conclusion that the person operated the vehicle, he has the legal right to proceed with the arrest. The arrested person then has the opportunity to present any evidence available to show that he was not operating the vehicle.