A Houston based online news source, KHOU.com, recently ran a story on a DUI suspect who claimed that he thought his was friend was driving despite leading police on a small chase with two flat tires. Although he may have some issues backing up his facts, he certainly had the right legal argument.
Many times cases are dismissed because there is insufficient evidence to show that driver was actually driving the vehicle at the time of the stop. This is true, even where the defendant actually admits driving. The rule is called the corpus delecti rule. It essentially states that a defendant's admission to driving is not admissible unless the court is satisfied that there is independent evidence to show that the defendant was actually driving. The body of the crime can't just be their words alone. The idea is that we don't want to convict people on words when the crimes didn't really even happen.
Most of time this issue comes up in accident cases where the defendant is already standing outside of the car when the police arrive. In that situation, law enforcement will try to note the following factors: registered owner; location of keys; seat position; presence of other individuals at the scene; and whether one or both airbags are deployed. Believe or not, even in the most egregious cases, prosecutors are willing to negotiate and reduce charges if these factors are missing. However, it's a little less persuasive when the police see you sitting in the driver's seat.