Prosecuting attorneys can use many "tools" when they put on or negotiate a DWI case. For example, to show your BAC was above 0.08, they can use the results of tests and show a video demonstrating a defendant's slurred speech. Witnesses can testify as to a driver swerving and drinking constantly at a party before getting behind the wheel.
Defense lawyers have many techniques at their disposal too. There are many potential problems with BAC tests, for example, and the same goes for witness testimony. Here is an overview of two common reliability issues that witnesses in a DWI case might have.
Their sobriety levels
Suppose there is a witness who might potentially testify he or she saw you drink five glasses of wine in the span of one hour (or saw you take drugs while drinking). Was that witness drinking as well? How much? A defense lawyer might be able to cast enough doubt on the credibility of the witness because he or she was too compromised by alcohol to truly have tracked anyone's alcohol intake.
People assume; it is just in their nature. It is even possible for 10 people at a party to swear up and down their colleague drank constantly throughout the evening when, in fact, this colleague was only drinking club soda or another non-alcoholic beverage. And even if you had some alcohol at an event, you could have switched to water or something nonalcoholic, and people, especially if they were intoxicated, might have assumed you were still taking in liquor.
Who are witnesses testifying for?
Many times, witnesses are testifying for the prosecution. Sometimes, though, the defense calls them. A witness who has certain facts in mind could testify, for example, that it was club soda you had been drinking at the party, not vodka, as another witness said. Similarly, a witness who was in the car with you could testify your speech was clear and you were not intoxicated. Of course, the prosecution could call their reliability in question, too. Credibility issues can cut both ways.