How do prosecutors prove DWI cases?
Generally, prosecutors prove a DWI case under three methods. The first -- and easiest -- method for the prosecutor to convict a person is by showing that the Defendant's blood alcohol level was a .08 or more. (This is usually done by a breath test). Under this "per se" method, if the breath test reveals a blood alcohol content of 0.08 or more, then the prosecutor need not prove anything else. The person is, by law, considered guilty of DWI. Simply stated, at that point, instead of being innocent until proven guilty, the Defendant is presumed guilty and must then prove his innocence.
The other method is used if there is no breath test or other chemical evidence. The prosecutor must prove that the person is guilty by showing that the Defendant was "under the influence" of alcohol or drugs or "impaired" by alcohol or drugs. As stated in the introduction, the prosecutor will use driving behavior and the field sobriety tests to establish that the Defendant was under the influence.