In a DUI case, you can expect for the prosecutor to call the arresting officer to the stand to testify about your driving the night of the incident. The police officer will discuss how fast (or slow) you were driving, whether you were weaving from one lane to the other, and whether you crossed the centerline. The officer will also bring up any other potential infractions or unusual occurrences such as running a red light or hesitation when going through a green light.
Your defense attorney should be able to argue that there are many different reasons why a person would drive like this that have nothing to do with intoxication.
The prosecutor will also ask the arresting officer to describe your appearance and behavior at the time of arrest. He or she may bring up that your speech was slurred, your eyes were bloodshot, you were stumbling, or your pupils were enlarged.
A good defense attorney knows that these signs may have nothing to do with being under the influence. For example, your allergies may have been acting up, you may have not had enough sleep, or your contact lenses may have been bothering you. Stress and nervousness may also produce the same signs as intoxication.
Field sobriety tests are often used as evidence in DUI cases. Unfortunately for the prosecutor, these tests are not scientific, and are in fact graded by the police officer. Many of the defenses that were listed above may also be used to explain why you did not pass a field sobriety test. There are also many physical problems that would make it difficult to pass these tests.
Blood alcohol tests are fraught with flaws and problems. If you have read my other entries on this blog, you will know that there are many shortcomings common with breath, blood, and urine tests.
Falsely high results could come from an officer being improperly trained on use of the breath-testing device, or the device itself being improperly maintained or calibrated.
As you can see, being charged with driving under the influence will not necessarily result in a conviction. These defense strategies, and many others, may lead to your acquittal.