Most state laws define crimes of drunk driving as follows: driving a motor vehicle on a road or highway while under the influence of alcohol. Newer statutes also provide for a per se offense, which a person commits when driving a motor vehicle on a road or highway with a blood-alcohol concentration of .08 percent.

Several state statutes require that a defendant was driving a vehicle in order to be convicted of a drunk driving offense. Other states use the terms operating a vehicle or being in physical control of the vehicle. These terms are not normally synonymous, and so it is important to determine how an individual state defines the term in the statute.

A number of issues may arise that relate to the "driving" element of a drunk driving offense. For instance, a person may be in a car but has not turned on the ignition. The question in some cases is whether the person was driving or operating a vehicle or whether the person was using the vehicle as a temporary shelter. Courts in various jurisdictions have identified several factors that may be used to determine whether someone has been driving a vehicle. Some of these include the following:

Field evidence may fall into one of five categories, including the following:

• Testimony regarding the defendant's unusual driving
• Testimony regarding the defendant's conduct or physical appearance
• Incriminating statements made by the defendant
• Testimony regarding the defendant's performance during a field sobriety test
• Tapes, film, and/or photographs taken at the scene where the defendant was driving and/or arrested

Police officers will often look at the defendant's physical appearance and symptoms of drunk driving in order to determine whether the defendant is intoxicated. The following are some of the more common symptoms of intoxication:

• The defendant's clothes are disheveled
• The defendant has not shaved or combed his or her hair
• The defendant's eyes appear to be red, glassy, or bloodshot
• The defendant's face appears to be flushed
• The defendant's breath smells like alcohol
• The defendant's speech is thick and slurred

The defendant's BAC level will be determined through one of three methods. The most common of these methods involves an analysis of the defendant's breath. Other tests analyze the blood or urine of the defendant. Refinements in the methods by which a defendant's BAC is determined have strengthened the ability of prosecutors to prove this BAC. However, these tests are not above reproach, and skilled defense attorneys can often successfully attack the methods by which the defendant's BAC was analyzed.