The crime of driving under the influence (often called DUI, drunk driving, or DWI depending on the state), actually encompasses two different acts, both of which can result in severe and often unexpected consequences. The first act, as the name implies, is driving under the influence. In every state, it is illegal to operate a motor vehicle if your reflexes, coordination, or general mental functioning has been impaired by alcohol or some other type of drug (whether legal or illegal). This rule applies regardless of how much alcohol or other intoxicating substance you have consumed. The second act covered by drunk driving laws is the act of driving with a blood alcohol content (b.a.c.) greater than .08%, which, for the average person, is the equivalent of having roughly four drinks in your system. This rule applies regardless of whether your coordination, reflexes or mental functioning have actually been impaired.

It is important to understand that only one of these conditions is necessary to be arrested and convicted for a DUI, not both. In other words, a person who has a b.a.c. of .04% (half the legal limit), but whose coordination and reflexes are still impaired by that small amount, would be guilty of a DUI. Similarly, a person who has a b.a.c. of .10%, but who was not impaired at all by that amount, would also be guilty of drunk driving. In addition, all states currently have “zero tolerance” laws for drivers under the age of 21. This means that for these individuals, driving with any amount of alcohol in the blood, no matter how tiny, is illegal.

In addition to criminal penalties, drunk driving cases almost always carry with them administrative sanctions and other indirect consequences.Many states automatically suspend the drivers license of individuals arrested for DUI, and the suspension can remain in effect even if the person is later acquitted or the charges are dropped. Drunk driving arrests are also a common basis for denying insurance coverage, or for raising premiums to extremely expensive levels. Those convicted of drunk driving are usually ordered to attend "DUI schools", which are a combination of alcohol rehabilitation and victim impact classes, in which offenders are confronted with first hand accounts of the dangers of drunk driving.

Field sobriety testing

Because there are two distinct ways to violate DUI laws, police officers who suspect a person of drunk driving will test for both. These tests are commonly referred to as “field sobriety tests”, and they are technically a criminal investigation like any other. Field sobriety tests usually begin with the police officer asking the driver to perform a series of activities that are designed to reveal whether his or her cognitive ability has been impaired. Common examples are asking the driver to walk a straight line, stand on one leg, or recite a portion of the alphabet. The “horizontal gaze nystagmus” test is another procedure that is widely used by officers because it tests an involuntary reaction, meaning that its results cannot be faked. The test involves the officer holding a pen or other object up to the driver’s eye level, and directing the driver to focus on the pen as the officer moves it to the edge of their vision. If the driver has been drinking (or taken other intoxicating substances), their eyeball will bounce or jerk when trying to focus near the edges of vision, and the officer will be able to tell that the person is drunk.

Even if a driver “passes” the cognitive field sobriety tests, officers will often ask for consent to conduct a chemical test in order to investigate the second basis for DUI, unlawful blood alcohol content. The most common investigative technique for this purpose is the “breathalyzer”. The breathalyzer is a portable testing device that can detect a person’s blood alcohol content when they blow into it. Although the results are not considered reliable enough to be used as evidence in most courts, they can provide probable cause to take the driver to a hospital for a urine or blood test, which are highly accurate and usually admissible.

Although field sobriety tests are technically criminal investigations, drunk driving suspects do not have exactly the same procedural rights as other criminal suspects. Unlike most criminal suspects who are generally free to refuse a search of their body or property without a warrant, DUI suspects are subject to “implied consent” laws, which require them to consent to chemical testing or face an automatic license suspension. States are legally able to do this because driving is not technically considered a constitutional right, so the states are free to limit the practice as they see fit. In DUI cases, this means attaching the condition that if an individual wants to drive in that state, they have to agree to give a chemical test if asked.