In every state, it is a crime for a driver to operate a vehicle while impaired by the effects of alcohol or drugs. The specific offense may be called driving under the influence (DUI), driving while intoxicated (DWI), operating under the influence (OUI), and even operating a motor vehicle intoxicated (OMVI). Whatever the specific title, DUI laws make it unlawful for a person to operate a car, truck, motorcycle, or commercial vehicle if:

• The driver's ability to safely operate the vehicle is impaired by the effects of alcohol, illegal drugs, prescribed medications such as painkillers, or even over-the-counter medications such as antihistamines; or

• The driver is intoxicated at a level above established DUI standards, such as blood-alcohol concentration (BAC).

Field Sobriety and Chemical Tests

When a law enforcement officer makes a vehicle stop and suspects that the driver may be intoxicated, the officer will conduct a "field sobriety" test on the driver, and may ask for his or her consent to some form of chemical test for intoxication.

Field sobriety tests usually involve a police officer asking a driver to perform a number of tasks that assess any impairment of the person's physical or cognitive ability. Examples of field sobriety tests include having the driver walk a straight line, heel to toe; having he or she recite the alphabet backwards; and the officer's use of the "horizontal gaze nystagmus" (eye and penlight) test.

Chemical tests can be conducted during the vehicle stop, using a Breathalyzer that measures a driver's blood-alcohol concentration (BAC), or at a hospital, where urine and blood tests can be performed. Many states allow a driver suspected of DUI to choose which type of chemical test is administered.

Refusing a Chemical Test: "Implied Consent" Laws

All states have "implied consent" laws that require vehicle drivers to submit to some form of chemical test, such as breath, blood, or urine testing, if suspected of DUI. The logic behind such laws is that, by assuming the privilege of driving a vehicle on state roads and highways, drivers have effectively given their consent to DUI testing when a police officer reasonably believes the driver is under the influence of alcohol or drugs. If a driver refuses to submit to such testing, implied consent laws carry penalties such as mandatory suspension of a driver's license, usually for six months to a year. Often, license sanctions for test refusal are more harsh than those imposed after DUI test failure. In most states a driver's refusal to submit to a chemical test may be used to enhance the penalties imposed if he or she is eventually convicted for DUI.

For a state-by-state listing of laws associated with DUI, go here.

"Per Se" and "Zero Tolerance" DUI Laws

All states have DUI laws that deem "per se intoxicated" any driver with a blood-alcohol concentration (BAC) above a set limit (now .08 in all states). This means that drivers with a BAC at or above .08 are intoxicated in the eyes of the law, and no additional proof of driving impairment is necessary.

All states also carry "zero tolerance" laws that target drivers under the legal drinking age. These laws penalize persons under 21 for operating a vehicle with any trace of alcohol in their systems (a BAC above 0.0), or with negligible BAC levels such as .01 or .02.

For a state-by-state listing of laws associated with DUI, go here.

Keep in mind that a driver may still be arrested and convicted for DUI without proof of "per se" intoxication, when other evidence of impaired driving is shown. For example, a driver with a .06 BAC level can be found guilty of DUI if an arresting law enforcement officer testifies that he observed the driver's vehicle swerving badly, and that the driver exhibited both slurred speech and severe inattention during questioning after a vehicle stop.

DUI Convictions: Criminal Penalties

A DUI conviction may carry criminal penalties including fines, jail time, probation, and community service. Some state laws impose certain minimum penalties for first-time offenses, then designate increased penalties for each offense thereafter. Severity of criminal penalties will vary according to the circumstances of the offense, including:

• Whether the driver has a history of DUI violations;

• Whether the driver was operating a commercial vehicle at the time of the DUI;

• Whether the DUI violation occurred while there was a child in the vehicle;

• Whether the DUI violation occurred simultaneously with another dangerous moving violation, such as reckless driving;

• Whether the DUI violation involved a car accident in which property damage occurred;

• Whether the DUI violation involved a car accident in which another person was injured or killed; and

• Whether the driver was under the legal drinking age at the time of the DUI violation.

For a state-by-state listing of certain penalties associated with DUI, go here.