Operating an automobile safely is difficult enough under the best conditions, but add the effects of alcohol or other mind altering chemicals and the task can become nearly impossible. A large portion of traffic deaths and serious accidents are caused by alcohol or drug-impaired drivers, although because of their loose state, many of them survive the incidents without serious injury. This has led all 50 states to establish strict laws concerning drivers who operate vehicles while under the influence of drugs or alcohol. In some states, violation of these laws is called DUI, ordriving under the influence. Other states refer to this violation as DWI, short for Driving While Intoxicated.

Many state DUI laws were developed in response to federal highway funding mandates. In order to qualify for federal road maintenance revenues, individual states were compelled to define a legal age for alcohol consumption and laws defining the legal level of intoxication (alcohol blood level) for drivers. Most states established a legal drinking age of 21 years, although some allowed the consumption of low-alcohol beers and wines at age 18. Later federal highway mandates encouraged all states to agree to a uniform drinking age of 21.

If a law enforcement officer observes suspicious driving behavior, they can legally perform a traffic stop of that vehicle. A strong odor of alcohol, slurred speech or general incoherence can lead to a field sobriety test to roughly determine intoxication. A DUI charge cannot be issued through suspicion alone, however.

If the driver fails to demonstrate the capacity for rational thinking and motor skills during these field tests, the officer can then ask permission to perform a blood alcohol content test, commonly abbreviated BAC. In a DUI case, the test must show a percentage of alcohol in the driver's bloodstream over a legal limit. In many states, this legal limit for DUI is .10% BAC, although many states have adopted a lower .08% BAC.

The legal blood alcohol level for drivers under the legal drinking age can be as high as .02% or as low as absolute zero. If any driver demonstrates a BAC over the legal DUI limit, he or she can be charged with a DUI immediately and booked into a holding cell for at least enough time to become sober again. Some law enforcement officers may use a special breathalyzer to determine BAC, which is seen as less invasive than the standard blood test. Because a person's BAC lowers over time as the alcohol is processed by the body, police officers must gather evidence quickly to establish a DUI charge. DUI suspects may also refuse to take a BAC test or call a lawyer for legal protection from interrogation.

Many states have a progressive DUI policy, which generally means that a first-time offender may not receive the maximum penalty allowed under the law. A judge can use some discretion when sentencing those convicted of DUI, especially if the accused pleads guilty and demonstrates remorse for his or her actions. Generally speaking, a first DUI conviction could lead to a revoked driver's license for up to a year, a substantial fine and/or community service, and several penalty points when your driving rights are eventually restored. More serious DUI penalties could include prison time, court-ordered alcohol rehabilitation and a lifetime suspension of your driver's license.

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