What is DUI?
Driving under the influence means operating a motor vehicle while your ability to drive safely is impaired. This impairment comes from alcohol, drugs, or a combination of both. You can be charged with DUI even if your blood alcohol concentration is below the legal limit if an officer believes you are too impaired to drive safely.
Blood Alcohol Concentration (BAC) Standards
In all 50 U.S. states, a BAC of 0.08% or higher is considered legally impaired for drivers 21 years and older. For drivers under 21, the limit is much lower, typically 0.00% to 0.02%, depending on the state. Commercial drivers have an even stricter limit of 0.04%. BAC is measured by analyzing breath, blood, or urine samples.
Types of Impairment
DUI laws apply to impairment from any substance, not just alcohol. This includes illegal drugs like marijuana and cocaine, as well as prescription medications and over-the-counter drugs that affect driving ability. Some medications can impair judgment, reaction time, or coordination just as much as alcohol.
How DUI is Determined
Police officers use several methods to determine if someone is driving under the influence. These include field sobriety tests (like walking a straight line), breathalyzer tests that measure BAC, and blood or urine tests. Officers may also observe signs of impairment such as slurred speech, poor coordination, or the smell of alcohol.
Consequences of DUI
DUI convictions carry serious penalties including fines, driver's license suspension, jail time, mandatory alcohol education programs, and increased car insurance rates. A first-time offense typically results in lighter penalties than repeat offenses. DUI convictions remain on your record and can affect employment, housing, and educational opportunities. 
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