How drunk or high does someone have to be before he can be convicted of driving under the influence?

In most states, it's illegal to drive a car while "impaired" by the effects of alcohol or drugs (including prescription drugs). This means that there must be enough alcohol or drugs in the driver's body to prevent him from thinking clearly or driving safely. Many people reach this level well before they'd be considered "drunk" or "stoned."

How can the police find out whether a driver is under the influence?

Police typically use three methods of determining whether a driver has had too much to be driving:

• Observation. A police officer will pull you over if he notices that you are driving erratically -- swerving, speeding, failing to stop or even driving too slowly. Of course, you may have a good explanation for your driving (tiredness, for example), but an officer is unlikely to buy your story if he smells alcohol on your breath or notices slurred words or unsteady movements.
  
  
• Sobriety tests. If an officer suspects that you are under the influence, he will probably ask you to get out of the car and perform a series of balance and speech tests, such as standing on one leg, walking a straight line heel-to-toe or reciting a line of letters or numbers. The officer will look closely at your eyes, checking for pupil enlargement or constriction, which can be evidence of intoxication. If you fail these tests, the officer may arrest you or ask you to take a chemical test.
  
  
• Blood-alcohol level. The amount of alcohol in your body is understood by measuring the amount of alcohol in your blood. This measurement can be taken directly, by drawing a sample of your blood, or it can be calculated by applying a mathematical formula to the amount of alcohol in your breath or urine. Some states give you a choice of whether to take a breath, blood or urine test -- others do not. If you test at or above the level of intoxication for your state (.08 to .10 % blood-alcohol concentration, depending on the state), you are presumed to be driving under the influence unless you can convince a judge or jury that your judgment was not impaired and you were not driving dangerously. Defense attorneys often question the validity of the conversion formula when driver's alcohol levels are based on breath or urine tests.

Do I have to take a blood, breath or urine test if asked to do so by the police?

• No, but it may be in your best interests to take the test. Many states will automatically suspend your license for a year if you refuse to take a chemical test. And if your drunk driving case goes to trial, the prosecutor can tell the jury that you wouldn't take the test, which may lead the jury members to conclude that you refused because you were, in fact, drunk or stoned.