However, even drivers who know about the law and intend to abide by it may have no idea what such percentages mean. Following are some basics about BAC--and ways to prevent impaired driving.

BAC refers to a measurable amount of alcohol in your blood. This amount is measured in grams per deciliter (g/dl). A positive BAC--.01 g/dl or higher--indicates that alcohol was consumed. And according to the law, a BAC of .08 g/dl or more indicates intoxication.

Until 2005, many states defined intoxication as a BAC of .10. However, that percentage was lowered due to a key discovery: Drivers with BAC between .08 and .10 are at least 11 times more likely to be involved in a single-vehicle crash than drivers with a BAC of zero.

BAC depends on many factors. These include your weight, gender, how much alcohol you consume, how much time you spend drinking, and how much food is in your stomach at the time.

"Three drinks over three hours should not impair a normal 150-pound man," says Stephen Simon, director of the Minnesota Criminal Justice System DWI Task Force. "However, that amount of drinking may impair a woman who weighs 100 pounds."

You cannot estimate your BAC. Since so many factors combine to determine BAC, don't try to estimate it yourself. Only a blood, urine, or breath test can pinpoint the exact amount. This fact is often ignored by people who have several drinks and then swear that they can "hold their liquor" well enough to drive. They forget that one sign of alcohol impairment is the inability to recognize their own impairment.

Impairment takes place at BAC levels below the legal limit. "Many crashes occur when a person driving a familiar route experiences some unexpected change, such as a car changing lanes, a child darting out into the street, or the car ahead of them stopping suddenly," says Simon. "Even the consumption of small amounts of alcohol detract from a driver's ability to respond quickly and appropriately to an emergency."

In short, you can endanger yourself and others by driving with a BAC of much less than .08. At .02 BAC, for example, drivers typically experience some loss of judgment and ability to divide their attention. At .05 BAC, they find it more difficult to steer and to visually track moving objects.

Also remember that drivers can be arrested with a BAC below .08 when a law enforcement officer has probable cause. Drivers who weave across lanes, stop short, or make exceptionally wide turns may find themselves in the trail of flashing red lights.

The bottom line: Don't drink and drive. If you plan to drink, then don't plan to drive. Instead, follow these suggestions from the National Highway Traffic Safety Administration (NHTSA):

• Choose a non-drinking friend as a designated driver.
• Ask ahead of time if you can stay over at your host's house.
• Take a taxi or check your local Yellow Pages for a free Safe Rides or Sober Ride program.
• Always wear your seat belt--your best defense against impaired drivers.

You can learn more about BAC from "The ABCs of BAC: A Guide to Understanding Blood Alcohol Concentration and Alcohol Impairment," a pamphlet published by the NHTSA. This publication is available online or by calling 1-888-327-4236.

--Published May 1, 2006