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Drinking and Driving

Driving involves multiple tasks, the demands of which can change continually. To drive safely, one must maintain alertness, make decisions based on ever-changing information present in the environment, and execute maneuvers based on these decisions. Drinking alcohol impairs a wide range of skills necessary for carrying out these tasks. This Alcohol Alert examines alcohol impairment of driving skills and describes some factors that increase motor vehicle crash risk.

Some Factors That Influence Crash Risk

Blood alcohol concentration. The proportion of alcohol to blood in the body is expressed as the blood alcohol concentration (BAC). In the field of traffic safety, BAC is expressed as the percentage of alcohol in deciliters of blood--for example, 0.10 percent (i.e., 0.10 grams per deciliter). A 160-pound man will have a BAC of approximately 0.04 percent 1 hour after consuming two 12-ounce beers or two other standard drinks on an empty stomach.

All State laws stipulate driver BAC limits, which now vary by State. According to these laws, operating a vehicle while having a BAC over the given limit is illegal. The BAC limit for drivers age 21 and older in most States is 0.10 percent, although some States have reduced the limit to 0.08 percent.

The many skills involved in driving are not all impaired at the same BAC's . For example, a driver's ability to divide attention between two or more sources of visual information can be impaired by BAC's of 0.02 percent or lower. However, it is not until BAC's of 0.05 percent or more are reached that impairment occurs consistently in eye movements, glare resistance, visual perception, reaction time, certain types of steering tasks, information processing, and other aspects of psychomotor performance.