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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.
Research has documented that the risk of a motor vehicle crash increases as BAC increases and that the more demanding the driving task, the greater the impairment caused by low doses of alcohol. Compared with drivers who have not consumed alcohol, the risk of a single-vehicle fatal crash for drivers with BAC's between 0.02 and 0.04 percent is estimated to be 1.4 times higher; for those with BAC's between 0.05 and 0.09 percent, 11.1 times higher; for drivers with BAC's between 0.10 and 0.14 percent, 48 times higher; and for those with BAC's at or above 0.15 percent, the risk is estimated to be 380 times higher.
Youth. Youthful age has been cited as one of the most important variables related to crash risk. Young drivers are inexperienced not only in driving but in drinking and in combining the two activities. In 1994, almost 7,800 persons ages 16 through 20 were drivers in fatal motor vehicle crashes. Twenty-three percent of these drivers, for whom drinking any quantity of alcohol is illegal, had BAC's of 0.01 percent or higher, compared with 26 percent of drivers age 21 and older.
According to Hingson and colleagues, each 0.02-percent increase in BAC above 0.00 percent places 16- to 20-year-old drivers at greater risk for a crash than older drivers. Roadside surveys indicate that young people are less likely than adults to drive after drinking; however, especially at low and moderate BAC's, their crash rates are substantially higher than those of other groups.
Driving inexperience and immaturity are considered to be the main causes of motor vehicle crashes among drivers ages 16 to 20, even when alcohol is not involved. In one study, Hingson and colleagues concluded that drivers in this age group have a greater risk than older drivers of being involved in a fatal crash even with a BAC of 0.00 percent. Young people's lack of driving experience renders them less likely than more experienced drivers to cope successfully with hazardous situations. This, combined with a penchant for risk-taking driving behavior such as speeding--along with a tendency both to underestimate the dangerous consequences of such behaviors and to overestimate their driving skill--contributes to the high crash rate among young drivers.
Gender. Twenty-nine percent of male drivers involved in fatal motor vehicle crashes had BAC's of 0.01 percent or greater, compared with 15 percent of female drivers. However, studies indicate that at BAC's ranging from 0.05 to 0.09 percent, crash risk may be greater for females than for males. Research shows that women metabolize alcohol differently from men, causing women to reach higher BAC's at the same doses. However, laboratory studies of alcohol impairment of driving skills among women are rare and the results are inconclusive.
For additional information please visit the MedicineNet.com Alcohol and Alcohol Abuse Center.
This information has been supplied in part by the National Institute of Health(NIH) and has been reproduced with its permission.
Last Editorial Review: 4/3/2002