Drinking and Driving

Around the middle of the twentieth century the technical means became available to measure the quantity of alcohol in a person's blood (the blood alcohol concentration, or BAC, usually measured in terms of grams of alcohol per milliliter of blood). Laboratory research using this technology showed that at BAC levels much lower than those normally associated with intoxication, tasks related to driving performance (such as divided attention tasks) were noticeably affected. Although the effects of BAC depend on such factors as an individual's weight, rate of drinking, and presence of food in the stomach, deterioration in performance becomes quite marked between BACs of .05 and .08. As a guide, the average man would attain a BAC of .05 or higher if he drank three "standard drinks" (e.g., three mid-size glasses of mid-strength beer) within one hour, without eating.

The alcohol-crash link was confirmed in a series of case-control studies that compared the BACs of drivers experiencing crashes with those of matched non-crash-involved drivers. These studies found that relative crash risks increase exponentially with BAC: at .05 the risk is double that for a zero-BAC driver, at .08 the risk is multiplied by ten, while at .15 or higher (the levels typically attained by drivers arrested for drinking and driving) the relative risk is in the hundreds. The curve is even steeper for serious and fatal crashes, for single-vehicle crashes, and for young people.

While it is likely that factors other than alcohol, such as a propensity to take risks, contribute both to the levels of drinking and to crash involvement, there is a near universal consensus that there is a direct and causal link between alcohol consumption and crashes, especially serious crashes. For example, eliminating alcohol would probably have prevented about 47 percent of fatal crashes in the United States in 1987 (Evans).