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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 (1).
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 (2). 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 (3). 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 (3-5). 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 (3,4,6,7).
Research has documented that the risk of a motor vehicle crash increases as BAC increases (3,4,8) and that the more demanding the driving task, the greater the impairment caused by low doses of alcohol (3). 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 (8).
Youth. Youthful age has been cited as one of the most important variables related to crash risk (9). Young drivers are inexperienced not only in driving but in drinking and in combining the two activities (9). In 1994, almost 7,800 persons ages 16 through 20 were drivers in fatal motor vehicle crashes (10). 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 (10).
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 (11). 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 (9).
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 (9). In one study, Hingson and colleagues concluded that drivers in th is age group have a greater risk than older drivers of being involved in a fatal crash even with a BAC of 0.00 percent (11). Young people's lack of driving experience renders them less likely than more experienced drivers to cope successfully with hazardous situations (9). 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 (12,13).
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 (10). 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 (8,14). Research shows that women metabolize alcohol differently from men, causing women to reach higher BAC's at the same doses (4,15). However, laboratory studies of alcohol impairment of driving skills among women are rare and the results are inconclusive (6).
Combining medications with alcohol and driving. Combining certain medications with alcohol increases crash risk. Sedatives and tranquilizers alone can impair driving skills (16) and can impair them even more when combined with alcohol (17-20). For example, low doses of flurazepam, a sedative-hypnotic prescribed for the treatment of insomnia, alone can impair a driver's ability to steer. The effect of this medication can be compounded with even a small dose of alcohol consumed the next morning (20). Driving skills can be impaired by other medications, such as codeine, as prescribed to treat moderately severe pain (20). When combined with alcohol, such medications' adverse effects on driving skills are exacerbated, as are the effects of some antidepressants, most antihistamines, certain cardiovascular medications, and some antipsychotic medications (20).
Alcohol tolerance. The repeated performance of a particular task in association with alcohol consumption can lead to the development of a form of adaptation referred to as "learned" or "behavioral" tolerance (21). Learned tolerance can reduce the
alcohol-induced impairment that would ordinarily accompany the performance of that particular task (21). However, when conditions change or when something unexpected occurs, the tolerance acquired for that task can be negated (22).
These findings may be applicable to the performance of tasks involved in drinking and driving (21,23). A driver who has developed behavioral tolerance to driving a familiar car over a particular route under routine circumstances may drive without being involved in a crash, despite consumption of some alcohol (21,23). However, when encountering a novel environment--for example, a detour--or an unexpected situation, such as a bicycle darting in front of the car, this same driver would be at the same risk for a crash as a novice driver at the same BAC, due to lack of prior learning opportunities for these unexpected events.
Legal sanctions, such as driver's license suspension and court-ordered alcoholism treatment, are designed to deter drinking and driving (24). Driver's license suspension and license revocation seem to be the most effective deterrents among the general driving population (24). However, a meta-analysis of deterrent strategies targeted to the drinking-and-driving population concluded that the most effective means for reducing rearrest for driving under the influence of alcohol (DUI) and crashes was a combination of license suspension and interventions such as education, psychotherapy/ counseling, and some followup (25).
Researchers contend that court-ordered treatment should be considered an adjunct, not an alternative, to license sanctions (24). According to Sadler and colleagues, a DUI conviction should serve to identify problem drinkers and guide or coerce them into alcohol treatment (26). Alcohol treatment for DUI offenders can range from short-term educational sessions to therapy programs lasting at least 1 year (24).
Treatment of convicted drinking drivers normally emphasizes modifying drinking behavior (24). The type and duration of treatment depend on factors such as the severity of the person's drinking problem and DUI history (24,27). DUI offenders with less severe drinking problems benefit most from educational programs (24,27), although no known model is thought to be most effective (24) in reducing recidivism or alcohol-related crashes. For repeat offenders or those with more seve re drinking problems, therapy that lasts for at least 12 months (24) and that includes intensive programs focused on the individual appears to be most effective (27).
The National Highway Traffic Safety Administration (NHTSA) credits State laws raising the legal drinking age to 21 with preventing almost 1,000 traffic deaths annually (11). Legislation to reduce the BAC limit to 0.02 percent or lower, referred to as the "zero tolerance law" for young drivers, has been passed by 29 States and the District of Columbia to reduce alcohol-related fatalities further (10,11). The National Highway Systems Act provides incentives for all States to reduce their BAC limits for drivers under 21 to 0.02 percent beginning October 1, 1998.
One study (11) examined the effectiveness of lowering BAC limits for young people in States where such laws have been in force for at least 1 year. The researchers found that after the BAC limits were lowered to 0.00 or 0.02 percent, the proportion of nighttime fatal crashes involving single vehicles in this age group dropped 16 percent.
Drinking and Driving--A Commentary by
NIAAA Director Enoch Gordis, M.D.
Progress has been made in reducing the consequences of drinking and driving; the percent of alcohol-related crash fatalities has declined from 43.6 percent of the total number of traffic crash fatalities in 1986 to 37.4 percent in 1992. Advances in technology (i.e., automobile engineering and road design), less public acceptance of drinking drivers, decreases in per capita consumption, and a growing willingness by the States to adopt public policies aimed at preventing alcohol-related injuries and deaths and enforce legal sanctions against drinking drivers may all be factors in this decline. Newer policies, such as the mandated "zero tolerance" for underage youth, have been shown to reduce crashes in this vulnerable age group. Additionally, increased attention to prevention programs that both impact on and affect adult behavior, such as server training, the designated driver concept, and intervention and education programs in secondary schools and colleges, have demonstrated some effectiveness in reducing alcohol-related driving fatalities.
While we have made progress, drinking and driving still claims about 15,000 lives annually. A variety of public policies, including law enforcement, prevention, and treatment efforts aimed at decreasing this unacceptably high rate, are being implemented by the States. Findings from research can provide information on which of these efforts, individually or in combination, are most effective in reducing drinking and driv-ing. For example, although license revocation combined with treatment has been shown to be effective in preventing repeat drinking and driving offenses, we do not yet know specifically which types of treatment are the most effective with which types of offenders.
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