The epidemiologic evidence linking alcohol and transportation accidents is supported by experimental studies of alcohol's effect on specific driving-related skills. Although the following discussion concentrates on highway traffic safety, most of the skills involved pertain to other forms of transportation as well. These skills may be divided into cognitive skills, such as information processing, and psychomotor skills (those involving eye-brain-hand coordination). Impairment is related to alcohol in terms of its concentration in the bloodstream. For reference, a blood alcohol concentration (BAC) of 0.04 percent might be achieved by a 150-pound man consuming two drinks in 1 hour.
The brain's control of eye movements is highly vulnerable to alcohol. In driving, the eyes must focus briefly on important objects in the visual field and track them as they (and the vehicle) move. Low to moderate BAC's (0.03 to 0.05 percent) interfere with voluntary eye movements, impairing the eye's ability to rapidly track a moving target.
Steering is a complex psychomotor task in which alcohol effects on eye-to-hand reaction time are superimposed upon the visual effects described above. Significant impairment in steering ability may begin as low as approximately 0.035 percent BAC and rises as BAC increases.
Alcohol impairs nearly every aspect of information processing by the brain. Alcohol-impaired drivers require more time to read a street sign or to respond to a traffic signal than unimpaired drivers; consequently, they tend to look at fewer sources of information. Research on the effects of alcohol on performance by both auto-mobile and aircraft operators shows a narrowing of the attentional field beginning at approximately 0.04 percent BAC.
The most sensitive aspect of driving performance is the division of attention among component skills. Drivers must maintain their vehicles in the proper lane and direction (a tracking task) while monitoring the environment for vital safety information, such as other vehicles, traffic signals, and pedestrians. Alcohol-impaired subjects who are required to divide their attention between two tasks tend to favor one of them. Therefore, alcohol-impaired drivers tend to concentrate on steering, becoming less vigilant with respect to safety information. Results of numerous studies indicate that divided attention deficits occur as low as 0.02 percent BAC.
The ability to divide attention is especially critical in aviation. Morrow and colleagues noted that radio communication during simulated flight was impaired significantly by divided attention deficit at BAC's as low as 0.04 percent.
The combined effects of these individual deficits on overall performance have been studied under simulated vehicle-operating conditions. A review of six ground-traffic simulator studies demonstrated consistently poorer performance at BAC's of 0.048 percent and above. In a typical study of the effects of pilot impairment, aircraft pilots completed eight sessions of simulated flight between San Francisco and Los Angeles in a Boeing 727-232 simulator. Planning and performance errors, procedural errors, and failures of vigilance each increased significantly with increasing BAC. Serious errors increased significantly at the lowest BAC, 0.025 percent, compared with performance at 0 percent BAC.
Results of epidemiologic and experimental studies permit certain conclusions to be drawn. First, the degree of impairment depends on the complexity of the task involved as well as the BAC. Second, the magnitude of alcohol-induced impairment rises as BAC increases and dissipates as alcohol is eliminated from the body. Third, at a given BAC, some skills are more impaired than others. Finally, investigators have not found an absolute BAC threshold below which there is no impairment of any kind. Certain skills important for driving are impaired at 0.01 to 0.02 percent BAC, the lowest levels that can be measured reliably by commonly used devices.
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