To maintain currency on the research in various areas of impaired driving problem, the National Highway Traffic Safety Administration (NHTSA) periodically conducts reviews of the research literature. This new review examined 112 studies conducted from 1981 to 1998 on driving related behaviors under low BACs focusing on two general questions:

1. What was the lowest BAC at which impairment reliably occurred?

2. What was the threshold of impairment for each of twelve separate behavioral areas?

The authors found that at BACs of .05, 34 percent of the studies reported impairment, and at BACs of .08, 94 percent of the studies reported impairment. They also found the threshold of impairment depended on the measure used, with some measures like divided attention showing impairment as low as .01 BAC and others, such as choice reaction time not showing impairment until BACs of .06 and above were reached.

Behavioral Areas of Impairment Summarized

Divided Attention
These tasks measure a person's ability to divide attention between two tasks at the same time, such as maintaining lane position and visual search. They are sensitive to alcohol effects. Impairment begins as low as .005 BAC.

Vigilance
These measures typically require a person to press a switch as rapidly as possible when a tone is heard. At BACs of .03 and above, impairment was consistently reported across all of the studies reviewed.

Tracking
These tasks require a person to match the movement of a stimulus in some way. Impairment is found at BACs as low as .0018 and consistently, depending on the task, at .005 BAC.

Perception
Studies in this category varied widely, from time estimation, auditory signal detection, visual search, pattern recognition, and traffic hazard perception. In general, impairment occurs at BACs around .08.

Visual Functions
These include studies of visual acuity, contrast sensitivity, oculomotor control and in general, impairment begins around BACs of .03.

Cognitive Tasks
These tasks also varied widely in information processing characteristics. Backward masking impairment was found at .03 BAC, digit-symbol substitution impairment was found at .08 BAC, memory tests found impairment beginning at .06 BAC.

Psychomotor Skills
The ability to maintain body balance was significantly impaired by alcohol at BACs of .04. At BACs of .08, 100 percent of the studies showed impairment. Performance of workplace tasks was a function of the difficulty and complexity of the task (operating a drill press was not affected until BACs of .06 were reached, whereas assembling electronic parts was impaired at .049 BAC).

Reaction Time
Simple reaction time is not affected by alcohol, but choice reaction time, where a person has several response possibilities, shows impairment at .06 BAC.

Critical Flicker Fusion
This measure has often been used in studies of psychoactive drugs. It is the point where a person perceives that a flickering light is constant. It, however, is an insensitive measure for alcohol, and impairment does not appear until .10 BAC.

Aftereffects
These measure the lingering effects of impairment after BAC has returned to zero. Six of 25 tests showed alcohol impairment; 19 tests did not.

Wakefulness
Although not a measure of skills performance, wakefulness is a requirement for safe driving. Wakefulness tests were found to be very sensitive to the effects of alcohol. The time to fall asleep was shorter with BACs of .10 and higher.

Conclusions

All driving-related skills showed impairment by .07 BAC, with the exception of simple reaction time and critical flicker fusion. In studies that only involved simulators (or on-road driving), divided attention, vigilance, and simulated piloting, 73 percent of the tests showed impairment by .039 BAC. Including tracking and wakefulness, 65 percent of the tests performed at BACs of .039 BAC and above showed impairment.

HOW TO ORDER

For a copy of A Review of the Literature on the Effects of Low Doses of Alcohol on Driving-Related Skills (58 pages), write to the Office of Research and Traffic Records, NHTSA, NTS-31, 400 Seventh Street, S.W., Washington, DC 20590 or send a fax to (202) 366-7096. Paul Tremont, Ph.D., was the contract manager.

U.S. Department
of Transportation
National Highway
Traffic Safety
Administration
400 Seventh Street, S.W. NTS-31
Washington, DC 20590

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