Some Effects of Alcohol on Car Driving on Two-Lane and Limited-Access Highways *

The purpose of this study was to investigate the effect of moderate doses of alcohol on a number of driving skills involved in steering and car-following, and passing decision-making. Forty drivers drove an instrumented vehicle on two-lane and limited-access highways either at night or in the daytime and either when sober or after having consumed alcohol to achieve a mean blood alcohol concentration of 0.085%.

Seven tests were conducted with each subject consisting of: passing gap time judgments, car-following with speed of lead-car constant, car-following with speed of lead-car varying, speed judgments, lateral position steering error, speed production, and speed maintenance.

It was found that subjects required about double the time gap to make a safe passing judgment at night than in the day, but no effect of alcohol was found. With the speed of a lead-car held constant, drivers under alcohol allowed a significantly greater headway than those of the placebo group. In addition, the variance in headway was significantly greater, both on the secondary road and the freeway, for the drinking drivers. When the speed of the lead-car was purposely varied in car-following, the mean headway and the variance in headway were significantly greater during the daytime than at night. In a limited set of the conditions there was also a significantly greater variance in headway attributable to alcohol. When subjects were requested to maintain the car at a fixed 80km/h over 11.3 km, with the speedometer covered, there was a significantly greater variance in speed for the alcohol group than the placebo group. There were no significant effects of alcohol found in: judgments of the speed of overtaking vehicles, lateral position deviations in steering, speed production without the use of the speedometer and the minimum safe time judged to be required to pass a vehicle ahead of them.