Twenty subjects completed an on-the-road driving experiment, consisting of two different tests conducted on two separate days. A two-part test was administered while subjects were under the influence of alcohol (BAC < = 0.05%); a four-part test was administered without alcohol consisting of a 2.5 h driving test under vigilance conditions on a quiet highway. The order of the tests was balanced across subjects. Changes in relevant physiological parameters, such as ECG and EEG, reflected changes in driver status and predicted driving impairment. Impairment of driving performance was measured in a standard driving test (SD lateral position and SD steering wheel movements) and in a recently developed car-following test (reaction to speed changes of a leading car).