Individual and group effects of 10 mg diazepam on drivers' ability, confidence and willingness to act in a gap-judging task.

Abstract

It has been suggested that an increasing number of people drive after taking diazepam, and while the clinical use of anxiolytics is expected to improve the patients' everyday level of performance, experimental studies of the effects of diazepam on driving have variously indicated performance decrements, no effects, or performance increments. Possible reasons for this inconsistency are variation between and within subjects, and differential effects of the drug on the various components of task performance. These factors were investigated by an experiment designed to compare the effects of a single 10 mg oral dose of diazepam between and within groups and between and within individuals, and to examine effects of the drug on separate behavioural components of a driving task. Twenty drivers took part in the study, which assessed their ability, their degree of confidence in their ability, and their willingness to drive through various sizes of gap. The drivers were divided into two groups of ten, and were tested individually on each of 2 days. Group 1's treatment was placebo-diazepam, and group 2's was placebo-placebo. When analysed by group, the results showed a significant increase (P<0.01) in group 2's willingness, and significant decrease (P<0.001) in group 1's willingness to attempt gaps, which suggests greater caution under diazepam treatment. There was no change in either group's confidence, but group 1 showed a significant increase (P<0.05) in their ability variance. When analysed by individual, a wide variety of significant changes (P<0.05 or better) were found in ability and confidence for group 1 individuals, some changing for better, and some for worse, under diazepam treatment. No significant changes in ability or confidence were found for group 2 individuals. All group 1 individuals were less willing to attempt gaps under diazepam treatment, while all group 2 individuals were more willing to attempt gaps under continued placebo treatment. The validity of these findings is discussed, in terms of real-life driver behaviour, and it is concluded that there may be a wide inter-individual variation in the effects of diazepam not necessarily apparent in grouped data, and that diazepam may have differential effects on the decision-making and perceptual-motor components of driver behaviour in a gap-judging task.