Driving under light and dark conditions: effects of alcohol and diazepam in young and older subjects

Objectives: Driving at night time increases accident risk due to visual con ditions, fatigue and impaired performance. In addition, the use of alcohol and benzodiazepines may enhance the risks related to nighttime driving. We studied these aspects of traffic safety in a simulated driving test with yo ung and older drivers. Methods: In a double-blind, crossover, placebo-controlled study, nine young subjects, aged 22-24 years, performed simulated driving in both 'light' an d 'dark' conditions, before and 1.5 h and 4 h after 0.8 g . kg(-1) ethanol (EOH) or 15 mg diazepam (DZ). Further, nine older subjects, aged 55-77 year s, were similarly tested, but their EOH dose was 0.7 g . kg(-1) and the DZ dose was 10 mg. The tests were vigilance assessment on visual analogue scal es (VAS), simulated driving under light and dark conditions for 6 min each and digit symbol substitution (DSS). Results: In the young subjects, both EOH and DZ similarly impaired DSS, wit h DZ causing more subjective drowsiness, clumsiness, mental slowness and po or overall performance than EOH. During simulated driving, both EOH and DZ impaired simple and complex tracking (EOH > DZ) and prolonged reaction time s (EOH = DZ). Impairment of performance was practically identical under lig ht and dark conditions. In the older subjects, objective performance on DSS was poorer (-30%) than that of the young ones, and subjective impairment w as marginal. During simulated driving, the baseline levels of variables in older subjects showed definite impairment (errors +100% to +500%) when comp ared with young subjects. Active drugs impaired several variables (EOH > DZ ), but the statistical significances were fewer than in young subjects. Inc rease in reaction errors reached statistical significance, especially while driving in the dark. Otherwise the driving results in light and dark were not notably different. Conclusion: Young subjects drew good baselines but were sensitive to EOH an d DZ, whilst the older subjects showed poor baselines but were less sensiti ve to EOH and DZ. Although the baseline driving and responses to treatments were different in young and older subjects, their driving and psychomotor impairment were unaffected by light conditions.