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 conditions, fatigue and impaired performance. In addition, the use of alcohol and benzodiazepines may enhance the risks related to night-time driving. We studied these aspects of traffic safety in a simulated driving test with young 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' and 'dark' conditions, before and 1.5 h and 4 h after 0.8 g x kg(-1) ethanol (EOH) or 15 mg diazepam (DZ). Further, nine older subjects, aged 55-77 years, were similarly tested, but their EOH dose was 0.7 g x kg(-1) and the DZ dose was 10 mg. The tests were vigilance assessment on visual analogue scales (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, with DZ causing more subjective drowsiness, clumsiness, mental slowness and poor overall performance than EOH. During simulated driving, both EOH and DZ impaired simple and complex tracking (EOH > DZ) and prolonged reaction times (EOH = DZ). Impairment of performance was practically identical under light and dark conditions. In the older subjects, objective performance on DSS was poorer (-30%) than that of the young ones, and subjective impairment was marginal. During simulated driving, the baseline levels of variables in older subjects showed definite impairment (errors +100% to +500%) when compared with young subjects. Active drugs impaired several variables (EOH > DZ), but the statistical significances were fewer than in young subjects. Increase 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 and DZ, whilst the older subjects showed poor baselines but were less sensitive 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.