Drowsy driving is a serious safety problem. A driver's vigilance declines not only with fatigue or sleep loss but also with a monotonous driving under low workload. However, the vigilance decline due to task monotony has not been analyzed comprehensively. In this study, a technique to evaluate vigilance level was developed and applied to determine the vigilance characteristics of 88 subjects during a simulated driving task. First, an indicator of vigilance level was defined based on the change in reaction time (reaction delay time) during a choice-reaction task that simulates car following on a highway. Then, by a driving simulator experiment, the reaction delay time was correlated with the danger level for a lane departure to quantitatively determine the subject's vigilance level. Next, by conducting another simulated driving experiment, large reaction delays, which would cause a lane departure, were observed in more than 70 percent of the subjects. These delays tended to appear in early stages of the task, and remained throughout the task. The subjects were categorized into five groups by the change patterns of the reaction delay, reflecting individual differences. Finally, a correlation is suggested between the vigilance decline level and the 'extrovert level' of the subject determined by a personality test.
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