There are many factors including environmental, mechanical and human factors that cause driver fatigue during driving. Among these factors, vibration is one of the major factors because drivers are usually exposed to sustained mechanical vibrations inducing mental as well as physical stress. This paper reports the influence of vertical vibrations lasting for a long time on the driver fatigue. For the assessment of the driver fatigue accumulated over an expanded period of time, experiments have been conducted in three steps: the extraction of a valid index among physiological variables, a vibration-load test using a mechanical vibrator, and a road test using an actual car. Statistical analyses of the relations between subjective ratings of fatigue and physiological variables indicated urinary adrenaline as a valid index. The results of the vibration-load test in terms of urinary adrenaline suggested that the vertical vibration at low frequencies of 1 to 2 Hz, corresponding to the resonance of head, induced a driver to graduall accumulate fatiguey and certainly. This fact has never been reported before in riding comfort studies. Moreover, it was confirmed in the road test that the suspension control capable of reducing the vertical vibration of 1 to 2 Hz reduced driver fatigue at a statistically significant level in actual driving.
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