Psychological fatigue is defined as a subjectively experienced disinclination to continue performing the task at hand. It generally impairs human efficiency when individuals continue working after they have become aware of their fatigue. It does not depend on energy expenditure and cannot be measured simply in terms of performance impairment. The interacting causal contributions to fatigue are the length of continuous work spells and daily duty periods, time available for rest and continuous sleep, and the arrangement of duty, rest, and sleep periods within each 24-h cycle. Empirical evidence for the separate and combined effects of these factors on fatigue, performance decrement, and accident risk are briefly reviewed, and the implications of these findings for driving and road safety are considered, with particular reference to the professional driver. This study shows that fatigue is insufficiently recognized and reported as a cause of road accidents and that its effects stem largely from prolonged and irregular working hours, rather than simply from time spent at the wheel.

Twenty-four men were given the task of judging whether to drive through gaps which might be larger or smaller than the car. They were also given a telephoning task of checking the accuracy of short sentences. Interference between the concurrently performed tasks was investigated. Telephoning mainly impaired judgments of 'impossible' gaps (p < .01). The control skills employed in steering through 'possible' gaps were not reliably degraded, although speed of driving was reduced (p < .01). Driving increased errors (p<.01) and prolonged response times (p < .005) on the sentence-checking task. It is concluded that telephoning has a minimal effect on the more automatized driving skills, but that perception and decision-making may be critically impaired by switching between visual and auditory inputs J The authors would like to thank the United Kingdom Ministry of Transport for providing some of the apparatus, most of Ss, and two research assistants. The General Post Office kindly provided the radiophone equipment and a technical assistant. The experimental car and financial assistance were supplied by the Medical Research Council. Thanks are also due to the Ministry of Defence (Air) for permitting part of an airfield to be used as a test track. The research was carried out uhder^the general direction of R. Conrad.

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