Eighty-three experienced drivers, evenly distributed over the age range 25-65, each drove for one hour over a pre-determined route in a Swedish town. 51 were men, 32 were women. The age distribution was the same for each sex. Each subject was randomly assigned to one of two thermal conditions: 21 degree or 27 degrees. The conditions alternated throughout each day, and all took place in daylight. The route included sections limited to 50, 70, 90 & 100 km/hour. The vehicle appeared normal to the subjects, but was in fact modified so that 21 different signals could be given, all of the kind a driver must monitor: warning lights, changes in the reading of the instruments, spontaneous operation of some of the controls, extraneous noises at front or rear, flashing blue lights visible in the rear-view mirrors only. Subjects were told about possible signals only in very general way. A computer administrated the signals, which occured randomly with equal probability, each preceded by a delay of random length from 30-180 seconds following a response or failure to respond. The computer recorded response times and missed signals, and subjects' verbal responses were subsequently analysed from tape. In spite of large inter-individual differences, and large variability in response due to different traffic conditions, there were significant negative effects of moderate heat on vigilance. The overall proportion of missed signals increased by 50% at 27 degree, and response times increased by 22% There were also significant differences between men and women in response time and missed signals. (A)
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