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Sun~nzary.-A scudy was carried out to determine the influence of low concentrations of alcohol in the blood upon performance in a simulatecl clriving task, under day and night driving conditions. Sixteen male Ss were administered three orange drinks, the last two containing pure alcohol. Mean peak bloodalcohol concentrations of 0.012% and 0.068% were obtained. After each drink S war given some vision tests and then received three trials lasting 12 min, each on the simulator at daylight and night illumination, and with glare. Four Ss also served as conrrols. The larger dose s~gnlf~cantly reduced tracking accuracy under all three driving conditions. The smaller dose detrimentally affected performance in the glare condition but not in the other two treatments. Visual acuity was significantly decreased for the large dose. Ir was suggested that the legal limit describing alcoholic influence in driving was too conservative, and that it should be reduced in night driving conditions.

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Data are presented on the ability of drivers to perceive and scale the relative velocity between their own and a lead vehicle. Experiments were carried out on four groups of subjects using Ekman's ratio-rating method. Only when the subtended angular velocity of the lead vehicle exceeded about .003 rad/s were the subjects able to scale the relative velocity. The threshold subtended angular velocity obtained in the experiments was very much affected by the ability of subjects to use the concept of a ratio – engineering students found this a simpler task than did subjects from the general population. The result of this was that the values used by engineering students were closer to the real values. The relative velocity was perceived non-linearly, with a Stevens' power law exponent of about 0.8. It was found that linear models gave as good a fit to the data. The implications of the results of the experiments are (i) traffic flow models that include human visual characteristics must consider the ‘dead zones’ in response produced by thresholds of subtended angle change and subtended angular velocity, (ii) it may be necessary to consider the non-linear relationship between perceived relative velocity and actual relative velocity (iii) In overtaking, the driver will not be able to scale the speed of the oncoming vehicle as the subtended angular velocity will be below the threshold value at the time of making the decision to overtake.