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Abstract

In the first experiment we investigated the effect of reduced visibility on the produced speed in a driving simulation. Participants were required to drive at a target speed of 90 km/h in different visibility conditions. We found that when realistic fog was simulated, the driving speed was reduced accordingly to the fog density. When a uniform reduction of the image contrast was implemented, no effects were observed on the produced speed. We speculated that fog reduces selectively the visibility of the distant region of the scene and leaves visible only the proximal area that contains high angular velocities. We hypothesized that the perceived speed is then biased by the available raw velocity signals from the visual field. In the second experiment we addressed the question whether the observed behavioral effect has indeed a perceptual origin. In a psychophysical task we asked the participants to estimate the speed of moving scenes when the sight was limited either to the periphery (high angular velocities) o r to the center (low angular velocities) of the field of view. According to our hypothesis, we found that when the central region was occluded, the speed at the periphery was perceived as being higher, and conversely, when the peripheral region was missing the speed at the center was perceived as being lower. We conclude that the speed reduction while driving in fog is due to a non-optimal perceptual compensation for the hidden central region with low angular velocities, which causes an overestimation of the driving speed.