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要旨:
Perceived speed of moving patterns is lower when visual contrast is reduced. In recent driving simulation experiments, this has been presented as an explanation why drivers might drive too fast in fog. In this simulation, contrast was equally reduced for all objects in the scene, independently of their distance from observer (distance-independent contrast reduction). In fog, however, contrast is reduced more for more distant objects (distance-dependent contrast reduction). We compared the effects of these two types of contrast reduction on both perceived and actual driving speed. In the first experiment twelve participants were presented with pairs of driving scenes—one with clear visibility, one with reduced contrast—and instructed to estimate which scene moved faster. Speed was underestimated with distance-independent contrast reduction but overestimated with distance-dependent contrast reduction. In the second experiment, ten drivers drove at target speeds under clear and reduced visibility conditions without tachometer. Participants increased speed with distance-independent contrast reduction but decreased speed with distance-dependent contrast reduction. These results show how visibility loss can lead to opposite perceptual and behavioural effects, depending on the nature of the underlying visual contrast reduction. In fog, indeed, the visual system fools drivers the safe way, advising them to decelerate.
