Effects of Reduced Contrast on the Perception and Control of Speed When Driving

Abstract

Misperception of speed under low-contrast conditions has been identified as a possible contributor to motor vehicle crashes in fog. To test this hypothesis, we investigated the effects of reduced contrast on drivers' perception and control of speed while driving under real-world conditions. Fourteen participants drove around a 2.85 km closed road course under three visual conditions: clear view and with two levels of reduced contrast created by diffusing filters on the windscreen and side windows. Three dependent measures were obtained, without view of the speedometer, on separate laps around the road course: verbal estimates of speed; adjustment of speed to instructed levels (25 to 70 km h(-1)); and estimation of minimum stopping distance. The results showed that drivers traveled more slowly under low-contrast conditions. Reduced contrast had little or no effect on either verbal judgments of speed or estimates of minimum stopping distance. Speed adjustments were significantly slower under low-contrast than clear conditions, indicating that, contrary to studies of object motion, drivers perceived themselves to be traveling faster under conditions of reduced contrast. Under real-world driving conditions, drivers' ability to perceive and control their speed was not adversely affected by large variations in the contrast of their surroundings. These findings suggest that perceptions of self-motion and object motion involve neural processes that are differentially affected by variations in stimulus contrast as encountered in fog.

Foggy perception slows us down

“…Previous studies suggested that the speed of visual motion in depth is underestimated when the global level of contrast is reduced (Snowden et al, 1998; Horswill and Plooy, 2008; Owens et al, 2010). These studies were directly inspired by classical vision research experiments that assessed the effect of contrast on the perceived speed of two-dimensional objects on a monitor (Thompson, 1982; Stone and Thompson, 1992; Blakemore and Snowden, 1999; Anstis, 2003).…”

“…These studies have shown that the perceived speed of two-dimensional moving objects—for example, plaid patterns on a computer screen—is underestimated when visual contrast is reduced. More recent studies based on driving scenarios have suggested that the underestimation of visual speed at low contrast applies also to perceived self-motion in three-dimensional environments (Snowden et al, 1998; Horswill and Plooy, 2008; Owens et al, 2010). This finding was proposed—and is still considered—as a possible explanation for excessive driving speed in fog.…”

“…Here, we tested the perceptual and behavioural effects of distance-dependent contrast reduction—that is, fog—on speed perception. We compared these effects with those of the distance-independent—that is, uniform—contrast reduction that has been used in previous studies to simulate fog (Snowden et al, 1998; Horswill and Plooy, 2008; Owens et al, 2010). To perform these experiments, we used a state-of-the-art virtual reality setup allowing us to realistically simulate fog in an ecological driving scenario (Loomis et al, 1999).…”

Glaucoma and Driving Risk under Simulated Fog Conditions

“…Previous studies have suggested that drivers tend to underestimate their speed when driving under fog conditions. 16−18 In foggy conditions, drivers may think they are driving more slowly than they actually are and, therefore, increase their speed. Due to their reduced field of view, speed misestimation could be even worse for glaucoma patients.…”

“…15 Previous studies have suggested that drivers tend to underestimate their speed when driving in fog conditions and, thus, drive too fast for the available preview. 16−18 This occurs because a reduction in optic flow and scene contrast suppress perception of speed under these adverse conditions. Since fog causes a reduction in contrast of the visual field, the apparent speed of the vehicle slows, causing the individuals to drive faster.…”