Stepping Feet Illusion

The stepping feet illusion is a motion-perception illusion in which two identically colored shapes, commonly described as a blue bus and a yellow bus, move at a constant, equal speed across a striped black-and-white background, yet appear to speed up and slow down as they pass over different stripes. The blue bus appears to move faster than its actual speed when it crosses the white stripes, where the contrast against its own color is higher, and appears to slow down against the black stripes, where the contrast is lower, while the yellow bus shows the opposite pattern, because higher-contrast movement is perceived as faster than lower-contrast movement at the same true speed. The psychologist Stuart Anstis first demonstrated the illusion in 2003 and proposed that the same contrast-based misjudgment of speed happens in real driving conditions, particularly in fog, where reduced brightness contrast between a vehicle and its surroundings can cause a driver to perceive another vehicle as moving more slowly than it actually is. The illusion takes its popular name from the way the two shifting shapes resemble a pair of stepping feet, and the effect disappears entirely once the striped texture of the background is removed.