The stepping feet illusion is a motion perception phenomenon involving two "buses," one blue and one yellow, moving horizontally across a "street" consisting of black and white stripes. Although both of the buses move at a constant speed, their perceived speed varies dramatically.
When the blue bus lies on the white stripes, the contrast is high (dark blue vs. white) and easily visible, so it appears to move faster than its actual speed. On the contrary, when the blue bus is against the black stripes, the contrast is low (dark blue vs. black) and harder to see, so the movement looks slower (see figs. 1 and 2). The opposite effects occur for the yellow bus. The two buses resemble a pair of moving of feet, giving the illusion its name.
Overall, the higher-contrast movements look faster than those with lower contrast. The effect disappears when the street's striped texture is removed because there is no contrast remaining, showing how an object's background can have a significant effect on its perceived speed.