Which of these "buses" is moving faster?
While they appear to be in a "race," they are actually moving at the same speed, as revealed when the background becomes solid gray. This GIF is a fine example of the Stepping Feet Illusion. From Wikipedia:
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. The opposite effects occur for the yellow bus. The two buses resemble a pair of moving of feet, giving the illusion its name.
According to a 2003 paper by UC San Diego psychologist Stuart Anstis, the illusion is why moving cars sometimes appear to be "slowing down in foggy conditions, because low contrast reduces perceived speed."
(via Cliff Pickover)