i found it already answered here
4 because 4 is godly
the psychological phenomenon that the child in the back seat is experiencing is a concept called optic flow.
to put it simply, optic flow is the visual motion that you experience as you move through the world. it refers to how you perceive things as you move around the world. the child is perceiving that the roadside is moving quickly because he or she is in a moving car, and thus it creates the perception that the roadside is moving faster than the buildings, which due to its size and position relative to the child, seems to move slower.