Have you noticed that at traffic lights people can’t press the button for the pedestrian crossing only once, but they feel the need to tap it several times? If the light doesn’t change immediately, they hit it like a budgie attacking a mirror?

Obviously, pressing the button doesn’t change the lights immediately; no one then notices that the pedestrian lights are incorporated into the normal flow of the traffic lights. The little green man only lights up when the pedestrian traffic will parallel that of the motorized sort.

Maybe aliens haven’t made contact with us because we lack the patience to wait for the other little green men who dwell among us.