Or the self-driving car in back could, you know, STOP before hitting the car in front of it. As for the car in front, even if there was enough gap to get up on the sidewalk, and the car didn't bottom out trying to do so, and it could rev up the curb in time to make a difference, and there was no pedestrian on the sidewalk to get run over (better to get rear-ended than to kill a pedestrian due to panic), the human in a properly stopped car at a red light would not be vigilantly looking back at every intersection for the one in a million chance of some moron about to run into him without even slowing down.