so you need a time machine? Accident happens, it goes back 5 mins in time and begins recording?
Also in most situations where you're bumped it would not be in the forward direction of the camera, the rear or sides. So really it is just for when you're moving and you drive into something in front. i.e. its YOUR fault. So then you'd want a camera to DELETE the video as it incriminates yourself. It would not be your fault if someone drove into you which is in general from the rear.