How’d they do this?

[Steve Roberts]

My guess:

Since I don’t see any jump in the foliage at impact, I assume that we’re looking at footage of the dummy sitting in place with masked footage of the live person, shot previously, comped over the shot. He may have been sitting right next to the dummy, directed to match the dummy’s position so as to hide it from camera. When the car meets the dummy, the live person layer is removed.