When combined speeds of collisions (that is, when the speeds of the two vehicles combined) get above 60 mph things get ugly. I don't need physics to know that the top of a skull flys about 20' from the vehicle, or that the eyes pop out of their sockets, or that the human neck snaps and there's not one damned thing first responders can do about it other than help the living, if any are around. Have a nice day, all, and slow down.