That's right, and the driver of a vehicle being hit from behind does not always win a lawsuit.
Example: if a driver slams on brakes and stops very suddenly while driving on an interstate and is hit from behind, a jury might find that the driver created a "sudden emergency". It would be unreasonable to expect the second driver to stop just as suddenly.
It's hard to win with this defense, but it can be done. It's usually necessary to show that the plaintiff did not stop suddenly for a good reason (like another car stopped in the road), or in realistic terms, have a plaintiff that the jury hates and is looking for a reason to find against.
When I was a trial lawyer (yes, (hanging head in shame) I used to be a trial lawyer) I won a case where my client hit a lady from behind. I did some checking on her background, found out she had pulled the sudden stop stunt before (and gotten some bucks out of it) and managed to get all that in evidence before the jury.
It helped that she was an obnoxious federal employee.
I knew a lawyer once who also did a great job -- he got a charge of sodomy reduced to following too closely.