I looked it up for California and the best I could find is that if someone stops or slows inappropriately they could bear partial blame. Partial as in not all of it, so some of the blame stays on the person doing the rear ending. To me this infers that you are not following the book if you don't leave enough space to stop in time.

>What is it about driving that makes threads personal?

How did I make this personal? By using the pronoun "you"? I was doing that in a general sense and not targeted, which I thought you were doing as well in your own post.