I get it. You'd prefer to use the phrase "car length" to not literally mean "length of a car" in this context. Apologies if I seemed pedantic but, as evidenced by this thread, your use of car length is going against the grain.
Safe following distance is often expressed in multiples of car length. What car? Sure, good point. Pick one; it's close enough! (If you're actually thinking about following distance in any terms at all, even roughly estimated, you're ahead of the safety curve and you clearly are!)
Ideally, following distance could be accurately estimated in metres and adjusted continuously by velocity and driving conditions. AI can do that. People just kind of eye it and say, "it was raining and I followed about four car lengths behind him" or "we were inching along and I was half a car length behind him". Good enough for meat brains!
Safe driving!