've never forgotten is that if you ram into someone from behind IT'S YOUR FAULT EVERY TIME. Not the guy in front, no matter what unexpected or capricious thing he did

A very good rule of thumb, but of course there are still the odd exceptions.

I was following a car with a trailer at a roundabout around a decade ago. He decided to go, then changed his mind, and stopped abruptly. I stopped several feet behind him. The guy behind me also stopped with a couple of feet gap. Then someone behind him failed to stop, hit his car, shunted it forward into mine, and mine forward into the trailer. Some crumpling of the front of my car, but the trailer got away with a cracked light cover.

The insurance companies, police and indeed everyone else, declared the final driver the only one responsible.

Now of course you could argue if I had left an extra foot it would have prevented the crumpling and rear light damage. But none of the people I mentioned - drivers, police, insurance - thought my positioning at fault in any way.