Horrifying pile up on the motorway

[BigV]

Nice chart.

You're friend's a menace.

The main reason we have troubles like these is because of drivers taking unexpected actions. In the OP, the unexpected action (by someone) was hard braking followed by being rear ended (repeat). Imagine someone running a red light, or changing lanes, or pulling out into traffic, etc. Those unexpected actions have a much higher likelihood of causing an accident than situations where drivers act in expected ways.

Having the driver ahead of me slow down, more and more and eventually stopping on the roadway (I'm presuming this is what you spoke of since if he'd pulled over to the shoulder passing him would have been the obvious choice and you said sometimes that didn't happen) is very unexpected behavior. Indeed the very act of slowing would at first exacerbate the very problem he is seeking to relieve.

Show him the chart and the suggestions to move over and let pass. Much more expected, much safer, and it completely solves his problem. Well, his tailgater problem. He's still a menace for stopping on the roadway for this kind of reason. But that's a different problem.