hir wrote:Nothing to do with overtaking.
OK. I'm not going mad. After some digging I now find numerous references in discussions on overtaking to this thought process / learning aid by Waremark on the old forum.
Waremark wrote:I say you should generally plan to be in by one-third of the distance you can see to be clear - one-third for the distance you will travel, one-third for the distance the person just out sight and coming towards you will travel, and one-third as a margin of safety and in case the one towards is going faster than you.
I've also found that David Heath from the IAM has been including it in his lectures on Overtaking while travelling around local groups, though I can find no slides or documents.