Exactly
That's the only time they're any use. In thick fog (and it's got to be fairly thick at that).

It's similar to rear high-intensity lights. Once visibility is clear for more than 100 yards or so, they aren't much use to anyone behine you, and you should certainly turn them off in close, slow moving traffic.

(Although if someone behind me is using their front fog lights when it's clear, I assume they think it's foggy and may just turn my rear fog lights on until they get the message)
(Kliper Filllets Getting noticed by twats on the internet since, Tue 25 Jan 2011, 7:41, Reply)