I've never understood the reason for doing this.
Any evening I go for a drive I can bet on seeing at least 5 or 6 people doing this - on dark country roads no less.

Fog lights aren't supposed to light up the road ahead, just the sides of the road as they are underneath the fog which generally starts a foot or so off the ground.

I've experimented a couple of times just using the fog lights on my car and you may as well not have any lights at all.
(Kliper Filllets Getting noticed by twats on the internet since, Mon 24 Jan 2011, 8:04, 1 reply, 14 years ago)

in thick fog
Out in the country, regular headlights create such a glare and full beam is just hopeless. For me, front spots and sidelights are best
( sandettie light vessel automatic New Twitter - @bollocksreally, Tue 25 Jan 2011, 0:58, Reply)

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)