Rule 226
You MUST use headlights when visibility is seriously reduced, generally when you cannot see for more than 100 metres (328 feet). You may also use front or rear fog lights but you MUST switch them off when visibility improves (seeRule 236).
I call them poor visibility lights rather than fog lights. I've been on roads in the rain, especially abroad where the spray has been so bad that if you weren't using the rear PVL's you were in serious danger of some one driving into the back of you.
The problem is that people don't turn them off or think they are somehow more visible or can see more in normal visibility conditions. Some, I think leave them on thinking they look more sporty, Colin McRae - esque if you know what I mean. It's no fun having a 4x4 coming towards you with all lights blazing when you're in the VX. You can't even flash them with the weedy lights on the VX, they just think you've got duff headlights.