Visibility was varying between 100-200 yards. Nearly all the cars had their headlights on; about half also had their fog lights on.
A few had no lights on at all. I assume these drivers are the ones who either think lights are for wimps, or who don't think at all.
But what bemuses me, every time, is the ones who have only their side-lights on. In fog, you can see a car before you see its sidelights. These drivers must have got as far as thinking "fog; put lights on" but not got as far as thinking "put headlights on because sidelights are invisible in fog, as I can clearly tell from all those other cars with only their sidelights on..."
I excuse those with sidelights comprising a little row of LEDs; I suspect these are visible from low earth orbit.
It's not just me. Highway Code rule 226 says:
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(Although I MUST say, translating the round number 100m to 328ft is ridiculously precise.)