Under what conditions must you switch on a lorry’s rear fog light when it is fitted?

Correct answer

In thick fog, very heavy rain or dense smoke, among other particularly adverse conditions.

Only in particularly adverse weather or environmental conditions, such as thick fog, very heavy rain, heavy snowfall or dense dust or smoke clouds, or equivalent conditions. Light fog or thin smoke alone is insufficient. Switch it off when those conditions end to avoid glare. Permission to use front fog lights on certain narrow roads with signed bends does not itself permit rear fog lights. Fitment depends on vehicle category; they are not compulsory on every vehicle.