Core: Fog-light rule: rain versus fog

In (heavy) rain you see less well and the road surface is more slippery (Book p.187). Always switch on your dipped beam in rain. In heavy rain that seriously impairs visibility, you may additionally switch on the front fog light — that is a 'may', not a 'must' for every rain shower. However, you may never switch on the rear fog light in rain, not even during heavy rain showers (Book p.188). The bright red light and its reflection off wet air actually hinder following traffic instead of helping. In weather conditions that seriously impair visibility — visibility less than about 200 metres, whether from rain, snowfall or fog — you may switch on the front fog light (Book p.191). The rear fog light is stricter: that is only allowed when visibility is less than 50 metres due to very dense fog or heavy snowfall, never due to rain. And even then: only use it for as long as it serves safety. Once following traffic has already seen you, the bright rear fog light actually hinders them.