Core: Fog: why your normal driving habits no longer apply

Fog forms mainly when warm, humid air flows over a cold ground surface (Book p.190). As soon as fog forms, you adjust your driving behaviour immediately. In very dense fog (visibility under 50 metres) four measures apply at once: keep three seconds' distance instead of two to the car ahead, reduce your speed, preferably do not overtake — not even on a motorway — and follow the right-hand lane as much as possible, so that in an emergency you can use the hard shoulder as a direct escape route (Book p.191). This speed reduction is independent of your lights: fog lights help others see you, but change nothing about your own, reduced sight — you reduce your speed regardless. The exact thresholds for front and rear fog lights (visibility <200 m and <50 m respectively) belong to a different atom (regen_wind.mistlicht_regel, Book p.187-188/191); this atom is about your driving behaviour once visibility is severely limited, not the precise lighting rules.