Fog, heavy rain, darkness and mountain roads all do the same two things: they reduce how far you can see and how well your tyres grip. The answer is the same each time: slow down until you can stop within the distance you can see, and double your gap.
The Motor Vehicles (Driving) Regulations, 2017 require speed suited to visibility, correct use of headlights and fog lamps, dipping high beam for other road users, and on narrow mountain roads the vehicle going downhill must stop on the left and let the vehicle going uphill pass (Regulation 21). This section adds practical techniques for fog, aquaplaning, floodwater, night driving and ghat roads, including low gears on descents.
Read Drive Safely at Night, in Fog, Rain & on Hills and Hill & Ghat Roads.