Avoid driving in fog unless your journey is absolutely necessary. Fog turns up every year. One moment you may be in bright sunshine, the next you may be in thick fog. It can turn up in dips in the road, or valleys, or on the tops of hills, or just around the next corner. Be ready to slow down and keep well back from the vehicle in-front. Rear fog lights should be used in any weather conditions (fog, mist, cloud, snow, rain ) where you cannot see more than 100 metres. Remember to switch off any fog lights when you can see more than 100 metres, or you have finished your journey.
Reduce speed because it gives you more time to see and assess a new situation, and react to it. You are less likely to aquaplane through puddles, and don’t forget that every time you reduce your speed to half, you reduce your braking distance to a third ( not by a third, but to a third!)