It's not just about what you can see, but how fast you are travelling and how quickly and safely you can stop. At 60mph it takes about 3.5 seconds to travel 100m.
Fog can bring other problems that may make it more difficult to stop than usual, especially if the surface is wet or cold temps mean your tyres don't grip like they would on a summers day.
Then there's the other vehicles around you. Not all of them will as modern and well maintained as yours is, and not all of them will be cars. If a lorry doesn't see you until it's too late then you're toast.