This is a particularly dangerous and often misunderstood case, because staying several car lengths away from the car ahead won't do any good if that car suddenly collides with a stationary car in front of it that's hidden by the fog. When the car you're following comes to a sudden stop, all the rules about safe following distance go right out the window. This is why professionals sometimes recommend just getting off the road when fog prevents a reasonable amount of visibility.

> These types of accidents are the most lethal, and they are all due to stopping distance (and could be avoided by driving at a lower speed)

Very true.