Near Waddinxveen on the A20 dense fog descends. You are not towing a trailer and your car is not heavily loaded. What applies to your following distance?
3 seconds — fog alone is already reason enough for the greater distance
4 seconds — double the normal 2-second rule
Just 2 seconds — the 3-second rule only applies with a trailer or heavy load
Explanation
Wrong — trap: doubling the margin. The book names 3 seconds in fog, not 4 — that is 1.5x, not a doubling (Book p.124).