Definition: driving in fog

It is not foggy if you can see four or five hundred yards while moving at seventy miles per hour down the highway. This is low cloud, not fog, and does not present any hazard. A weatherman might call conditions foggy, but he's not on the scene driving. He is many hours in the past, trying to imagine what is happening on the road at a given time.

Fog is a little hazardous when you can't see five or more carlengths ahead if you are maintaining your interval of a carlength for every ten miles of speed between you and the car ahead, and, that the rest of those on the road are also maintaining their intervals.

Fog is actually hazardous when it gets down to three carlengths of visibility. Now it's time to slow down, especially if you are losing the tail lights of the nearest vehicle in front of you. You run the risk of overcompensating due to the visual tunnelling effect of losing your reference points at the sides of the road.

Another hazard is caused by motorists threatened by the idea of fog. Drivers who are used to crystal clear views from horizon to horizon feel threatened by low clouds and so drive erratically in safe conditions, sometimes at normal speeds and suddenly slowing for the idea of dense fog sweeping across the freeway, or some such nonsense. These people are a hazard to be avoided.

Turn on your lights, drive normally, stay alert. Thanks for your attention.