Fog Lights at Night Reduce Forward Vision

In my motel room last night watching SkyNew there was an interesting very short item from BBC News about the use of fog lights. It seems that using your fog lights for night driving in good weather on the open highway reduces your forward vision.

Assuming there is no fog and you have your vehicle lights on high-beam, you should have your fog lights extinguished (off) in order to ensure that you can see as far as possible into the night in front of your car to limits of your high beam illumination.

Fog lights, which are designed for use in fog (oddly enough), point down to the ground and to the outside of your vehicle. They light up about a three to five metre circle directly in front of the vehicle and to the sides with bright light. They are designed and aligned to work in two ways. Firstly they light up a zone directly in front of your car ‘below’ the fog level so that you have some degree of forward and side vision when the fog closes in. The assumption being that, as there is fog about, you are driving cautiously and, most likely, slowly. Secondly they make your car obvious to any person or vehicle approaching from the front. This is done both from direct light and from strong reflected light, because, remember, fog lights point down at the ground directly in front of the vehicle.

The problem with having fog lights on when driving at night in good weather is that these lights are pointing strong light directly onto the ground just in front and to the sides of your vehicle. This strong reflected light coming off the road and the sides of the road causes the pupils in your eyes to close down about a half to a third of an f-stop. This means that less light is entering your eye, which then in turn reduces how far ahead ‘into the dark’ you are able to see.

The actual amount of forward night vision that you lose is dependant on how bright your fog lights are and how far your head-light high beam reaches. The brighter the reflected light from the fog lights the more your pupils will close down.

It seems that for the ‘average’ modern vehicle driving with your fog lights on at night on the open road reduces your forward vision by up to 100 meters.

Who would have thought?

Having seen this BBC item I figure this is probably one of the reasons why, in Western Australia anyway, having your fog lights on at the same time as your headlights is a bookable traffic infringement and carries a fine. It’s a safety issue.