Originally Posted By: Hokiefyd
One has to be careful with fog lamps. Too much light close to the ground can be detrimental to both you and other drivers. You don't want to flood the foreground with light, as that takes away your distance vision. And the round-bounce glare to other drivers with even standard fog lamps can be pretty disabling on wet pavement...especially since many people use fog lamps inappropriately.
You have it completely backwards. Fog lamps are specifically designed to add lots and lots of foreground lighting, where it's most useful in foul weather, which is when fog lamps should be used. A good fog lamp will focus the hot spot about 5 feet in front of the vehicle, spread the beam out 100+ degrees, have a razor sharp cutoff, and only be useful to about 100 feet or less.
Yes, driving with the fog lamps on all the time (in fair weather) will result in unnecessary foreground lighting and affect distance vision. At that point, the driver is using the wrong tool for the job.
Since Daniel Stern was mentioned, here's what he has to say about fog lamps (emphasis is mine):
Originally Posted By: Daniel Stern
That's the key point: fog lamps are meant to be used in heavy fog, rain, or snow to help the driver see the edges of the road close to the car so s/he can safely make progress through foul weather at very low speeds.
The fog lamps' job is to show you the edges of the road, the lane markings, and the
immediate foreground. When used in combination with the headlamps,
good fog lamps weight the overall beam pattern towards the foreground so that even though there may be a relatively high level of upward stray light from the headlamps causing glareback from the fog or falling rain or snow, there will be
more foreground light than usual without a corresponding increase in upward stray light, giving back some of the vision you lose to precipitation.
When used without headlamps in conditions of extremely poor visibility due to snow, fog or heavy rain,
good fog lamps light the foreground and the road edges only, so you can see your way safely at reduced speeds.
http://www.danielsternlighting.com/tech/lights/fog_lamps/fog_lamps.html
To illustrate, here are a pair of isolux diagrams for Hella 500 lamps, a good auxiliary lamp offered in both fog and driving patterns.
Fog lamp:
Driving lamp:
For comparison, a Hella 90mm projector headlamp (low beam):
