What good are fog lights, really?
Many of today’s vehicles have front fog lights. What good are they? The quick answer is not much. Even good fog lights, which are relatively rare, are of very limited use to most drivers. To the extent, fog lights should be turned off almost all the time and you will forget the car has them. Sometimes the original bulbs last years and years because they’re almost never used.
Of course, fog lights are meant to be used in heavy fog, rain, or snow to help the drivers see the edges of the road close the car so they can safely make progress through foul weather at very low speeds. That is all these lights are designed, intended, and able to do, and most of the ones available aren’t even capable of doing that.
However, there are a lot of drivers who are accustomed to use fog lights in dry weather. Leaving the fog lights on at all the time does not actually improve the lighting safely performance or the driver’s ability to see, though many people do so in the mistaken belief that they can see better this way at normal road speeds in dry weather. According to a systematic study, in the United States more people inappropriately use their front fog lights in dry weather than use them properly in poor weather. This is because we generally can’t accurately tell how well or how poorly we see. We have subjective impressions, reactions, and feeling about how “good” or “bad”.
Many drivers have the subjective impression of taking good headlights as foreground light, and remember, that’s what fog lights produce. In clear conditions, though it makes us feel more secure, more foreground light is not a good thing, it’s a bad thing. Generally speaking, what’s in the foreground is close enough to be within the foreground light for you to avoid hitting at normal speeds. If you increase the foreground light, such as by turning on the fog lights, you pupils react to the brighter pool of foreground light by constricting, which in turn substantially reduces your distance vision. This is also the reason why it is not appropriate to have fog lights lit with the high beam headlights. If you are going fast enough to need high beams, you definitely don’t want to spoil your distance vision by overly lighting the foreground.
So, what is a good fog light? A good fog light produces a wide, bar-shaped beam of light with a sharp horizontal cutoff at the top of the beam, and minimal upward light above the cutoff. Almost all factory-installed or dealer-optional fog lights, and a great many aftermarket units, are essentially useless for any purpose, especially for extremely demanding poor-weather driving. Many of them are too small to produce enough light to make a different, produce beam patterns to help, lack a sufficiently-sharp cutoff, and throw too much glare light into the eyes of other drivers, no matter how they’re aimed.
All in all, good fog light is essential for driving, and what kind of weather you want to choose to use the fog light is important too.