So What Are Fog Lights Actually For?
Drive anywhere in Ireland in the dark, and you will notice the phenomenon of drivers using their front fog lights when there is no fog. Perhaps they do not realize they have them on, perhaps they think they make their car look cool, or perhaps they genuinely think it makes driving safer, but a brief analysis of the performance of these lights, and how our eyes work, may make you think twice about using them in anything but fog in the future.
A fog light is designed for use in conditions of significantly reduced visibility, and is used to show you the edges of the road, the lane markings, and the immediate foreground, so you can see your way safely at reduced speeds. A good fog light outputs 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.
Firstly, what are the disadvantages to you, the driver, in driving with your fog lights on when there’s no fog? You may think you can see better with them on, but in clear conditions more foreground light is not a good thing, it's actually a bad thing. Some foreground light is necessary so you can use your peripheral vision to see where you are relative to the road edges, the lane markings and that pothole 10 feet in front of you. But foreground light is far less safety-critical than light cast well down the road into the distance, because at any significant speed (much above 40 kmph), what's in the foreground is too close for you to do much about. In other words, if you can see it with your fog lights, you’re going to hit it! If you increase the foreground light, your pupils react to the bright, wide pool of light by constricting, which in turn substantially reduces your distance vision. It's insidious, because high levels of foreground light give the illusion, the subjective impression, of comfort and security and "good lighting", even though you actually can’t see as far down the road as with your fog lights switched off.
Secondly, what about the effect of fog lights on other road users? Well, a lot of fog lights on the market do not have correct beam patterns, or are not adjusted correctly, and cause a lot of glare for other road users when used in clear conditions. Also, a common misconception is that fog lights should be used when it is raining, but their extra downward light hitting a road surface shiny with water creates high levels of reflected glare for other drivers. Since we're all "other drivers" to everybody else on the road, it's well to think of roadway safety as a cooperative effort. In most driving situations, fog lamps are neither useful nor necessary, but more people use their fog lamps when the prevailing conditions don't call for their use, than use them when the conditions do call for their use.
It should also be noted that a lot of the factory-installed or dealer-optional fog lamps, and a great many aftermarket units, are essentially useless as fog lights, as many of them are too small to produce enough light to make a difference, produce beam patterns too narrow to help, lack a sufficiently-sharp cutoff, may actually be driving lights - with a spotlight beam pattern, or throw too much glare light into the eyes of other drivers, no matter how well they are aimed. The industry also needs to look at the example they are setting for road users in the promotion and design of their cars. It is common practice to show launch pictures of a new car with its fog lights switched on, promoting the practice further, and the novel design of the sidelights on some modern cars, such as the Audi A5 and BMW 5-series, encourages drivers to put off using their dipped beams for as long as possible to show them off, even though they can’t see where they are going! This often then prompts the driver to use the fog lights instead, thus committing the double crime of driving with no lights suitable for seeing objects in the distance, and also ruining their night distance vision with the glare just in front of their car.
Your car doesn’t look cool because it has its fog lights on, and glare is dangerous, so do yourself and everyone a favour: choose them carefully, aim them properly, use them thoughtfully, and leave them off except when they're genuinely necessary.
