Through the mist
Clearing the fog on fog lights
Are fog lights useful or do they just look cool?
Many modern cars, particularly the top end models are now fitted with fog lights, but there appears to be limited direction as to their appropriate or effective use.
The very name suggests that they should be used in…well…the fog, but as many an online forum will testify, it can be a contentious subject indeed. The web is awash with discussions (many using language that I dare not reproduce) from those who use them all the time because they think they look cool to those who abhor their use for any or every reason and openly deride those who have them.
Back in the olden days when my dad was a boy, fog lights used to be yellow in colour but these days they are very often white. They are typically mounted low in the bumper (to spread light ‘under’ the fog) and are sometimes confused with day running lights (DRLs). If the lights are wired so they can be turned on by themselves, but turn off automatically when the parkers or headlights are switched on, then they are DRLs. If they only work in concert with the parking lights or headlights, they are classified as fog lights.
First of all, it is important to note that the use of fog lights in other than inclement weather or low visibility is actually an offence in most Australian states and territories due to the glare to oncoming drivers. While it seems to be enforced quite randomly, a number of people have made sizable donations to the annual police ball by using them to show how cool they are.
Fog lights are great for...well...fog
Some drivers claim that fog lights are useful for suburban night driving for the light they provide directly in front and to the sides. Though, as a study conducted by SAE International in the US discovered, our perceptions as to how good or bad our headlights perform are actually quite subjective and not always accurate. They found that fog lights are more frequently used inappropriately in clear weather than they were used appropriately in inclement weather.
There are a range of factors that affect the safety performance of automotive headlights, that is, how well they enable a driver to see what they need to see in order to avoid a collision, and the foreground light produced by fog lights is a very long way down that list. While foreground lighting is necessary for your peripheral vision in order to see where you are relative to the road edges and makes us feel more secure, the problem with too much foreground lighting is that it is unhelpful. Foreground light is far less safety-critical than the light that is cast further down the road by your headlights because at any significant speed (40km/h or more), what's in the foreground is too close to avoid. It may be cold comfort to simply get a good look at what you’re about to hit.
Turning on your fog lights increases the foreground lighting, causing your pupils to react to this bright pool of foreground light by narrowing, which substantially reduces your distance vision—especially since there's no increase in the intensity of distance lighting to compensate for the increased foreground light. That is not going to help you see things in time to avoid them. This is also why you shouldn’t use fog lights with high beam, because if you're going fast enough to need high beam, you don't want to spoil your distance vision with too much foreground lighting.
Highbeam and fog lights may look cool, but it's ultimately unhelpful.
When we use our fog lights inappropriately, we perceive that our vision is better than it actually is and subconsciously adjust our driving to match, which is not a good combination. So my advice is to use your fog lights for their intended use, driving in foggy conditions (and to make your car look cool in photos), but do yourself and everyone else a favour by switching them off when driving in clear conditions.