Monday, January 20, 2014

The benefit of front foglights

Too many people, it seems, don't really know how to properly use front foglights. The natural assumption is that if it's foggy, you should use foglights, right? Well - sort of. Foglights are there to help with a particular problem - when the atmosphere in front of you is highly reflective, your own regular headlights can be blinding to you. Whether it's fog, heavy rain and spray, or driving snow, the headlights are angled and positioned such that you can end up with near total white-out as the light reflects back at you. What makes this worse is when you flick from dipped to full headlights, because now you've pretty much doubled the amount of light shining back at you. This is dangerous because it's dazzling to you, and it means you can't see very far, and this is where fog lights come in. They're normally mounted lower down on the front of a car and typically cast a wider but shorter beam. The idea is that, being low down, they reflect less light back at you from whatever is outside. Here's where the confusion sets in - people will tend to put their foglights on in addition to their main dipped headlights, and that really has no benefit. What I prefer to recommend is switching your headlights down to side lights / marker lights, and then turning on the foglights on. There's still plenty of light out of the front of the car for others to see you, but now all that light is low down to the ground. With the main headlights off, the backscatter off the fog or snow is largely eliminated, the dazzle has gone and you can see much more clearly what's going on ahead of you. The picture here is mocked up in photoshop in an attempt to show the difference between driving on dipped main beams (top) and marker lights with fog lights (bottom).
Final tip : if your car has rear foglights, use them in low vis - it makes you FAR more visible to everyone behind. But turn them off again when the inclement condition clears up otherwise it's dazzling. I'm looking at you, every driver in England ....

It's important to note the distinction between fog lights and driving lights, though. Driving lights are normally mounted at the same height as the headlights (or higher), and are there for added illumination in normal conditions. They normally have different lenses to the main headlights to give a flat, wide beam to illuminate the sides of the road (where normal headlights have a narrow, long beam for distance). Turning driving lights on in fog is the same as flicking to high beams - dazzling and pointless.


Anonymous said...

What a good idea would to make it mandatory to give this lesson as part of gaining a drivers licence. I never see fog light used in the correct fashion. To many people, especially juvenile males with every light thats available on the front of the car lit up. And if they have high intensity rear lights those morons have them as well.

Brian said...

I drove a 2004 Subaru Forester in the Colorado mountains for many years, and I needed to use the factory-installed fog lights many times. Unfortunately for Subaru owners, the cars are wired so that the driver can only use the fog lights while the low-beam headlights are on, even though the fog lights have a separate switch.

Here are Subaru's options: low beams only, high beams only, secondary lights (corner markers, etc) only, or low beams with fog lights. The fog lights will not illuminate with the secondary lights or high beams. I thought this was a manufacturing defect, and when I asked the dealer to correct this, they knew nothing about it, could find no relevant service bulletin, could not explain why it worked like this, and sent me away with my repair order marked, "Vehicle performs as designed."

In subsequent reduced-visibility situations I either used the secondary lights only or the low beams. The built-in fog lights were useless.

Chris said...

That's pretty much how they all are. You typically don't ever want to use high beams in fog which is why the foglights turn off with high beams. The peculiarity with Subaru is that they won't allow the fog lights with side lights. As you said you have to have at least the low beam on. Honda don't do this, VW don't do this and my current Range Rover doesn't do this. It allows foglights and side lights without the main headlight being on (i.e. No low beam)