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.