Intent and Design
For the new owner of a European auto, the rear fog light is often a source of great mystery, chances are they’ve never seen it before on their domestic car or truck. A common question we are asked is “Why does my European car have only one fog light”.
Or an equally common question, “I think I have a rear fog light out, only the driver’s side seems to work.”
While not common in the U.S., rear fog lights are mandatory in Europe, and are especially common on German and Swedish autos. Typically, the rear fog light is a bright red light that is the same brightness as your brake lights. In Europe, two rear fog lights are common, but in the United States, this is not currently allowed. Why does the United States not allow dual rear fog lights? I’ve heard a number of reasons, the most common answer is so drivers don’t mistake your two rear fog lights for brake lights. The reasoning seems to be that in foggy conditions, an overtaking driver might mistake the dual rear fog lights as the car in front of him as brake lights, then hit his own brakes, creating a potential for a rear collision.
Purpose
The job of the rear fog lamp is to reveal the position of your vehicle when driving in the fog, snow or other conditions that limit visibility. Your normal taillights are fairly dim, so they are not as likely be seen in foggy weather. Rear fog lights are often placed on the left side of the vehicle, to help mark the corner of the car when vehicles behind are overtaking (passing) it. This bight light (looks like a brake light on) lets other drivers know approximately where the vehicle is located in a travel lane. Like the fog lights on the front of your car help make you more visible to oncoming traffic, rear fog lights make you more visible to people behind you. There are several types of rear fog lights. Some take xenon or halogen headlight bulbs, but other rear fog lamps are made up of LED lights. LED lights are bright, long lasting and common on newer vehicles with rear lamps.
Example
In the case of the 2016 Volvo S60, pictured here, a couple rear lights illuminate when you push the button, making the Volvo much easier to spot on the highway in the fog and other poor conditions. Volvo lights are one of the most common examples of rear fog lights. The rear fog lamp is a useful safety feature, but issues arise when drivers are unaware of the feature and switch on the fog lights in normal driving conditions. Sure, you might get a little extra forward road illumination, but you’re also unknowingly blinding drivers behind you and sometimes even making them think you’re braking. Therefore, these optional fog lights should not be used as normal driving lights. If your vehicle has rear fog lights, only switch them on when you are driving in fog, snow or other conditions that may prevent another driver from seeing your vehicle.