During low visibility conditions such as foggy and precipitation, driving becomes more dangerous and more risky. The standard set of headlights won't do the job to keep the road visible that is why Volkswagen Passat is equipped with another set of lights that promises to make the road visible and less dangerous.
Fog lights are set of lights that are located at each end of the front of the vehicle which main purpose is to provide a wider beam pattern thus making the road more visible during precipitation and foggy weather. Typically these light are situated below the headlights or at the bumper about 10-24 inches from the ground. This is so because fog lights project more powerful beam than the regular headlight thus making it more dangerous for the oncoming traffic when it is at the higher location. This is why there are two main points to consider when mounting or configurating the fog lights. First, fog lights should minimize the amount of return glare into the driver's eyes and second is to minimize glare into oncoming eyes.
Fog lights are very useful to keep the road safe from vehicular accidents. As thick fogs dominate the road, high speed driving is very dangerous while low speed could subject other vehicles to collide on your rear. To keep accidents from happening, the vehicles should be visible at all times. Fog lights should be turned on.
When is the right time to use the fog lights? Normal road visibility should entitle you to see more than a hundred yard from where you are. If the road visibility drops from that, say less than a hundred, it is time to turn those additional lights on. Rear fog lights on the other hand are used when the visibility is less than 50 yards. But there is a main difference between the front and the rear fog light. Rear fog lights are usually a single red light as bright as a brake light. They are called fog lights because rarely does your visibility drop to such a dangerous level in normal rain or snow like it can in fog.