Volvo: 2009 Volvo S80, volvo s80, rear fog

Question
This car has only one rear fog light for what reason?

Answer
Mario,
The use of one fog light in the rear is so that it does not appear that the drive has applied the brakes or the the brake lights are stuck on. When the rear fog is on and the brakes are applied a set of brake lights turn on so as to not confuse traffic behind.