There are special bicycle carriers that you attach to the tow bar of a car. You can transport bicycles here. You are not allowed to transport anything other than bicycles on a bicycle carrier. The bicycles should not protrude more than 20 centimeters from the sides of the car. If the bicycles protrude more than 10 centimeters on the side, you do NOT have to use a marking board. In some theory books and many other websites, it is stated that this is the case. But this is absolutely not true! A marking sign is never required on a passenger car or a commercial vehicle.
Furthermore, the number plate, retro-reflectors, and lighting of the car must remain visible. If this is not the case, the bicycle carrier must be fitted with a white number plate, retroreflectors, and additional lighting.
You may not transport more bicycles than the bicycle carrier is made for. Three bicycles on a bicycle carrier for two bicycles is prohibited. A bicycle carrier does not fall under the trailer category and therefore a rear fog light is not required.