Aside from being the international signal for stopping at road intersections with traffic lights, red lights are also the colour globally associated with sex for sale.
A red light is commonly seen outside brothels and you find red lights in front of prostitutes' houses in the red light district.
Another place you find red lights is in front of doctor's surgeries in some countries. I have no idea why the same type of lights adorn both a doctor's practice and a bordello but I do remember one of my first times in a red light district where a friend pointed out the red lights in the front of some houses and mentioned (he was a doctor) "those are not doctor's surgeries". | 
