Areas of vice are referred to as "red light districts" in reference to:

In reference to the signal lanterns of railroad construction workers. There is no certainty as to where the term red-light district came from. But one of the explanations dates back to the 20th century. It is associated with the railroad and railroad workers. During this time, the workers would carry their red lanterns which they used for lightning purposes as well as signaling to their fellow workers. It was believed that these workers brought their lanterns with them when they went to prostitute houses near the railroad lines. The number of red lanterns created a red light area. 

*blinks* youforgotyourname**