From Wired magaine, an interesting then and now comparison – 1991 vs 2010 – mapping changes in the distribution of New York prostitution. The article explains:
Red-light districts were commonplace in big cities throughout the 20th century. But in recent years, New York City has seen the midtown areas with high concentrations of sex workers empty out. The trade has moved indoors and diffused into more-upscale districts downtown.