New York Subway graffiti
Subway graffiti has been a theme in many cities, though New York has to be in the front of that theme. They have had numerous books printed covering the history behind New York subway graffiti. Additionally you can purchase books that are based mainly upon images of subway graffiti.
If you really take a moment and think about it, it makes since. Where the rest of the country has train cars that have been tagged, New York has subways. Add to that the culture and feel of New York and you have to agree that New York would be on top of the graffiti we now refer to as Subway Art.