The Watts Towers were designated a National Historic Landmark in 1990 by the NPS. On the plaque it states "These structures possess National signifigance in commemorating the history of the United States of America." The Watts Towers is located adjacent to the Watts Tower Arts Center, which includes a small park, parking lot, 2 buidlings for art galleries, events, and youth workshops. http://www.wattstowers.us/
Watts Towers on its south portion is lined by 20 homes on 107th street. In recent years Watts Towers has seen a signifigant increase in funding, and some of this funding has went toward a beautification project of the community surrounding the Towers, particularly these 20 homes lined up directly behind the Towers. The only other area surrounding the Towers is the Railroad. I don't know if these homes would require a more signifigant buffer zone than just the street which currently seperates them from the towers. Watts Towers themselves are surrounded by a fence that is only unlocked if you're on a tour.
Being a National Historic Landmark now for 20 years, my guess is the residents located on 107th street are long used to visitors, and as I understand it many of them are directly involved in activities and events occuring at the Watts Art Center. Many of the homes are painted and adorned with art, which works well alongside the eclectic work of Simon Rodia.
- and yes I am very interested in how my country develops their T-List, thanks