An analysis of the impact of a vacant lot greening program in Philadelphia found that neighborhoods where vacant lots were converted into small parks and community green spaces are associated with reduced crime when compared to neighborhoods with unimproved vacant lots. The greening of 4,436 vacant lots from 1999 to 2008 was associated with consistent reductions in gun assaults across all four sections of Philadelphia studied and with consistent reductions in vandalism in one section.
Branas, Charles C., Rose A. Cheney, John M. MacDonald, Vicky W. Tam, Tara D. Jackson, and Thomas R. Ten Have. (2011). A Difference-in-Differences Analysis of Health, Safety, and Greening Vacant Urban Space. American Journal of Epidemiology, 174(11), 1296–1306.