An analysis of the impact of greening 4,436 vacant lots in Philadelphia from 1999 to 2008 found that vacant lot greening was associated with residents' reporting significantly less stress and more exercise in select sections of the city. Because newly greened vacant lots may serve as safe havens, residents may have felt less stress or may have seen greater outdoor opportunities for exercise in a cleaner, more attractive, and safer environment.
Branas, Charles C., Rose A. Cheney, John M. Macdonald, Vicky W. Tam, Tara D. Jackson, and Thomas R. Ten Have. “A Difference-in-Differences Analysis of Health, Safety, and Greening Vacant Urban Space.” American Journal of Epidemiology 174, no. 11 (November 2011): 1296–1306. https://doi.org/10.1093/aje/kwr273.