A Difference-in-Differences Analysis of Health, Safety, and Greening Vacant Urban Space
Greening vacant lots was “associated with consistent reductions in gun assaults” across the entire city. In some parts of the city, the greened lots were also associated with consistent reductions in vandalism, lower stress levels, and even higher rates of exercise.
Branas, Charles C. et al.(2011). A Difference-in-Differences Analysis of Health, Safety, and Greening Vacant Urban Space. American Journal of Epidemiology. doi: 10.1093/aje/kwr273
The authors conducted a decade-long difference-in-differences analysis of the impact of a vacant lot greening program in Philadelphia, Pennsylvania, on health and safety outcomes. “Before” and “after” outcome differences among treated vacant lots were compared with matched groups of control vacant lots that were eligible but did not receive treatment. Control lots from 2 eligibility pools were randomly selected and matched to treated lots at a 3:1 ratio by city section. Random-effects regression models were fitted, along with alternative models and robustness checks. Across 4 sections of Philadelphia, 4,436 vacant lots totaling over 7.8 million square feet (about 725,000 m2) were greened from 1999 to 2008.
Regression-adjusted estimates showed that vacant lot greening was associated with consistent reductions in gun assaults across all 4 sections of the city (P < 0.001) and consistent reductions in vandalism in 1 section of the city (P < 0.001).
Regression-adjusted estimates also showed that vacant lot greening was associated with residents’ reporting less stress and more exercise in select sections of the city (P < 0.01).