A Difference-in-Differences Analysis of Health, Safety, and Greening Vacant Urban Space

Abstract

Greening of vacant urban land may affect health and safety. 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 m(2)) 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). Once greened, vacant lots may reduce certain crimes and promote some aspects of health. Limitations of the current study are discussed. Community-based trials are warranted to further test these findings.

Nature-Based Strategies for Improving Urban Health and Safety

“…With a few exceptions, 79,91,101,104,[127][128][129] prior studies have not applied experimental or quasi-experimental approaches in testing whether urban greening affects health and safety and therefore have limited ability to establish causal mechanisms. 64 Despite implementation challenges and longer study periods, new studies should aim to provide the highest level of evidence through randomized controlled trials.…”

“…64,66,78 However, one quasi-experimental study of a vacant-lot greening program in Philadelphia, PA, found an association between greened vacant lots and increased physical activity. 79 Green exposure has also been shown to prevent and mitigate stress, anxiety, and depression, [80][81][82][83] especially in urban environments. 81,84,85 Access to green views has been associated with improved mental health in general 86 and more specifically has been shown to reduce mental fatigue and improve coping with stressful settings.…”

“…90 In a blighted urban environment, the vacant-lot greening study mentioned above found decreased levels of self-reported high stress around newly greened vacant lots. 79 In addition, a small randomized trial found decreased ambulatory heart rate, as a dynamic measure of stress, among participants who walked in view of Bclean and greenedv acant lots compared to participants who walked in view of untreated vacant lots in their neighborhood. 91 Social isolation is a strong predictor of mortality and morbidity.…”

“…104 In one quasi-experimental study, gun violence was significantly reduced around previously vacant land that had been transformed to well-maintained green space trees. 79 The authors postulated that cleaning and greening physically decreased opportunities for hiding guns and other illegal activity by removing uncontrolled growth of weeds and buildup of large trash items on vacant land. 79 In addition, there is evidence that temperature moderation could be a pathway by which vegetation affects crime.…”

“…79 The authors postulated that cleaning and greening physically decreased opportunities for hiding guns and other illegal activity by removing uncontrolled growth of weeds and buildup of large trash items on vacant land. 79 In addition, there is evidence that temperature moderation could be a pathway by which vegetation affects crime. Many cities are subject to the urban heat island effect, where pervasive impervious surfaces elevate temperatures, which has been shown to increase resident discomfort.…”