摘要：Objectives. We investigated the health and safety effects of urban green stormwater infrastructure (GSI) installments. Methods. We conducted a difference-in-differences analysis of the effects of GSI installments on health (e.g., blood pressure, cholesterol and stress levels) and safety (e.g., felonies, nuisance and property crimes, narcotics crimes) outcomes from 2000 to 2012 in Philadelphia, Pennsylvania. We used mixed-effects regression models to compare differences in pre- and posttreatment measures of outcomes for treatment sites (n = 52) and randomly chosen, matched control sites (n = 186) within multiple geographic extents surrounding GSI sites. Results. Regression-adjusted models showed consistent and statistically significant reductions in narcotics possession (18%–27% less) within 16th-mile, quarter-mile, half-mile ( P < .001), and eighth-mile ( P < .01) distances from treatment sites and at the census tract level ( P < .01). Narcotics manufacture and burglaries were also significantly reduced at multiple scales. Nonsignificant reductions in homicides, assaults, thefts, public drunkenness, and narcotics sales were associated with GSI installation in at least 1 geographic extent. Conclusions. Health and safety considerations should be included in future assessments of GSI programs. Subsequent studies should assess mechanisms of this association. Exposure to trees, vegetation, nature, or green space in urban areas has been connected with multiple public health benefits, including reduced mortality, 1–3 morbidity, 4 stress, and mental fatigue. 5–7 In addition, a growing body of research has investigated the relationship between urban nature and public safety, although with mixed results. Multiple studies have found that dense vegetation may promote crime by providing criminals a place to hide themselves or illegal goods. 8–10 Other studies have found that urban nature is associated with reduced crime. As a broad measure of urban nature, vegetation abundance has been linked to reductions in violent crimes, property crimes, 11 assault, robbery, and burglary. 12 Other studies have used more specific measures. For example, larger crown spreads of street trees and residential lot trees have been associated with fewer total crimes, property crimes, and vandalism. 3,13 Another study found that increased tree canopy cover was associated with reduced incidents of shooting, theft, robbery, and burglary, especially on public lands. 13 A study of the cleaning and greening of vacant lots found significant reductions in gun assaults and vandalism. 14 One challenge in interpreting these studies for management purposes is that they often use coarse measures of both nature and safety. Public safety outcomes have traditionally emphasized measures such as total crimes, violent crimes, and property crimes. Few have investigated the effects of urban nature on specific crimes or health behaviors such as drug use and possession, illegal dumping, vandalism, and public drunkenness and disorderly conduct. In addition, with few exceptions, 11,14,15 previous studies have not applied an experimental or quasiexperimental approach to test whether urban greening can improve health and safety. Little is known about the mechanism of association between urban nature and crime. The broken windows theory 16 holds that disordered and disinvested urban environments promote criminal activity. Blighted urban environments like this can erode a sense of mutual regard among residents and passers-by, signaling that no one cares and that illegal activity will be tolerated in a space. 16,17 Greened, openly visible, and ordered spaces may contribute to defensible space, indicating territory, surveillance, and care for a space, which may reduce opportunities for violence and crime. 18–20 Other mechanistic pathways that may connect urban nature and crime are social cohesion and psychosocial stress. Environmental factors, such as vacancy, physical decay, noise, pollution, and crowding, can provoke a physiological stress response that can aggravate aggression and violence. 21 Green space may help prevent and mitigate stress, anxiety, and depression. 22,23 Access to green views has been shown to reduce mental fatigue and improve coping with stressful urban environments. 5,6 Greening has also been associated with a stronger sense of safety and feeling of security. 15 Green stormwater infrastructure (GSI) is an emerging form of urban greening initiative in the United States and other countries. Approximately 700 cities across the United States have outdated combined sanitary and storm sewer systems that are subject to overflow during heavy rain events. 24 The US Environmental Protection Agency estimates that 850 billion gallons of combined sewer overflow discharge occurs each year in the United States. State and federal regulations, in addition to concerns about public health, ecosystem health, and climate change, have put cities under mandate to reduce combined sewer overflows. Compared with traditional “gray” approaches such as installing separate stormwater drainage systems, GSI approaches are increasingly seen as less expensive alternatives to reducing combined sewer overflow. These GSI approaches refer to a variety of in-ground installments that allow infiltration, evapotranspiration, and capture and use or reuse of stormwater. 25 GSI has largely been implemented on a small scale and in conjunction with new development or redevelopment projects. A lack of legislative or regulatory support has been a barrier to the broader implementation of alternative stormwater management strategies. 26 However, in April 2007, the Environmental Protection Agency signed a statement of intent supporting and encouraging the municipal use of GSI to meet federal regulatory standards. With these new federal standards, some municipalities, such as Philadelphia, Pennsylvania (population 1 547 607), have begun to plan for or implement GSI citywide. Environmental benefits are a main driver of the GSI approach. GSI installments and other urban vegetation have been shown to be effective at reducing stormwater flows and improving stormwater quality, 27 reducing the heat island effect, 28 and improving air quality. 29 Economic benefits are also a key driver for cities’ decisions to invest in GSI and other green space remediations; for instance, proximity to urban parks 30 and street tree plantings 31 can increase property values. Another main impetus for Philadelphia’s approach is what the city refers to as the “triple bottom line” 32 : that investment in GSI not only helps meet environmental engineering standards but has social and economic benefits as well. The greening of urban hardscape, or replacing gray impervious surface with green or otherwise pervious surfaces, is expected to improve health and safety conditions, but limited evidence supports this. Because the city of Philadelphia undertook one of the largest GSI efforts in the United States, an opportunity arose to study the impact of this program on health and safety. We therefore sought to fill various gaps in knowledge by testing the effects of GSI installments on health and safety across Philadelphia using a difference-in-differences analysis technique similar to that used in a previous observational study of vacant lot greening in Philadelphia. 14 Although our study used similar methods, the programmatic and treatment aspects of GSI and vacant lot greening are quite different. Vacant lot greening functions as a public–private venture that reproduces the same low-cost aesthetic cleaning and greening treatment of vacant lots that on average measure 1000 square feet. On the other hand, the primary goal of GSI projects is stormwater recapture, there are a variety of GSI treatments, and surface elements of GSI projects are typically much smaller than 1000 square feet. The difference-in-differences methods we employed allowed us to uniquely study and isolate the effects of these GSI projects.
