Historically, cities and towns have utilized “grey” infrastructure such as water pumps and tanks to manage stormwater. These methods, however, use traditional construction materials and do not reduce or treat the quality of stormwater.

GI, which mimics natural processes, captures and treats stormwater at its source. The capture and treatment processes vary widely depending on the GI feature (e.g. permeable pavement vs. stream restoration). In addition to flooding reductions, GI enhances water quality by preventing stormwater from picking up and carrying pollutants throughout the local habitat.

All GI features reduce stormwater flooding and enhance water quality.

Urban greenspaces support vibrant habitats for a wide variety of flora and fauna. GI projects fundamentally contribute to habitats by simply restoring and/or expanding the habitat’s greenspace with an increase in the functional landscape plants. This can provide a diversity of plant and animal species and return heterogeneity to the local ecosystem. Choosing plants for the ecosystem benefits they provide can create a healthier greenspace and increase the ecosystem functions of the space. Planting one tree not only can stabilize soil and infiltrate water but also increase native insect populations as well as the number of birds and mammals. Enhanced biodiversity leads to healthier and more resilient ecosystems.

GI makes a crucial contribution to animal life by increasing pollinators (e.g., hummingbirds, bees, insects). These pollinators, attracted to reintroduced native plant species, boost the park or greenspace by preserving the annual blossoming of plant life and ensuring a healthy urban ecosystem.

Several studies have shown how trees and even expanded plant life in urban greenspaces can enhance air quality and lower temperatures. GI removes pollutants and chemicals from the air. Enhanced shade and evapotranspiration ease urban heat island impacts and cause temperatures to decline. On a long-term, macro-level, large greenspaces such as urban forests can impact climate change by removing greenhouse gases and reducing the urban heat island effect.

Although many GI elements positively impact local air quality and temperature, projects that emphasize the expansion of greenery (e.g., urban tree canopies) will have a more pronounced effect than those deliberately limited in size (e.g., green roofs). However, even green roofs can substantially impact air quality, especially when installed on a city-wide scale. One recent study examined the equivalent of 19 football fields of green roofs in Chicago and found that the roofs removed nearly two tons of air pollutants over the course of one year. Replacing every 'grey' roof in Chicago with green roofs, would produce an annual pollutant removal of two thousand tons.

Deborah Cohen and Bing Han have extensively studied how public parks increase physical activity. Their decade-long study of 80+ parks throughout Los Angeles (appendix, no. 14) found that those parks made a “substantial contribution” to moderate-to-vigorous physical activity levels (MVPA) in neighboring communities.

Even a small GI project can expand the potential for physical activity in a local park. For example, bioretention features along a walking trail will make the trail more attractive to residents. A small-scale project still may produce impressive benefits, findings show that even pocket parks increase

While exercise in any setting leads to better health, green exercise — exercise in a natural environment — achieves results with respect to obesity, heart rate and other key health measures that go beyond those in indoor settings (“grey” exercise). So, your GI project not only can increase your community members’ exercise levels, but also can lead to better quality exercise.

In addition to increasing exercise levels, many GI projects can result in lowered localized temperatures and better air quality. Several studies over the years — including one for the District of Columbia’s Department of the Environment — have concluded that lowered temperatures and enhanced air quality attributed to greenery expansion can lead to reductions in heat- and pollution-related mortality rates and hospital visits.

The same GI features which lead to enhanced access to exercise also can lead to enhanced mental health for individuals, mainly by reducing stress and lowering anxiety. These benefits result from individuals simply being in the greenspace itself, whether he/she is active or not. Green roofs, for example, may boost stress-reduction in individuals, even though they do not enhance exercise opportunities.

While further research needs to be conducted to better understand why greenspace positively impacts mental health, reductions in stress and anxiety from the use of/ proximity to greenspace are well-documented, even in laboratory settings where subjects only viewed greenspace in photographs. 

“Social Cohesion” can mean many things. NRPA’s definition comes from two sources:

• Forrest and Kearns noted that community cohesion occurs when those in the community have strong feelings and behaviors associated to trust, common values and a sense of belonging.

• The Center for Active Design’s ASSEMBLY Civic Engagement work highlights civic appreciation and trust, participation in public life and stewardship of the public realm as crucial tentpoles for social cohesion.

Research shows more community members engage in social and volunteer activities and community cohesion increases over time in cities and towns with expanded GI and greenspaces rather than those with only concrete or "grey" spaces. Rain gardens, green roofs, urban tree canopies and other features provide opportunities for a community to interact in new ways.

A community’s social cohesion — and the socialization of individuals in that community — is a key driver of physical and mental health. Socialization in parks also can boost community members’ physical activity as more and more are drawn to a park due to its GI project.

According to analysis from the Center for Active Design, when community residents see that public parks and other spaces are well-cared for and rehabilitated, their trust in their local government — and even in each other — increases over time.

 
During the last few decades, social science researchers have examined the links between various forms of greenery on violent and nonviolent crime in urban neighborhoods. A landmark study from 2001 (appendix, no. 26) found that Chicago apartment residents in greener surroundings reported less crime than did those living in greyer properties. Further, researchers have been investigating how GI features can impact neighborhood safety:

• In the early 2000s, Philadelphia installed GI features at many sites around the city to reduce stormwater flooding and save resources. A 2015 study (appendix, no. 27) examined more than 50 such sites and found statistically significant reductions in drug possession, burglary and other crimes compared to the level of crime in city neighborhoods with no GI.

• In Youngstown, Ohio, the greening of select vacant lots throughout the city also was associated with a statistically significant reduction in a wide variety of crimes compared to untreated lots.

Although researchers are still investigating why GI and other forms of green space are correlated with reduced crime, they propose two arguments:

1. As GI attracts more community members to exercise, socialize and engage in all sorts of activities in the park, more eyes are “patrolling” the space, thus discouraging criminal activity.

2. GI can lead to reduced stress and anxiety over time for community members, which could contribute to crime reduction.

Community activation, as explored in this Framework, refers to the GI-related actions that your project can inspire throughout the community; these actions can include increased use of GI on residents’ properties or community involvement in your project’s development, construction and opening.

The main goals of the GI movement in the United States consists not only of expanding projects in parks and other public spaces, but also helping community members learn more about GI and implement it on their own. Your efforts in raising the public's awareness of your project ideally will lead to a proliferation of private projects across your community, creating more support for future GI work.

Nationwide, nearly 250,000 workers are involved in installing GI features in local communities every year. The unique demands of GI installation involve more than 30 occupations ranging from landscape architecture to construction as well as several other disciplines.

Research by Jobs for the Future — a nonprofit advocating for conservation-related jobs in underserved communities — looked at several cities throughout the country and found tens of thousands of local citizens employed in hundreds of GI projects. Portland, Ore., alone posts more than 10,000 jobs across thousands of GI installation projects throughout the city each year.

GI has the potential to enhance both local business activity and neighborhood property values:

In one of her studies measuring consumer purchasing habits, Kathleen Wolf has looked at the effect of GI and other greenspace on urban business activity. Her subjects indicated they were willing to spend 8-to-12 percent more in business districts with urban tree canopies and other GI features (appendix, no. 32).

With respect to property values, a study focused on the Battery Park neighborhood in New York City showed that buildings with green roof installations sometimes captured rents 16 percent higher than the average in the area, and dozens of studies have documented the rise in home and retail property values due to adjacent public parks and greenspace (appendix, no. 33).

Inclusive Healthy Places (IHP) Framework 

The Gehl Institute recently published this toolkit for park and recreation departments and related organizations to plan, build, and evaluate public-space projects (like green infrastructure) in ways that are better connected to community needs.

The IHP divides dozens of measures into different Drivers - factors that lead to more inclusive, healthy places. See the measurement wheel on page 45 of the resource, where you will find the four Drivers below that are relevant for GI projects:

• Characteristics of People Present: Is your project serving the needs of the entire community?

• Access: How has your project improved access to the park?

• Use & Users: How is your project encouraging connection, as well as physical activity, among community members?

• Safety & Security: Has your project improved the safety of park users, with respect to environmental improvements or a decrease in criminal activity?

NRPA includes data points for each of the above in our Benefit Measurement Tables. Resources can be downloaded from the GI Evaluation Framework home page.

The Value of Green Infrastructure

This guide from the Center for Neighborhood Technology (CNT) shows how to calculate environmental benefits across several different features (e.g., air-quality benefits from green roofs), which is invaluable if you are looking to measure your project’s results in these areas.