How Green Space Fosters Safer Communities
5 questions with researchers Mardelle Shepley and Naomi Sachs
So far this year, over 35,000 people have died due to gun violence in America. In 2020, guns became the leading cause of death for American children and teens, for the first time taking more young lives than car crashes. These tragedies affect communities across the country and are prompting new conversations, policies and actions to stem the loss of life.
As communities look for ways to prevent further tragedy, research has revealed a promising, but perhaps less intuitive, option: investment in parks and green space. Recently, a team of researchers collaborated to uncover whether green space reduces violent crime and what types of interventions appear most impactful. We spoke with Dr. Mardelle Shepley of Cornell University and Dr. Naomi Sachs of the University of Maryland, two authors of the study, to understand how green space reduces violence and how cities can support community safety by investing in parks, trails, trees and more.
This conversation has been edited for length and clarity.
Q: To start, could you share some of the top takeaways from your research? How does civic infrastructure like parks and trails impact community safety?
Mardelle Shepley [MS]: One primary conclusion from our research is that there’s evidence green space can be a strategy for reducing crime. Another is that there are specific mechanisms at play, though we don’t yet know which are having a major role. It may be all of them or just a couple of them. Third, there could be cost benefits. We’d like to think everyone’s primary motivation is to increase people’s health and wellness, but there’s evidence these interventions can also save money.
Naomi and I co-authored a second paper with Hessam Sadatsafavi, Michelle Kondo and Ruth Barankevich — a meta-analysis to determine how much money could be saved by greening vacant lots. While mowing on its own did not have an impact, greening and gardening interventions reduced firearm violence by more than 5%. And the cost savings from lowering firearm crimes recovered between 8% and 21% of greening expenses depending on the size of the urban area.
Our motivation for doing this work is that violent crime, particularly gun violence, is a serious issue in our country. So we’re very motivated to figure out how we as designers can stem the tide of this terrible trend.
Naomi Sachs [NS]: Yes, exactly. There is evidence that urban green space reduces violent crime, but it’s not across the board. It’s not, just plant a million trees and everything will be fine. There are nuances, and we don’t yet know what all of those nuances are.
Q: Why is crime lower when people have access to a nature-rich built environment? What changes for people in the situations where green space is working well?
MS: We have hypotheses about this, and one is that it creates a place — a designated space for people to interact, feel like they’re part of a community and have a sense of security. Another is exercise. People exercise in most of these green spaces, and exercise impacts endorphins and improves people’s health. There’s also the idea of biophilia, which refers to our innate affiliation with nature. From an evolutionary perspective, the more you understand the world around you, the better your chances of survival. This phenomenon is parlayed into preferences. Nature itself has stress-reducing properties. Some data suggest that when you look at water, your blood pressure drops.
Also, parks can define a territory that had no definition before. Parks, green space or even a clean lot look like they belong to somebody. And if you’re in a community that’s been recently endowed with a green amenity, that may give you a sense of pride in your community, and hope. The last item is the notion of climate modulation. Instead of asphalt, you have greenery, which helps reduce the heat island effect. The trees themselves can filter particulate matter and improve the quality of the air. And those health-laden characteristics actually may result in a reduction in crime.
NS: In the first paper we talk about the idea of broken windows theory — the perception that something is being taken care of, those cues to care, as Dr. Joan Nassauer would say. So the opposite of broken windows is important. Also, there’s a lot of great research demonstrating that exposure to green space reduces stress, and that stress reduction can translate to individuals and communities in a lot of good ways. Blood pressure goes down, the heart rate slows and cortisol, which is a marker of stress, goes down. Emotionally, anxiety is also reduced. Rumination, which is a specific form of anxiety, decreases. And things that go up include well-being, altruism, empathy and a sense of connection to others. Cognitively, we see increased memory performance, problem solving and creativity.
In terms of specific behaviors, people are more encouraged to go outside to a green space, or walk by a place that maybe before was a scary vacant lot. People participate in activities they didn’t feel comfortable doing before. And we’ve heard anecdotally about people having block parties and picnics and utilizing a new green space that was previously a space to avoid. Now it’s a place for people to come together with their dogs, their kids, their neighbors.
Finally, Charles Branas and his team in Philadelphia wrote several papers on the Transforming Vacant Land program managed by the Philadelphia Horticultural Society. The vacant lots were overgrown, and the trash and weeds were providing places to hide guns and drugs. When these lots were cleared out, there was nowhere to do illicit activity anymore. And when people in the neighborhood occupied the site again, there were eyes on the street.
Q: When it comes to public spaces, which characteristics are particularly impactful for fostering safer communities?
MS: In our study, we focused on five areas: parks, community gardens, vegetated streets and walkways, trees and ground cover, and undeveloped green space. Out of 14 studies, nine indicated that having trees and grass reduced violent crime. Four studies were inconclusive, but no studies found evidence contrary to that hypothesis. So that starts amounting to something significant. For the other categories, there was typically a large number of situations in which things improved, but the evidence was not quite as strong as trees and ground cover.
Q: Based on your research, what do you think cities should invest in if their goal is to reduce violence? How would you suggest putting the insights of your research into action?
NS: I think both adding green space and improving green space are incredibly important. And there is good research that the people who benefit most from improved green space, and from green space in general, are people who least often have access to it, which is disadvantaged communities, often Black and brown and communities with people living in poverty. They benefit greatly from introducing green space, whereas in richer communities, there’s already green space, and the benefits reach a plateau.
Some examples of investments include cleaning and greening vacant lots, making existing places safer and greener — like adding nature to a playground at a school that’s just asphalt with basketball courts — or opening up a playground at a school to the whole community.
One thing we found but could not explore in our study is the idea of co-benefits. In addition to reducing crime, greenery is helpful in many other ways. It’s what we call an “ecosystem service.” Trees and other green spaces reduce the heat island effect. Trees purify the air, produce oxygen, improve stormwater conditions and stabilize the soil. They are aesthetically pleasing. They’re even fun to climb. So there are additional benefits that green space and especially trees provide.
But again, the details matter. Even the kind of tree matters. Some trees, such as silver birch, yew and elder, are far better at filtering emissions and reducing pollution than others. Different tree species also produce different amounts of pollen, worsening people’s allergies. For example, although silver birch trees filter emissions, they also produce a lot of pollen. Factors like these make decisions about which trees to plant rather nuanced.
Similarly, there are very successful tree planting programs, and there are some that are unsuccessful, and it’s often because the community hasn’t been engaged. In Detroit about a decade ago, around a quarter of residents eligible for free trees were saying no. A study by Dr. Christine Carmichael at the University of Vermont revealed important reasons why, and it most often had to do with negative experiences with trees, or with city workers or outsiders who did not care for the trees. People feared that trees would damage their properties, leaving them with the costs. This shows that the idea of “let me do this for you” does not work nearly as well as, “How can we work together and be respectful and provide the best solution?”
Q: Investing in nature-rich environments to improve safety may be counterintuitive for a lot of people. What are some of the best arguments for investing in public space as a way to improve public safety?
NS: The more we amass evidence that is rigorous and not just anecdotal, the more convincing that argument becomes. And it goes to the co-benefits as well. We know the heat island effect and rising temperatures also affect people’s aggression. There’s more crime when it’s hotter. By planting trees and making a place greener, we’re also reducing the heat island effect and other harmful impacts of climate change. And it’s also, according to our literature review, helping to reduce violent crime. But I will say again that it has to be done right.
Community engagement is really important. And so are things like limbing up trees and providing clear sightlines, not providing places where people can hide in bushes. These are cues to care. So even if it’s more like a Piet Oudolf landscape with wildflowers and grasses, and it’s a little messy, there need to be clear cues that the place is being taken care of and maintained. Someone is caring for this space and people are using it for playing with their kids and walking their dogs and having picnics or just hanging out. That’s really important.
MS: Something that’s come up a couple of times is the idea of working with communities directly to find out what’s happening and including members of the community as paid members of the research team. So instead of academic researchers coming down and saying exactly what to measure, we go to the community and say, “This is what’s happening. How do you think we should be measuring this? Who should we be talking to and how should we be talking to them? What tools should we be using?” That’s where my research is right now.
NS: Gun violence is a huge topic, and the greatest percentage of gun violence is suicide. And here, the co-benefits — reducing stress, improving a sense of well-being and community pride, ameliorating climate change — are really important. They indicate that access to and engagement with green space can help improve mental health, which helps communities dealing with gun violence from suicide. Right now I am working on Baltimore’s Project VITAL study, and this connection is one aspect of community safety that we are investigating.
Dr. Kati Peditto and Christine Tanner Fournier of Cornell University and Dr. Hessam Sadatsafavi of Johns Hopkins Medicine collaborated with Drs. Shepley and Sachs on their meta-analysis The Impact of Green Space on Violent Crime in Urban Environments: An Evidence Synthesis.
Reimagining the Civic Commons is a collaboration of The JPB Foundation, the John S. and James L. Knight Foundation, The Kresge Foundation, William Penn Foundation, and local partners.
