With mass shootings on the rise, an unexpected group of professionals is trying to make it easier for people to avoid getting killed in the melee: landscape architects. These are the people who place just so many oversized planters on a pedestrian thoroughfare to prevent an attack by car bomb from hitting the crowd or the buildings behind the barricades. And one of their first design challenges is the new Sandy Hook Elementary School, built to replace the site of the infamous 2012 massacre.

During the school day, visitors have to pass through the gap in a rain garden to get to the hardened entrance, and they can be observed from inside during the approach, thanks to the Svigals + Partners design, as well as specific landscape architecture from Richter & Cegan. Once inside, the doors close, and classroom doors deadbolt automatically. Windows are coated with a special hardened glaze, so that even if the window is shot, it would take an attacker 10 to 15 minutes with a sledgehammer to force open a hole big enough to crawl through. Delay animates everything about the design, to buy time for the people inside and mitigate the harm an attacker can commit while waiting for rescuers to arrive.

Delay animates everything about the design

While urban planners and architects can’t hope to stop all the forces that lead to a mass shooting, they can understand how the crowd flees. And they can design spaces to discourage crimes of opportunity and reduce the damage an attacker can do, as this elementary school has. When people have to shelter in place, it is the past work of architects that determines just how safe those places are. Outdoor spaces designed by architects and built with crowd dynamics in mind might save lives when the next concert suddenly turns into a bloodbath.

Landscape architecture already adapted to the post–9/11 fear of terror attacks. In 2004, the American Society of Landscape Architects held a symposium on this new reality, called “Safe Spaces: Designing for Security and Civic Values..” The symposium featured experts talking about everything from “hardening historic walls without sacrificing original materials” to using dense clusters of trees as a barrier for cars that wouldn’t stop people on foot from walking around.

When it comes to outdoor spaces, security focuses on what happened in the past. That can mean large barricades to stop cars from entering, bomb-sniffing dogs to check abandoned backpacks, and bag checks at gated entrances. Yet, all these defenses share a failure in common: there are protections against what previously worked, against ways past attackers entered schools or ambushed marathons or scattered concerts and hurt people. It is hard to prepare for new attacks, from novel directions. Like the attack in Las Vegas, where a man on the 32nd floor of a hotel with a room overlooking a concert used almost two dozen semi-automatic rifles to shoot people on the ground below.

“Whatever risks we evaluate, people get crazier and crazier, or cleverer and cleverer,” says Jay Brotman, a partner at Svigals + Partners. “Vegas is a good example. They probably had all these layers of barriers, people watching everything else, but somebody did something totally different that took them all totally out of play.”

“Whatever risks we evaluate, people get crazier and crazier, or cleverer and cleverer.”

To try to help people face down shooters, the Crime Prevention through Environmental Design makes interior spaces to protect those threatened inside until they can escape. For outdoor spaces, there are some tools, like low hedges to make it easier to stop approaching people, or one-way gates that unlock from the inside but require an electronic sign-in to open from the outside. With big, temporary outdoor installations, there are fewer tools available.

The main thing to take into account for these designers is how people move — or perhaps, more accurately, stampede — in response to threats. Researchers draw from studies of how people move, observations of real-life tragedies, and computer modeling in order to determine how people behave in crowds: how they get stuck, trampled, or endanger others in their attempts to escape.

Studies on crowd dynamics show that when people are stressed and fleeing, they can flee more slowly, and they might be less flexible in what route they pick if under a time constraint. (Both conditions that likely apply to a crowd trying to escape an active shooter.) There are limits to what researchers can observe through participant study; high-pressure, high-stress situations, like how intoxicated people might flee sudden loud noises, are ethically off limits.

“To analyze a threat that can come from anywhere — north, south, east, or west — and then to plan for egress in all directions is,” says Brotman, “near-impossible.”

There are lessons that can be drawn from computer simulations, like modeling the entire evacuation of a stadium under ideal conditions. And there are crowd flow games, like this one about navigation and evacuation designed by Bode. There’s also testing models with real-world volunteers, like the aforementioned Boy Scouts. An advantage of experiments with volunteers is that humans can wear color-coded outfits, showing how multiple sections of stadium seats intermingle down the exit, or how traffic moving in different directions down a hallway eddies and flows.

Outside of computer simulation and participant studies, researchers look at observations from real emergencies and disasters. A 2007 study looked at the crowd flow during the Hajj in early 2006, where millions of pilgrims came to Saudi Arabia. Foreign visitors gathered in a tent city, and then navigated from streets of the tent city to a narrow bridge. By studying where crowds intersected and how the dynamics changed at a critical density, the authors were able to recommend changes that ensured the Hajj in 2007 went off without a similarly tragic incident. (However, that same tent city would see another fatal crush killing at least hundreds in 2015.) Using a mathematical model to simulate the flow and turbulence, the researchers could apply some principles of fluid dynamics to the crowd and its movement.

A generation ago, urban planning didn’t focus on terrorism

The biggest limitation of observation is that it can be hard to separate the variables: did people panic because of the heat, or did the crush of bodies in the intersection overheat everybody unfortunate to get caught in the stampede? Experiments by academics can be expensive to run, and can be so artificial as to also produce unrealistic results. Besides that, there are good ethical reasons not to test crowds in high-pressure and high-stress experiments. Still, planners looking to make outdoor events easier to survive, observation study of horrific situations like fatal stampedes or people fleeing a massacre, it may be the only option.

A generation ago, urban planning didn’t focus on terrorism; from the 1970s through the 1990s, the design focus was crime. “Safer City Centres: Reviving the Public Realm” features an exhaustive selection of ways to deter crime (primarily theft, though also vandalism and gun violence) through actions individuals and society can take. Nestled in that list of options, as one factor among many to save a public center from crime, is “gun control.”

Update October 19 5:00PM: Updated to add credit to landscape architects Richter & Cegan in the second paragraph.