Have you ever sat in a park bench and wondered why it was so…uncomfortable? Often times, that's not an accident.
Arm rests in the middle of a bench prevent someone from sleeping there, and short backs are designed to discourage people from lingering too long. Then of course there's the New York City subway's notorious example of “leaning bars,” which don't allow anyone to sit at all.
This is called “defensive design,” and it's used to alter human behavior and/or limit the ways in which an object can be used, whether it be a bench or a part of a building.
Defensive design isn't just limited to benches. Strategically-placed, highly-visible security cameras are another example, as are rounded edges on ledges and ridges on handrails meant to discourage skateboarders. Recently there's been discussion about benches, planters, and bollards that are designed to make vehicle-based terrorist attacks more difficult.
There's also defensive design aimed at animals, like spikes on buildings or ledges that make roosting impossible for birds.
Even streetlights are a form of defensive design—a classic example, in fact. When they became popular in the 19th Century, streetlights changed cities forever. They made it safer to go out at night so more people spent more time out, which in turn drove economic development.
Most defensive design tries to solve some problem, like detering crime. However, some designs specifically target people who are experiencing homelessness, and/or make life more difficult for some people with disablities. As our cities grow, we need to think about who we're designing our public spaces for, and who we treat like a problem and exclude.