(NEXSTAR) – Once you see it, you can’t unsee it. Examples of hostile architecture, also known as hostile design or defensible architecture, are everywhere in cities and public spaces.

You’ve likely seen it outside businesses, at bus stops, along sidewalks, and in parks, as it can take many forms. Hostile architecture is “hostile” because it subtly (or sometimes quite plainly) keeps people away.

“Through design you are making them uncomfortable so that they leave, or you’re making particular activities that they tend to do uncomfortable,” explained Anastasia Loukaitou-Sideris, a professor of urban planning and dean at UCLA’s Luskin School of Public Affairs.

One common example of hostile architecture is simple: a bench divided in two, usually to discourage someone from lying down. Even the makeup of the bench (slats of metal with gaps in between) is comfortable enough for a short sit, but is a subtle way to discourage lingering for hours.

Some transit stations eliminate benches altogether, offering riders a surface to lean against instead.

Other pieces of hostile design are more obviously hostile in appearance. Metal spikes, for example, can be seen along sidewalks or under awnings to prevent people from sitting or lying on the ground.

But sometimes it’s less obvious, like when large, steel planter bins were installed on San Francisco sidewalks. Residents of the neighborhood said it wasn’t just for beautification, but also to leave less room for people to set up tents on the sidewalk.

Another San Francisco neighborhood tried to do the same thing but with large boulders, which were ultimately removed.

The Seattle Department of Transportation once installed bike racks under an overpass. Critics said there was little need for bike parking there, and the way they were positioned was more likely designed to keep people from setting up tents, the Urbanist reported. The city ended up removing the bike racks.

Another more subtle example of hostile design are metal bars or divots that interrupt long benches or railings. These are usually used as a tactic for deterring skateboarders.

Hostile design can also be less about the physical space. “Another example is there was a Skid Row park in Los Angeles where the authorities would start the sprinklers at night so people could not sleep in the park,” Loukaitou-Sideris said. There are also stories of businesses blasting loud music outside to discourage gathering or loitering too long.

In recent years, much of the conversation around hostile architecture has centered around issues of homelessness. It’s part of a larger conversation about how public spaces are designed and who they are for.

On one side of the debate, Loukaitou-Sideris said: “You can consider bus stops public spaces because everybody can sit there and they should be open and accessible to the public, right? And unhoused people often use the bus — sometimes they use it as shelter but sometimes they use it to go to work, or reach a destination. So by excluding them from these spaces, does it retain the publicness of the bench?”

But on the other hand, “Local governments would say that if the space is occupied constantly by an unhoused person, then other legitimate riders will not be able to use it.”

While many pieces of hostile design target homeless people, in particular, they’re not the only people impacted. An uncomfortable bench is uncomfortable for everyone, not just someone trying to sleep on it. If steel planters or boulders take up half of the sidewalk, you’ll have less room to walk or push a stroller. If there are spikes on the ground under an awning outside a store, you’ll have a harder time standing there to shelter from the rain.

The debate is nothing new — there are examples of hostile architecture that date back more than 100 years — and it rages on in state legislatures, academic spaces and online. What may seem like a relatively small design choice can encompass issues of discrimination, access to public resources, social control and individual freedoms.

Let’s go back to the example of the bench for a moment.

Selena Savic, co-editor of “Unpleasant Design,” describes it on an episode on the “99 Percent Invisible” podcast:

“A classic is the bench with armrests in between, which of course let you rest your arm on the armrest, but at the same time they restrict any other kind of use than sitting upright. The only way this bench can be used is by three people sitting next to each, not looking at each other — which is not the only thing you can do on a bench. Especially, it’s not the only normal and legal thing you can do on a bench.”

By trying to keep someone from sleeping on the bench, designers may end up keeping people from wanting to use it at all.

Savic continued, “When we expect people to do bad things to the bench before we think of anything good that people might do to this bench, I think that’s a very sad approach to public space.”