Hostile architecture is an urban design trend in which public spaces are constructed or altered to discourage people from using them in a way not intended by the owner. Also known as defensive architecture, hostile design, unpleasant design, or defensive urban design it is most typically associated with aggression against the homeless in the form of "anti-homeless spikes" — studs embedded in flat surfaces to make sleeping rough uncomfortable and impractical. Hostile architecture often targets the city's most vulnerable, both intentionally through anti-loitering and anti-skateboarding measures and unintentionally by making the cityscape hostile to all parts of the public, especially seniors, people with disabilities, and children.
Other forms of behaviour which are commonly designed against by hostile architecture include skateboarding, littering, loitering, and urination which are deterred with methods including sloped window sills to stop people sitting, benches with armrests positioned to stop people lying on them, and water sprinklers that "intermittently come on but aren't really watering anything." Although the term "hostile architecture" is recent, the use of civil engineering to achieve social engineering is not: antecedents include 19th century "urine deflectors". Its modern form is derived from design philosophy, Crime Prevention through Environmental Design which aims to prevent crime or protect property through three strategies: natural surveillance, natural access control, and territorial enforcement. The Seattle Department of Transportation installed bicycle racks to prevent the homeless from camping. A gas station in New Kensington, Pennsylvania installed blue lighting in their bathrooms that make it hard for drug users to find their veins.
Critics of hostile architecture argue that it makes contrarianism impossible, that it replaces public spaces with commercial or "pseudo-public" spaces and uses architecture "to enforce social divisions".
In 2018 British artist Stuart Semple created a social media public awareness campaign encouraging the public to place identifying stickers on instances of hostile design in their environment.
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- HostileDesign.org, Project homepage of Stuart Semple sticker campaign.