We hold certain truths about cities to be self-evident. That includes the theories put forth by urbanists like Jane Jacobs decades ago—like the idea that design elements such as glazing and street lighting make streets safer, or that architectural diversity and pedestrians are both keys to healthy neighborhoods. But up until recently, it’s been tough to test those theories broadly and empirically; every city is different, and collecting enough data to study them all has been arduous.

That’s changing, as technology like computer vision and deep convolutional neural networks have become more common in urban design—and as researchers studying cities have begun using them to test the 70-year-old ideas of seminal urbanists like Jacobs, Kevin Lynch, and others. This nascent field is introducing big data to ideas about design and urbanism that have existed mostly as theories for decades.

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So, what about a specific street makes it appear safe or unsafe? The authors describe a further experiment carried out on the Google Street View images. They obscured different parts of each image and tested how the neural network perceived the safety of each section of a street, building up a library of elements that either contributed or detracted from the safety of a given street. Fascinatingly, there were specific and repeated design elements that scored as unsafe, including parked cars, blank walls, open pavement, and darkness. Other elements were associated with safety—green space, glazing, open areas, and sidewalks.

Hidalgo is careful to note that finding a correlation between aesthetics and the perception of safety isn’t the same as finding causation between aesthetics and crime, as policies like the Broken Windows Theory assume. Instead, he wants to develop more empirical ways to study cities and the way they’re perceived—and, in turn, provide better science to the policy-makers who shape legislation.

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In a new paper that will be presented at the ACM Multimedia Conference this fall,Hidalgo and coauthors, Marco De Nadai and Bruno Lepri, test two established ideas about urban design. One is Jane Jacobs’s theory of "natural surveillance," where street lights, windows, open spaces, and diverse uses all contribute to local residents being able to keep their neighborhoods safe. The other is "defensible space theory," an idea posed a decade later by Oscar Newman, who argued that architectural details, like an archway or a set of steps leading up to a front door, create semiprivate spaces that locals are more likely to surveil and defend.