When city officials and urban planners measure street safety, they usually do so by counting the number of crashes: How many times has a driver hit another driver, or hit a pedestrian crossing the road, at this particular intersection? To Megan Ryerson, a transportation engineer and urban planner at the University of Pennsylvania, that view of safety is too narrow. We can count crashes and deaths, but those numbers don’t tell us anything about how people feel as they travel around a city.

That metric also limits how we even begin to measure the concept of “safety.” A map of crash hot spots doesn’t tell us what routes people avoid entirely because they don’t feel safe on that road. It also means a safety measure might be put in place only after a certain number of crashes, so there’s a “literal human cost” to the way we measure or plan for safety, Ryerson says. Instead, she created another metric, based on biometric data that can identify dangerous or challenging parts of urban infrastructure before a crash ever occurs.

For her study, a group of cyclists biked around Philadelphia wearing eye-tracking glasses and a gyroscope that collected data on when and where they moved their eyes, and how often and at what points in their ride they swiveled their heads around. Those movements indicated the cognitive workload and stress levels the cyclists felt as they traveled through different types of road infrastructure, like riding in a protected bike lane for one stretch, and then in a “mixing zone”—an area where there is no physical separation between bikes and cars—the next.

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