We often talk colloquially about the "fast pace of city living," and that pace actually has a default speed: We’ve long assumed that people cross the street walking at about 4 feet per second.
Crosswalks are timed with this number in mind, so you don't get clipped by a creeping car when the red hand starts flashing at you midway through an intersection. But the older we get, the more likely we are to slow down. Most 80-year-olds just don’t move at 4 feet per second.
This bit of infrastructure trivia wasn't all that relevant just a few decades ago. "In the '60s, a majority of people weren't living past 70, or 75," says Hilde Waerstad, a physical therapist and research associate with the MIT AgeLab. As the vast baby boom generation now prepares to age well beyond 75, the demographics of entire cities will effectively age, too. "We're entering into this new era," Waerstad says, "that we just have not seen before."