The first video 21-year-old Roh posted was in July. In about 20 seconds, he follows five different walkers, quietly unpacking their gait to camera before rating them for speed, direction, footwork and phone use. He has since rated walkers in New York in more than 30 videos, and has recently diversified to other US cities. This is his first time in the specific chaos of central London. “So far, it seems comparable to New York,” he says. Just then, a Lime bike almost collides with us outside a cinema. “Come on!” he cries. “Those city bikes are a new thing. That’s an automatic zero, buddy.”