Self-driving cars have it rough. They have to detect the world around them in fine detail, learn to recognize signals, and avoid running over pets. But hey, at least they’ll spend most of their time dealing with other robot cars, not people. Now, a delivery robot, on the other hand, it roams sidewalks. That means interacting with people—lots of people—and dogs and trash and pigeons. Unlike a road, a sidewalk is nearly devoid of structure. It’s chaos.