Bridges are hard in part because of the way that Uber’s system works. Over the past year and a half, the company has been creating extremely detailed maps that include not just roads and lane markings, but also buildings, potholes, parked cars, fire hydrants, traffic lights, trees, and anything else on Pittsburgh's streets. As the car moves, it collects data, and then using a large, liquid-cooled computer in the trunk, it compares what it sees with the preexisting maps to identify (and avoid) pedestrians, cyclists, stray dogs, and anything else. Bridges, unlike normal streets, offer few environmental cues—there are no buildings, for instance—making it hard for the car to figure out exactly where it is. Uber cars have Global Positioning System sensors, but those are only accurate within about 10 feet; Uber’s systems strive for accuracy down to the inch.