The Google car has now driven more than half a million miles without causing an accident—about twice as far as the average American driver goes before crashing. Of course, the computer has always had a human driver to take over in tight spots. Left to its own devices, Thrun says, it could go only about fifty thousand miles on freeways without a major mistake. Google calls this the dog-food stage: not quite fit for human consumption. “The risk is too high,” Thrun says. “You would never accept it.” The car has trouble in the rain, for instance, when its lasers bounce off shiny surfaces. (The first drops call forth a small icon of a cloud onscreen and a voice warning that auto-drive will soon disengage.) It can’t tell wet concrete from dry or fresh asphalt from firm. It can’t hear a traffic cop’s whistle or follow hand signals.
And yet, for each of its failings, the car has a corresponding strength. It never gets drowsy or distracted, never wonders who has the right-of-way. It knows every turn, tree, and streetlight ahead in precise, three-dimensional detail. Dolgov was riding through a wooded area one night when the car suddenly slowed to a crawl. “I was thinking, What the hell? It must be a bug,” he told me. “Then we noticed the deer walking along the shoulder.” The car, unlike its riders, could see in the dark. Within a year, Thrun added, it should be safe for a hundred thousand miles.