Uber says a single metric isn't a clear indication of an autonomous car's safety

In the days since a self-driving Uber vehicle killed a pedestrian in Tempe, Ariz., local police and federal agencies have yet determine whether Uber's technology was at fault. But new documents obtained by The New York Times show Uber's technology had made little progress in the last year. The story details the series of setbacks the company faced in trying to get self-driving cars to market, including careless safety drivers who are supposed to take over test cars in case of emergencies. The key stat underpinning the report is what's known as "miles per intervention," or the number of miles the car can drive on its own before the safety driver has to take over the car. The Times story cites internal documents showing Uber was unable to meet its goal of driving an average of 13 miles without a driver having to take back control as of March.