Uber on Wednesday rolled out a fleet of self-driving cars in San Francisco, the second U.S. city to participate in its pilot project. Pittsburgh was the first. Riders who request an UberX — the lowest-priced Uber vehicle — will be matched with one of the self-driving Volvo XC90 SUVs if one is available and if the customers are willing, according to Anthony Levandowski, head of Uber’s Advanced Technology Group.