On a three-lane test track along the Monongahela River‚ an 18-wheel tractor-trailer rounded a curve. No one was on board. A quarter-mile ahead‚ the truck's sensors spotted a trash can blocking one lane and a tire in another. In less than a second‚ it signaled‚ moved into the unobstructed lane and rumbled past the obstacles. The self-driving semi‚ outfitted with 25 laser‚ radar and camera sensors‚ is owned by Pittsburgh-based Aurora Innovation Inc. Late this year‚ Aurora plans to start...