Late this year, Aurora Innovation Inc. plans to start hauling freight on Interstate 45 between the Dallas and Houston areas with 20 driverless trucks. Within three or four years, Aurora and its competitors expect to put thousands such self-driving trucks on America’s public freeways. The goal is for the trucks, which can run nearly around the clock without any breaks, to speed the flow of goods, accelerating delivery times and perhaps lowering costs. But the vehicles have drawn skepticism from safety advocates, who warn that with almost no federal regulation, it will be mainly up to the companies themselves to determine when the semis are safe enough to operate without humans on board.