The U.S. Postal Service says it has found a way to cut costs and speed up mail delivery: self-driving trucks! The government agency recently tested autonomous tractor trailers on a 1,000-mile route between Phoenix, Arizona, and Dallas, Texas. The 22-hour trip normally requires multiple drivers because of limits on how long employees can spend behind the wheel. On the test runs, a human safety engineer and a backup driver rode in each self-driving truck. But the test showed that self-driving vehicles could one day handle the route solo. That would make the trip faster and save money on drivers. As for local deliveries, self-driving trucks could debut in the next decade, the agency says.