The U.S. Postal Service is making a move into autonomous long-haul trucking. In a limited trial, it will run five round-trip routes on interstate highways between Dallas and Phoenix.

• The U.S.P.S. is partnering with a San Diego autonomous driving technology company, TuSimple

• Trucks will carry someone who can take the wheel if something goes wrong

• The U.S.P.S. is interested in driverless trucking as a way to cut costs and speed deliveries

• Trucks might be a natural fit for automation — routes are simpler and more repetitive than those traveled by passenger vehicles

• Autonomous trucking is seen as a possible solution to a chronic and growing shortage of long-haul truckers