You've probably heard about self-driving vehicles, which usually are cars or vans. Actually, we're also closer to a time when even trucks will be driverless. A fresh sign of that comes from the U.S. Postal Service, which is in the midst of a two-week test transporting letters and packages 2,100 miles round-trip between Dallas and Phoenix -- across three states – via autonomous trucks that don't need human steering, acceleration, braking or turn signaling. Nine cameras and radar-like sensors let "robo-rigs" scan the road about 3,280 feet ahead.

For now, a "safety driver" and a monitoring engineer sit in the front seat – which also keeps other motorists on interstate highways from being freaked out by a truck driving itself. If all goes smoothly, this would be a big advance for the autonomous driving industry and a possible solution to driver shortages that freight haulers face nationwide. The Postal Service spends more than $4 billion per year on highway trucking services through outside contractors. New-era trucks could save hundreds of millions by eliminating drivers and rules that limit their hours. The mail service also is working with the University of Michigan on a self-driving truck for rural routes.

Three hands-off trucks in the Southwest test that began last week come from a new San Diego company called TuSimple that made the how-it-works video below. It earns commercial shipping rates from the Postal Service, and says it'll soon start a paid pilot program with a large federal agency to make 1,000-mile runs autonomously.

Because driverless trucks perform best on highways – with no traffic signals, pedestrians or bicycles -- the vision is to have them haul cargo long-distance between depots on the outskirts of big cities and then transfer the loads to smaller, manned trucks for local delivery. Four companies based in the San Francisco area also are racing to develop autonomous big rigs, and a Swedish firm is road-testing driverless electric delivery trucks.

Postal service says: "We are conducting research and testing as part of our efforts to operate a future class of vehicles which will incorporate new technology." – Kim Frum, spokeswoman

TuSimple president says: "Performing for the USPS on this pilot in this particular commercial corridor gives us specific-use cases to help us validate our system, and expedite the technological development and commercialization progress." -- Xiaodi Hou, company founder

Driver shortage: The American Trucking Association estimates a shortage of about 60,000 operators each year, due to an aging workforce and the difficulty of attracting younger drivers.