Starting on Tuesday morning, letters and packages moving between Phoenix and Dallas will travel on customized Peterbilt trucks run by TuSimple, an autonomous startup based in San Diego. There will be five round trips between the two cites, with the first haul leaving from Phoenix this morning. It’s the first time that the Postal Service has contracted with an autonomous provider for long-haul service.

“This pilot is just one of many ways the Postal Service is innovating and investing in its future,” the USPS said in a press release that cited the possibility of using “a future class of vehicles” to improve service, reduce emissions and save money. After the initial trial, which is expected to last about two weeks, the Postal Service will assess whether to continue working with TuSimple.

Two years ago, the USPS Office of Inspector General outlined a step-by-step approach for the adoption of autonomous vehicles, and then earlier this year put out a request for ideas on using autonomous technology in its delivery fleet. While self-driving mail trucks are still years—if not decades—away, autonomous long-haul trucking might be realistic much sooner.

The Postal Service spends more than $4 billion per year on highway trucking services through outside contractors. Those costs have been rising due to a national shortage of drivers. Self-driving trucks could save hundreds of millions by eliminating human drivers and the hours-of-service rules that keep them from driving round the clock.

For now, however, TuSimple will have a safety driver behind the wheel for the 1,000-mile trip between Phoenix and Dallas, as well as an engineer in the passenger seat monitoring the autonomous systems. In the future, the startup aims to provide “depot-to-depot” service without drivers.

“When the vehicle can operate truly driverless, it will be much more efficient,” said Chuck Price, chief product officer at TuSimple. “We think we complete a coast-to-coast run in two days, where today it takes five.”

TuSimple has raised $178 million in funding since its founding in 2015. For its most recent round in February, the company was valued at $1 billion. Its trucks have been carrying cargo for customers in Arizona since last year.

Price said that drivers who encounter a TuSimple truck on the interstate between Phoenix and Dallas probably won’t notice: “It’s polite. It uses turn signals. It merges properly. It does all of the things that a professional driver is trained to do.”

And TuSimple, he added, would rather that nobody knows about the robot driver. “We are actually trying to minimize marking,” Price said, “because we find that people tend to either get distracted in amazement or distracted in the devious way and try to mess with us.”