Autonomous vehicle trials are getting bigger and bigger.

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UPS announced Thursday that is has been testing self-driving tractor-trailers in Arizona for months with TuSimple, which is developing the autonomous technology. UPS said TuSimple has been using self-driving trucks to transport its goods between Phoenix and Tuscon since May.

A driver and engineer have been riding in each vehicle because a driver is legally required, UPS said. The companies monitor the distance and time the trucks drive themselves, plus safety data and transportation time.

UPS announced the tests when it said it had also taken a minority stake in TuSimple. The companies didn’t share the terms of the deal.

TuSimple said it’s working on technology that will allow shipping companies to operate self-driving tractor-trailers of the heaviest class.

Scott Price, chief strategy and transformation officer at UPS, said the company is committed to developing and using technologies that help them operate more efficiently.

“While fully autonomous, driverless vehicles still have development and regulatory work ahead, we are excited by the advances in braking and other technologies that companies like TuSimple are mastering,” he said. “All of these technologies offer significant safety and other benefits that will be realized long before the full vision of autonomous vehicles is brought to fruition — and UPS will be there, as a leader implementing these new technologies in our fleet.”

The deal could also be a money saver for UPS. The company currently contracts with third-party trucking companies during its peak shipping season, but TuSimple said it can cut average purchased transportation costs by 30 percent.

“TuSimple is confident that it can accelerate bringing the first self-driving truck to market to increase road safety,” said Xiaodi Hou, TuSimple’s founder, president and CTO.

A number of companies, including Tesla and Alphabet’s Waymo, have been testing self-driving cars. In Texas, Kodiak Robotics has also been testing self-driving trucks on highways.

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