Autonomous trucking developer Embark Technology said today it teamed up with vehicle rental and leasing giant Ryder System Inc. last month to conduct four self-driving truck tests, the first steps in offering the technology in the commercial space within a few years.

In a series of test drives beginning in October, the partners assigned humans to drive tractor-trailers in the first- and last-mile sections on surface roads. The trailers were then, transferred to an autonomous tractor for the highway portion, Ontario, Calif.-based Embark said. A human driver was behind the wheel whether the vehicle was operating in an autonomous or traditional driving mode.

The partners created a relay of handoffs between human-controlled and autonomous tractors hauling trailers loaded with Frigidaire refrigerators between distribution centers in El Paso, Texas, and Ontario, Calif. The 650-mile route covered stretches of Interstate 10 in Texas, New Mexico, Arizona, and California.

In each test, a human driver from Ryder's Dedicated Transportation Solutions division picked up the trailer at the Texas DC and brought it to a transition point along the interstate, where the driver unhooked and connected the trailer to Embark's automated truck. The Embark tractor then hauled the load along the highway portion before handing the trailer off to a second Ryder driver, who covered the final miles and made the delivery in California.

The Embark trucks operated at a "Level 2" automated driving setting, where computers handled accelerating, braking, and steering. In a Level 2 environment, the driver's hands can be off the wheel, and the driver is prepared to intervene at all times. Level 0 describes human-controlled driving, while Level 5 is full autonomy.

Technology behind self-driving vehicles continues to advance at a rapid pace. However, self-driving trucks won't begin cruising American roads until 2025 at the earliest, due largely to regulatory hurdles, according to a recent study from consulting firm Frost & Sullivan.

Automakers like Volvo and Daimler AG have conducted tests of self-driving trucks on U.S. and European roads. The San Francisco-based ride-hailing pioneer Uber Technologies Inc. jumped into the ring in 2016 when it acquired autonomous trucking startup Otto for $680 million.

Google Inc. sister company Waymo says it is on track to release an autonomous minivan by 2025. The automated vehicle systems developer Peloton Technology is working with fleet management services provider Omnitracs LLC and transport and logistics giant FedEx Corp. to roll out its truck "platooning" technology that links truck-driving controls so a human driver in one truck can lead autonomous vehicles following close behind.

By partnering with Embark, Ryder is building its familiarity with self-driving vehicle technology, and working through two phases of adopting it for eventual commercial use, Chris Nordh, Ryder's senior director for advance vehicle technology, said in an interview.

The first phase includes the recent Embark tests, which rely on a suite of "driver assistance technologies" as opposed to fully automated driving, he said. "We're already incorporating autonomous and semi-autonomous technologies into our logistics fleet and our rental fleet today, such as forward-looking radar, emergency braking, and lane departure warnings. Those are building blocks," Nordh said.

The second phase of bring autonomous trucks to market will require changes in the regulatory environment and in social acceptance of the technology by other drivers on the road, Nordh said. Those hurdles may take time to clear, but as the public increasingly sees fully autonomous, "Uber-type" vehicles driving in urban environments, they will become more accepting of the idea of sharing the highway with autonomous trucks, said Nordh.

As Embark continues to push its autonomous trucking technology to market, it will focus on highway driving, which has more structured rules than local roads, allows for faster commercialization, and could help solve a looming driver shortage challenge for the freight industry, according to Embark CEO Alex Rodrigues.

"Trucking is facing a workforce problem,"" Rodrigues said in a statement. "More than 50 percent of all drivers will retire in the next two decades and there aren't nearly enough young drivers joining the industry to replace them. By allowing automation to work together with local drivers to handle less desirable long haul routes, we will be able to increase productivity to address the current shortage of 50,000 drivers while creating new local driving jobs that attract younger drivers for the industry."

Embark will also continue to work with retailers like Frigidaire, which said the technology could help it ensure a reliable delivery network, according to a statement from AB Electrolux, Frigidaire's Swedish parent. "Every month we're making big strides," Rodrigues said. "We're working towards commercializing this technology and aim to have it operational within just a few years."