The most optimistic analysts project that trucks with empty cabs and a computer at the wheel will travel on U.S. highways in as little as two years with no escort or safety driver in sight now that the Trump administration has signaled its willingness to let tractor-trailers to become truly driverless.

The U.S. Department of Transportation this month announced that it will "no longer assume" that the driver of a commercial truck is human, and the agency will even "adapt the definitions of 'driver' and 'operator' to recognize that such terms do not refer exclusively to a human, but may in fact include an automated system."

The statements were part of a roughly 70-page document outlining the agency's latest interpretation of the existing federal laws and regulations that govern autonomous vehicles. The document falls short of formal rulemaking, but it is authoritative, experts say, signaling where the federal government will – and will not – intervene in the rollout of driverless trucks.

"It's pretty significant," says Richard Bishop, an automated vehicles industry analyst. "They're saying, as far as federal law is concerned, you can do it now."

The release of the new guidelines will almost certainly turbocharge the testing process and ramp up the competition between companies that have logged tens of thousands of miles in testing to prepare truly driverless trucks for the open road.

"Previous to this document, it was like, 'OK, guys, you've got 50,000 miles on your trucks, we'll wait for 250,000 miles, and it doesn't matter because the feds don't give us any clarity anyway,'" Bishop says. "Now it's, 'OK, you're at 50,000 miles, let's look really hard at what it takes for us to make the safety case and determine if those parameters have been met.' It's accelerating that process rather than sitting back and letting the guys test it."

Already, automated truck developers such as Embark and TuSimple have made freight deliveries where the computer takes control on the highway, overseen by a human "safety driver." Companies have also successfully tested "platooning," where a truck with a human driver leads a convoy of as many as five computer-driven trucks following at close distance to reduce drag and save fuel.

The technologies promise big savings, with driverless trucks potentially slashing 40 percent from the cost of long-haul freight – much of it in saved labor expenses – and platooning cutting 10 to 15 percent in fuel costs.

Going fully driverless, however, had been seen by some analysts as a milepost that might not be reached until 2030 or even the mid-2040s. The long rollout was partly attributed to ongoing technological challenges, as well as strong skepticism from the driving public, a sentiment stoked by a handful of fatal crashes involving automated passenger vehicles.

But there was also the issue of legal ambiguities, which left companies guessing how U.S. regulators would interpret federal laws and regulations that refer to a "driver" and when, or even whether, the Trump administration would try to harmonize the existing patchwork of state measures that set different standards for automated vehicles.

With the document last week, the Department of Transportation sent a strong signal that it plans to take a hands-off approach to regulating driverless trucks – one the agency also indicated it plans to make official through a formal rulemaking process that will almost certainly pre-empt any state measures, such as those in California that prohibit driverless trucks altogether.

"That seems to be the implication: that there is a political commitment from U.S. DOT to facilitate interstate commercial trucking and that, second, the agency will be able to pre-empt state laws," says Bryant Walker Smith, a professor at the University of South Carolina School of Law who studies law and technology. "What it reflects is a striking political commitment to automated trucking from this DOT."

The Transportation Department's document quickly sparked a backlash from some consumer rights groups, which raised concerns about driver and pedestrian safety as well as the persistent threat of cyberattacks.

Autonomous trucks have recorded no fatal accidents in the U.S., but at least four people have been killed in crashes involving semi-autonomous passenger vehicles, including most recently in March, when a woman crossing a road in Tempe, Arizona, was fatally struck by a self-driving vehicle owned by the ride-hailing company Uber.

"Despite deaths, injuries, and crashes involving a variety of semi-autonomous and autonomous vehicle technology across the country, DOT continues to insist that eliminating regulation is the way to achieve safety," the Center for Auto Safety said in a statement. The Transportation Department's document "perfectly captures this administration's approach to protecting people: get out of the way and let industry drive."

Ryan Calo, a professor at the University of Washington School of Law, allows that the document "concedes a lot to industry," but he adds that "it also reflects some mature thinking on the part of the DOT."

The document, notably, in a departure from each of the previous versions issued in 2016 and 2017, acknowledges that autonomous vehicles present a new kind of safety risk, even if they'll reduce the number of collisions overall. Previous guidance, by contrast, had simply highlighted the need for developers to minimize risks to public safety.

"They understand that things may happen with autonomous vehicles that wouldn't happen with human drivers," Calo says. "They're saying there may be fewer accidents, but there will also be different accidents, and we need to plan for that."

Whether that acknowledgment convinces the public is another matter. More than half of Americans, for example, are "very" or "somewhat" concerned about sharing the road with a driverless vehicle, according to a May 2017 survey by Pew. Another poll this summer, by Advocates for Highway and Auto Safety, a group that last week expressed concern that the Department of Transportation was taking "its hands off the wheel when it comes to safety" with its latest guidance document, found that more than two-thirds of Americans remain skeptical of autonomous vehicles.

"One of the big hurdles this technology needs to take, both for trucks and for cars, is consumer acceptance," says Stephan Keese, senior partner at the consulting firm Roland Berger. "As long as the vehicles stay safe, we will probably see a relatively bullish approach from the administration. If something were to happen – god forbid, but let's be realistic. This is a brand-new technology. I'm afraid at some point something will happen – we could see a very quick change in mindset driven by public opinion."

Autonomous trucks generally officially operate at what's known as Level 2, an engineering standard that includes technologies such as automatic braking, acceleration, and some amount of steering. (Basic cruise control, by contrast, would amounts to Level 0, and certain features such as lane-assist or adaptive cruise control would be Level 1). However, autonomous trucks are often effectively operating at Level 4 – or "high automation," with their safety drivers generally only taking over on local roads.