A semi-truck drives through a puddle on the construction site of the American Center for Mobility in Ypsilanti, Mich., on Aug. 15. Rep. Debbie Dingell (D-Mich.) and Rep. Robert E. Latta (R-Ohio) visited the center to meet with experts in the autonomous vehicle industry. (Sean Proctor/Bloomberg)

The House passed a major self-driving-vehicle bill Wednesday. But a big unanswered question remains: what to do about commercial trucks.

Driverless trucks are seen as one of the most promising — and fraught — elements of the coming autonomous future on U.S. roads. Convoys of robo-trucks guided across the country by a single human driver — or none at all — could become a major economic force. They could be a boon to safety, or a particularly potent hazard, opposing advocates say.

They could also gobble up plenty of good-paying jobs.

And so lawmakers seeking bipartisan backing for the Self Drive Act made clear that their definition of a "highly automated vehicle . . . does not include a commercial motor vehicle," as the legislation puts it.

That means it doesn't cover trucks bigger than 10,000 pounds, or vehicles meant to carry more than 10 passengers or hazardous materials.

The bill blocks states from regulating "the design, construction, or performance" of automated vehicles, clarifying that such power is in federal hands. Many technology and car companies have warned that state legislators are leaving behind a "patchwork" of regulations that could dampen innovation and thwart travelers crossing state lines.

Some state officials, meanwhile, argue that federal guidelines on autonomous vehicles, which are voluntary, do too little to guarantee safety.

The U.S. Transportation Department has been working on changes to the Obama-era policies, and Transportation Secretary Elaine Chao will travel to Michigan next week to describe updated guidelines.

The House legislation passed Wednesday also allows automakers and tech companies to seek exemptions, totaling in the tens of thousands per company, from federal vehicle safety standards, as long as the companies can ensure that a car's safety won't be downgraded. That would allow, for example, an automaker to ditch the steering wheel to allow more creative driverless designs. The bill also instructs Chao, within two years, to require "safety assessment certifications" that demonstrate driverless vehicles "are likely to . . . function as intended and contain fail safe features."

The legislation calls for the creation of an advisory council to wrestle with a long list of outstanding questions on autonomy, including cybersecurity concerns, environmental impacts, and access for people with disabilities and those living in "rural, remote, mountainous, insular, or unmapped" areas.

"While the National Highway Traffic Safety Administration can't write a safety standard to make us all perfect drivers, it can work to advance lifesaving technologies to avoid collisions, and that's part of what this bipartisan legislation will put in place," Rep. Greg Walden (R-Ore.), chairman of the House Energy and Commerce Committee, said before Wednesday's vote.

The Self Drive Act came out of the commerce committee with unanimous support this summer, offering a rare bipartisan win on a high-profile issue on which members are eager to show results.

A separate House committee, Transportation and Infrastructure, has jurisdiction over trucking, which meant that backers of the Self Drive Act could avoid the touchy and potentially perilous driverless truck issue. But the Senate committee on Commerce, Science, and Transportation, which is crafting its own bill, has wrestled with trucking, and it's not clear how the House and Senate approaches will eventually mesh.

The Senate committee's chairman, John Thune (R-S.D.), on Wednesday said trucking "has emerged as a pivotal issue" for Congress and announced a hearing on the matter next week. The issue has gummed up efforts to write legislation in the Senate.

The International Brotherhood of Teamsters was among the groups pushing Congress to stay clear of trucks.

James P. Hoffa, president of the Teamsters, said many issues remain with the House bill. But the union, which represents 600,000 drivers, commended members of Congress "for recognizing that a starting point for any discussion on this subject was that no legislation should impact commercial motor vehicles or traditional commercial drivers," Hoffa said in a statement.

Hoffa said the Teamsters must be at the center of any separate discussion on autonomy and trucking to make sure that technology is "not used to put workers at risk on the job or destroy livelihoods and chip away at the middle class."

But Michael Cammisa, vice president of safety and connectivity at the American Trucking Associations, said the industry doesn't think that "it makes sense to write legislation without it applying to all vehicles, and that includes commercial trucks which account for 33.8 million registered vehicles and 450 billion miles traveled annually."

Cammisa added that "it continues to be our belief that the technologies being developed today will assist, rather than supplant, drivers on the road."