The U.S. Supreme Court on Thursday further weakened the Port of Los Angeles’ Clean Truck Program, overturning two minor provisions that had been upheld by a lower court.

In a 9-0 decision, the court said the port cannot require trucking companies to submit parking plans for all trucks, a provision intended to ensure trucks would be parked in lots rather than on streets. The justices also threw out a provision that would have required companies to put placards on all trucks, listing a phone number for environmental or safety complaints.

Those sound like minor provisions, but they stuck in the trucking industry’s craw. When the 9th Circuit Court of Appeals upheld those elements of the plan, judges ruled the Port was acting as a participant in the trucking market, not as a regulatory body. Only the federal government is allowed to regulate truckers on most matters.

The American Trucking Associations, which sued the port over the plan initially and appealed the 9th Circuit’s ruling, said allowing the Port to make rules for trucking companies could open the door to wider regulation at other ports across the country.

“What they are doing is regulating us, and that’s something the court said 9-0 that they cannot do,” said Curtis Whalen, executive director of the ATA’s Intermodal Motor Carriers Conference, which represents port trucking firms. “Had the court said the Port was acting as a market participant, that would have been a huge defeat.”

Still, despite the ruling, parts of the Clean Truck Program remain in place. Older trucks remain banned from the harbor, and both Port officials and Mayor Antonio Villaraigosa said Thursday that emissions have been cut substantially since the program went into effect in 2008.

“Our Clean Truck program has reduced harmful truck emissions by 91 percent,” Villaraigosa said in a statement. “We are reviewing the Supreme Court’s decision, but we intend to continue our efforts to clean L.A.’s Port to the extent the law allows.”