London judges ruled in favor of a legal challenge into the U.K.’s travel rules but stopped short of forcing the government to explain how it works out the troubled traffic light system.

The judges said that while government ministers had failed to properly tie the quarantine requirements for travelers returning home with the U.K.’s color-coded system, they would not force the government to reveal its reasoning behind the categories. It was enough, they said, that ministers continue to review the rules.

The challenge was brought by the owner of Stansted and Manchester airports and supported by a number of airlines including Ryanair Holdings Plc, British Airways’ parent IAG SA, Virgin Atlantic Airways Ltd and EasyJet Plc. The group said travelers need to understand how the government takes decisions on the traffic light system.

“The way decisions have been taken to date has not been transparent and has created huge confusion and uncertainty,” the group said in a joint statement after the ruling.

The group’s lawyers had asked to see the government’s data that informed the U.K.’s risk assessments for each country during the pandemic. They argued that the government’s “fundamental lack of transparency” means that the basis for categorizing territories between the green or amber lists “appears arbitrary.”

The U.K. went further last week in its traffic light border system, creating a new “amber plus” category that makes it harder to travel to France, the only country with that designation.

“Our traffic light system continues to cautiously manage the risk of new variants, as we balance the timely reopening of international travel while safeguarding public health and protecting the vaccine roll-out,” the transport department said in an emailed statement.