On a busy stretch of travel weeks, the Federal Aviation Administration is pressing pause on one part of how San Francisco International Airport conducts arrivals, cutting the rate of incoming flights during a period of runway work. The FAA said the change is meant to address safety concerns tied to how planes are handled on closely spaced parallel runways and to the complexity of the Bay Area airspace.

FAA spokesman Ian Gregor said the agency concluded that SFO’s “longstanding practice” of landing two planes at the same time on closely spaced parallel runways that are just 750 feet apart—along with congested airspace—was too dangerous. He told reporters officials would not explain why that practice had been allowed to continue.

SFO operates with two sets of parallel runways, and the temporary runway project is already reducing capacity. The north-south runways are out of commission for six months for a repaving project that accounts for nine of the 18 flight-per-hour reductions, with the new FAA rule change affecting the remaining nine flights.

The FAA said the change is unique to San Francisco rather than tied to a broader set of safety problems after the March crash at New York’s LaGuardia Airport involving an Air Canada jet and a fire truck. In describing the FAA’s decision, the agency said the rule change was not triggered by broader safety concerns and only affects SFO.

SFO airport spokesman Doug Yakel said the airport expects delays to land on some passengers even as it works with the FAA to improve arrival times. Yakel said about 25% of arriving flights are expected to be delayed by 30 minutes or more, and he did not provide additional details about whether any flights might be canceled.

Airlines responded to the changes with early indications that schedules were already being tested. United Airlines said it was reviewing the rule change to determine whether any adjustments were needed to its flight schedule, and Alaska Airlines said the situation was changing day to day, with 15 flights delayed out of SFO on Monday and none delayed on Tuesday.

The immediate operational constraint is partly tied to the timeline of the runway work. Yakel said the runway under construction is set to reopen on Oct. 2, a date the airport expects will ease at least some of the delays once the construction phase ends.