The Metropolitan Transportation Authority, which operates New York City’s subways, buses and commuter lines, cut a planned debt refinancing by about 40% as tax-exempt bond yields hovered near their highest levels in at least 15 years.

The MTA plans to sell $650 million of bonds as soon as Oct. 19, down from an originally planned $1.05 billion, according to Olga Chernat, the agency’s deputy chief of financial services. The MTA scaled back the sale because higher borrowing costs reduced the potential savings from refinancing debt backed by fare and toll revenue, Chernat said.

Yields on benchmark 30-year tax-exempt bonds stood at 5.11% on Wednesday, according to data compiled by Bloomberg. They reached 5.26% on Sept. 29, the highest since at least 2011, the data show.

“With market rates going up, we adjusted the bond size to achieve our savings targets,” Chernat said in a statement. “Refunding candidates that no longer meet our savings targets will be saved for future refunding opportunities.”The MTA refinances existing debt to lower its principal and interest payments, which account for about 14% of its $22.8 billion operating budget this year, according to financial documents. The agency had $49.8 billion of debt outstanding, including $16 billion of transportation revenue bonds, as of Sept. 17.

Moody’s Ratings rates the MTA’s transportation revenue bonds A2, while S&P Global Ratings assigns them an equivalent A rating. Fitch Ratings rates the bonds AA.

Higher yields have made it harder for states and local governments to refinance debt at a savings. New Jersey’s Transportation Trust Fund Authority pulled a $1.7 billion refinancing earlier this month because higher rates prevented the deal from generating the required savings, according to Darryl Isherwood, a spokesman for the state’s Treasury Department.