To forestall looming deficits in RTD’s annual operating budget, agency directors Tuesday night approved a five-year budget plan that calls for a 10 percent reduction in transit service.

The one-time cut — set to start Jan. 1 — is aimed at saving the Regional Transportation District $12 million a year in each of the next five years.

Buffeted by lower-than-expected sales-tax collections and higher expenses, including the cost of diesel fuel, RTD finance officials said the agency’s operating budget would likely run in the red for a good part of the 2012-17 period without the relief.

The new budget plan approved Tuesday, which includes 10 percent across-the-board fare increases scheduled for 2014 and 2017, should result in RTD staying in the black over the next five years, said Doug MacLeod, the agency’s manager of financial reporting.

The aim of the five-year plan is to provide RTD with more “sustainable” financial projections that should allow the agency “to avoid big fluctuations in service that we’ve seen over the years,” MacLeod said.

Spokesman Scott Reed added that RTD approved service increases in recent years that were “not sustainable.”

Reed said RTD’s staff and the agency’s board of directors still need to identify which routes should be targeted for the 10 percent service reduction scheduled for January. Initial ideas for the cuts will be presented at a board study session Tuesday night, he said.

The budget plan also calls for RTD to get $2.4 million in annual savings over the five years from a hiring freeze and by not filling vacancies.

The planned cuts in transit service mean RTD will need 120 fewer positions in the agency’s bus-operations department, but officials said such reductions can be achieved from the normal attrition rate of 10 to 15 bus operators who leave the agency each month, and by the transfer of about 35 bus operators to the West Corridor light-rail line that will run to Lakewood and Golden from Denver.

Testing of trains on the $710 million West line, now under construction, is scheduled to start next year, Reed said, and the rail line is to start passenger service in May 2013.