Slumping sales tax revenues - down 9.7 percent in 2008 compared with 2007 - have already forced cuts in bus and light-rail service from the Regional Transportation District. And with RTD's contract with its union due to expire Saturday, some concessions on both sides probably need to be made.

RTD has asked members of Amalgamated Transit Union Local 1001 to accept a wage freeze during a three-year contract. Meanwhile, the agency has put in place a one-year hiring freeze and eliminated pay hikes for salaried employees - including top management - in 2009.

For its part, the union is insisting on what it calls a "modest" wage increase and has notified the state Labor Department of its intent to strike if a contract isn't sealed by Saturday.

It's hard to justify wage increases given the current revenue collapse. But if RTD management expects union workers to live with no raises for the duration of the contract, its managers should make a similar pledge for a three-year freeze. Board members who won significant pay raises last year should volunteer to leave some of that money on the table as well.

To be sure, board members had been woefully underpaid until the 2008 legislature gave a 400 percent salary hike to those elected last November or later - from $3,000 to $12,000 annually. But new board members who forgo at least some of their higher compensation until the economy rebounds would be sending the right message to the union.

By formally threatening a strike, the union is compelling the Labor Department to determine whether a work stoppage would interfere with public peace, health and safety. The department hasn't made such a ruling when the past three contracts were in dispute - not even in 2006, when the union went on strike for a week. It shouldn't reach that conclusion this year, either.

While such a ruling would avert a strike and keep trains and buses running, it would also subject the contract to binding arbitration. That's potential dynamite for taxpayers and transit riders, and RTD is wisely resisting that outcome. It would leave the fate of the contract in the hands of an outside arbiter, who could impose a solution allowing increases in compensation that could necessitate even greater reductions in service and possibly layoffs.

The union handles all light-rail service and more than 40 percent of the bus routes. So in the event of a strike, RTD would have to assign management personnel to operate limited bus service and might close light rail altogether until a new contract is signed.

Moreover, the 50-plus percent of RTD bus routes that are under contract to private companies have unionized drivers. The transit agency does not have the authority to redirect those privately operated buses along RTD's own routes. (The door-to- door access-a-Ride service for the disabled is under private contract as well, and should not be diminished by a strike.)

Weathering this fiscal downturn won't be easy. But RTD needs to do it without imposing burdensome new costs on the public that further jeopardize existing service.