DON'T SET your watch by the bus schedules yet--but it looks as if the masterminds of Metro finally may be stumbling onto a secret that, up to now, only its riders knew: that reliability of service is critical. Only yesterday did a Metro board committee reach the seemingly obvious conclusion that top attention should be given to how well its buses and trains really run. And because that has not been the situation in recent memory, this decision is not the least bit silly. It is desperately overdue.

Out of necessity in its early stages, Metro concentrated on construction of the subway system. To build it required years of frantic maneuvering around the region, up on Capitol Hill, down at the White House and in the capitals of Maryland and Virginia just to raise the start-up money--and the keep-going money after that. There were tracks to be routed, equipment to buy, stations to be designed and built and countless interjurisdictional wrinkles to be ironed out.

Then came the acquisition of bus companies--a relatively uncoordinated collection of policies, equipment and personnel that was supposed to mesh with the subways. That hasn't happened to anybody's great satisfaction yet; but at least now it is a key part of this latest rededication to reliability of service.

With operating deficits up and ridership stagnating, Metro cannot afford any further deterioration of service and still expect to attract the people it should be transporting. Slight inconveniences are one thing, daily torture another. When buses never show, routes constantly change and/or shrivel and the number of transfers cries out for help from a travel agency, the odds of anybody with the slightest option taking Metro are lower than the Rosslyn station.

Still, a mere vote for better service means nothing --and the committee has addressed this by calling for specific performance standards. For example, the commitee notes that in Montgomery County, more than 30 percent of the Metrobuses run behind schedule; it is calling for a 90 percent on-time record as the minimum acceptable standard. Other standards would seek to guarantee the proper number of subway cars in operation during rush hours and to reduce employee absenteeism.

The recommendations now await full board consideration. But, like Metro's riders, they shouldn't have to wait long. They deserve attention now.