STEP RIGHT UP, say the Metro mathematicians, and put another nickel in--that is, if you still care enough to send yourself around town by bus and subway instead of car. The staff and management of Metro, in that order, are looking at fare increases, which you can also read as rider decreases. Does this mean that somewhere between Farragut Square and infinity Metro's fares will go so high that nobody will climb aboard?

Certainly there are limits to how much people who have options will shell out to mass transit-- particularly if "mass" overtakes "transit" in terms of service quality. Antiquated bus routes, dirty vehicles above or below ground, bad heating or cooling systems and missed runs aren't the stuff of which popular transit systems are made--and one can look at Seattle or Atlanta for examples of well-run systems with their financial houses in reasonable though not necessarily bargain-fare order.

Right now there are other factors feeding an anti- transit climate: while Metro is talking higher fares, parking lot rates seem to have leveled off at least temporarily, gasoline prices have been dipping and cars with better mileage have been taking to the streets. So Metro has to compete--and to do so costs money that elected officials of the local governments in this region are reluctant to put into the Metro pot.

So back you come to user fees--fares. A base fare of 65 cents, as proposed by the Metro staff, would not be outrageous by national standards. Already, the 75-cent base fare is becoming the pattern. But in this region, the serious internal split in Metro has to do with the long-ride subway fares into the suburbs. Maryland's members of the Metro board have argued for higher base fares and lower mileage fees for rail; and as the subway lines continue to crawl farther out into the counties, these arguments are sure to become more shrill.

Meanwhile, board members from the District and Virginia are saying they would support the proposed basic fare structure while opposing a plan to charge 10 cents for bus transfers, which they understandably cite as just another fare increase rather than a plan to stop free riders who use other people's transfers.

If the quality of service is kept high, we suspect that the majority of riders will adjust to slightly higher fares. But to keep up service and to keep these fare increases slight instead of devastating, Metro's board should move on three fronts: 1)work toward, not away from, more uniform and understandable fare structures; 2)disconnect fare increases and salaries from direct, full tie-ins with the cost-of-living index, which has resulted in skyrocketing labor costs at Metro; and 3)work with the participating state and local governments to enact a regional tax that can be used for transportation without having to undergo round-robin, intramural and highly politicized negotiations.