WASHINGTON'S METRO fares may increase on December 5 by about 10 percent. The head of New York City's transit authority, Richard Ravitch, testified last week before a Senate subcommittee on just how grim the transit fare situation in his city may become if no more federal aid for mass transit is forthcoming. At the same hearing, Rep. Harold Ford (D-Tenn.) said the transit system in Memphis may have to be closed in November for want of financial help. Transit fares are also up in Boston, Chicago, Richmond and in 60 percent of the nation's major cities since 1980, according to the U.S. Conference of Mayors.

Most of the increases reflect the impact of inflation on gas prices and the cost of maintenance. But there is another factor: more people are riding buses and trains. Ridership increases during the 1979 gas shortage have been sustained. With the added riders have come more coins in the farebox, but not enough to pay for the added wear and tear on vehicles or the added buses new riders ask for. So transit fares have had to be raised.

Fares will apparently have to be raised again soon. The Reagan budget proposed and the House and Senate have both agreed to reduce rederal aid for operating mass transit by a third in 1982, two-thirds in 1984 and altogether by 1985. Federal assistance for buying new buses, trains or building more subway lines will be cut by a quarter in the 1982 budget. This will put more pressure on local governments to up transit fares and cut or eliminate services.

It is true that local governments will then be under pressure to eliminate poorly used routes and make better use of what staff and facilities they have. But cities can't do what they have to alone. They need help.

One important help would be an overhaul of the federal regulations that increased mass transit costs. The administration's reworking of the handicapped transportation regulations, issued this week, greatly reduces one potential burden. But the labor protection provisions of the Urban Mass Transportation Act wre left unamended in the budget process.These provisions have strengthened the already strong hand of transit unions in bargaining for very generous wage packages and lenient work rules by giving the unions and effective veto power over any federal transit grant. Since wage costs now account for 70 to 80 percent of the operating costs of mass transit systems, only a stronger management position at the bargaining table can produce the savings needed to offset the loss of federal operating subsidies.

Even if operating costs could be met locally, most cities cannot afford the expense of buying new vehicles without some outside assistance. As a result of the proposed cuts, several cities -- Los Angeles, Detroit and Baltimore among them -- will not be completing subway lines that are under construction. Others won't be able to buy new fleets of trains and buses. By delaying construction of rail system and replacement of broken down buses and trains while the cost of riding mass transit is rising, the federal government is pursuing a policy that will get people back into cars -- something that is costly for the whole country in terms of air pollution and oil consumption. In Washington, for example, last year's rare increase meant 2.4 percent less riders. It would be better for the federal government to maintain subsidies for constructing subway lines and other capital projects as it decreases aid to perating budgets. That will allow mass transit systems to remain strong and serve their well-meant purposes without getting fat.