It costs only 15 cents to ride the bus anywhere in the Atlanta area, and the buses sparkle because each one is washed every day. Atlantans are riding in ever-increasing numbers. A new subway is under construction and on schedule.

It's wonderful what you can do with financial security, and the Metropolitan Atlanta Rapid Transit Authority (MARTA) has it. A 1-cent general sales tax is collected in two counties here and earmarked for public transportation.

Gov. Jimmy Carter supported and signed the bill from the Georgia legislature in 1971, which let Atlanta-area citizens vote on whether to approve a sales tax for running the buses and building a subway. Then transportation Commissioner Bert Lance campaigned hard for the tax, and it passed narrowly.

The benefits of having a predictable source of revenue to pay for running buses, buying new equipment and matching federal construction grants are obvious to visitors familiar with the political brinkmanship that characterizes the financing of Washington's Metro system.

In Atlanta, there are no areawide squabbles over how much transit service to keep the property taxpayer happy because the property tax does not pay for transit.

Washington Metro officials themselves recognize the advantages of such a situation and the Metro board has taken the lead, although somewhat tentatively, in attempting to build the case for regional taxes earmarked for transit. MARTA is just one of several transit systems that can be studied to see what such taxes can do.

MARTA is not perfect. It has done no better at beating inflation than anybody else and the 15-cent fare cannot survive much longer.

Atlanta's proposed 53-mile rail system was supposed to cost $1.3 billion. By 1974, the estimate had risen to $2.1 billion. Now MARTA general manager Alan F. Kiepper refuses to make estimates.

John Wright, MARTA board chairman, and Kiepper both said Carter was supportive if not wildly enthusiastic about the Atlanta rail system (only 10 miles of it will be subway) and the sales tax proposal.

The big hurdle, Kiepper said, was to persuade the Georgia legislature to let a sales tax proposal go to referendum. "The state considered the sales tax its primary source of revenue," Kiepper said. "To permit the localities to draw on it represented a significant change in position."

With that hurdle cleared, the campaign began in DeKalb and Fulton counties to win voter approval for the sales tax. Two promises were made by the tax backers:

MARTA would build a 53-mile system.

MARTA would acquire the existing privately owned bus company, reduce the fare from 40 cents to 15 cents, and hold the fare at that low level for seven years.

The promised fare reduction was needed, it was felt, to win votes in Atlanta's poor, black community. "A number of comparisons were done to show that the average person would save more per week in bus fares than he would pay out in sales taxes," Kiepper said. "It was a persuasive argument."

The public takeover of the bus company came in 1972, and the 25-cent drop in fare resulted in an immediate surge of new bus riders. "We had to buy second-hand buses just to keep up," Wright recalled. MARTA has 22 million more riders a year now than it had at the time of the takeover, an increase of 38 per cent.

The sales tax permitted MARTA not only to reduce fares, but also to improve service. Kiepper said improvements including new routes, shorter intervals between buses and increased emphasis on security have all helped. MARTA sought and got a strong antilittering ordinance to stop smoking and eating in buses, then lobbied the courts to rigorously enforce the ordinance. They did.

The result of all this is that Atlanta's bus operation today is generally regarded as one of the nation's best. It has won awards from the American Public Transit Association (APTA) as the safest system for three years running.

While the tax has made possible clean buses, improved service and low fares, the fares can't stay that low forever. In fact, according to board chairman Wright, the seven-year promise has become something of an albatross around MARTA's neck, "although it did pass the referendum," he said.

The Georgia legislature subsequently imposed a requirement on MARTA that no more than half of the tax revenue could be spent for operations: the rest had to go for buying equipment or matching federal construction grants. Last year, 46 per cent of the tax revenue was used for operations.

"When we reach 50 per cent, the board will either have to raise the fare or cut service," Kiepper said. "I do not believe we can retain the 15-cent fare beyond July 1, 1978." That will be after six years and three months of 15-cent fares - nine months short of the promise.

The reasons that MARTA's costs are rising are no different than those of any other transit system. Most prominent among them are the fact that wages for bus drivers and mechanics have doubled in six years and the cost of diesel fuel is escalating rapidly.

The other promise made to pass the referendum - that a 53-mile rail system would be built - is also on the skids.

The federal government and inflation have combined to force MARTA to drop its plans for an instant full system and to proceed with shorter, fully operational segments completed one at a time. Federal funding for 80 per cent of the construction costs is assured for only 13.7 miles of the 53.

MARTA is also fighting to keep its sales tax whole. Under the original legislation, the tax was to be reduced to 1/2 cent on the dollar in 1982. MARTA obviously would like to keep it at a penny and is campaigning to do so.

But MARTA's problems seem insignificant when compared with other transit systems that have been forced for budgetary reasons to put the quality of service in the back seat.

"I think the sales tax is the fairest most reliable tax that can be levied for public transit," Kiepper said. "Everybody pays, and it responds to inflation. We know about what it is going to produce and we can budget our service to it instead of going through an annual battle of adjusting service to the politics of the property tax."