While many cities are raising bus and subway fares to make up for cuts in federal subsidies for mass transit, the Los Angeles Rapid Transit District (RTD) today cut bus fare from 85 to 50 cents.

"Los Angeles is going in the opposite direction of the rest of the country," said Albert Engelken of the American Public Transit Association in Washington. "The rest of the country is increasing fares. But Los Angeles," he said, "because of the wisdom of its voters, has lowered the fares.... Los Angeles now has affordable mass transit."

The "wisdom of the voters" to which Engelken referred was exercised in 1980 when 54 percent of the voters in Los Angeles County approved Proposition A, which increased sales taxes to pay for a mass transit system. Last April the state Supreme Court upheld the constitutionality of the measure.

So, beginning today, residents of Los Angeles County are paying an extra half cent on the dollar in sales taxes, and bus riders are boarding for 35 cents less than they did yesterday.

A locally imposed tax dedicated to transit has been the solution for several successful transit systems around the country, notably Atlanta's and Seattle's. The Reagan administration has been trimming and is trying to eliminate operating assistance for local transit authorities. That has left them with three choices: to raise fares, raise taxes for subsidies or cut service.

Most have chosen to raise fares, but that is becoming increasingly difficult politically because a fare increase falls heavily on the poor, many of whom are out of work. New York City's transit authority just yesterday decided to hold the line on fares for another six months.

The Los Angeles Rapid Transit District, according to Marc Littman of the RTD news bureau, expects a 20 percent increase in ridership as a result of the lower fare. In May, the system averaged 1,080,000 weekday trips, the lowest in three years, Littman said. He added that the RTD lost nearly 300,000 riders after the fare was increased last year to 85 cents.

The new bus fare, however, probably won't last beyond 1985. The first year of the new sales tax is expected to generate $288 million in mass transit revenue, the bulk of which will go to subsidize the lower bus fares. By the fourth year, revenues are expected to rise to $350 million.

But, beginning in 1985, as mandated by Proposition A, at least $100 million must go to building a rail rapid transit system. Los Angeles is the largest American city still without a rail system; although plans are in the works for two systems, federal funding for them is in doubt under the Reagan administration's policy of "no new starts" in rapid transit construction.

Rebecca Reardon of the Los Angeles Transportation Commission, which oversees revenues from the new tax, says she expects "a bit" of a fight between her commission and the RTD three years from now, when the greater part of Proposition A funds begins to go toward rail systems and the commission will have to decide how much, if any, of its discretionary funds will go to subsidize bus fares.

But Engelken, for one, is not worried about 1985. "We're not dead yet on this funding," he said. "Congress will not allow operating assistance to be phased out. By 1985, I think the administration will get religion and realize the importance of rapid transit in this country."