In Washington, rank connotes more than prestige. More often than not, it also means a free parking place.

Take Hogan & Hartson, a prominent downtown law firm at 555 13th St. NW, adjacent to the Metro Center subway station. Lawyers at the firm park at the building free; clerical employes and support staff must pay for the privilege of driving to work or take public transit.

Such rewards are a major cause of congestion on Washington area roads, according to a new study by the Metropolitan Washington Council of Governments. By encouraging people to drive to work, the study said, parking policies are undercutting efforts to coax people out of their cars and into van pools, trains or buses.

"We've got to look at disincentives to single-occupant automobile travel," said Ron Kirby, COG's director of transportation planning. "It costs me $8.25 to park in my building, which is why I take Metro." Kirby suggests that parking should be taxed like other employe benefits.

According to the COG study, which was released yesterday, the number of people driving alone into Washington's central employment district -- an area that comprises the District, Rosslyn, the Pentagon and Crystal City -- has actually increased in the last 10 years, from 122,100 a day in 1977 to 146,000 this year. The proportion of solo drivers to commuters who get to work in other ways has stayed roughly constant, at 33 percent.

"Why, after a major investment in Metrorail, expansion of high-occupancy vehicle {HOV} lanes, and promotion of ride sharing, have we not been able to reduce the proportion of persons traveling to the core area in single-occupant automobiles from the 33 percent observed in 1977?" the study asked.

The practice of providing free or subsidized parking for ranking employes is widespread in Washington. At the Justice Department, Ninth Street and Pennsylvania Avenue NW, all 1,000 parking spaces are provided free to employes who qualify, according to a spokesman. Most big law firms provide free parking, at least for partners. The Washington Post, at 15th and L streets NW, provides a limited number of parking spaces for $45 a month, far below the commercial rate of roughly $125.

For some commuters, driving to work alone is the only alternative.

Alfred A. DelliBovi, acting head of the Urban Mass Transportation Administration, where parking costs just $10 a month, acknowledges that he often drives to work from his home in Burke. But he said he rarely finishes work in time to catch the last bus to his home from the Pentagon, which leaves at 6:35 p.m.

"Most of the time, I can't use mass transit to get home," he said. "The area has an unpredictable work force. This is not a factory town where everyone goes in with their lunch buckets at 8:30."

Allen Snyder, a partner at Hogan & Hartson, described the free parking spaces at his firm as a fringe benefit. But he, too, observed that a lawyer's long and irregular hours make driving necessary.

"If I have to work late, trying to hook up between the subway and the late-night bus would be very difficult, and I think a lot of people are in that same situation," he said.

Nevertheless, the COG study notes that for commuters who have a choice between driving and taking public transit, cost is often the deciding factor. In 1981, for example, the number of people driving alone into the central employment district dropped to 118,000, a dip that Kirby attributes to a sharp increase in fuel prices.

In 1975, when the Canadian government eliminated free parking for 40,000 employes in Ottawa, the number of people driving to work alone declined by 20 percent, according to the study. And in 1979, when President Carter temporarily suspended free parking for government employes in Washington, "reductions of 5 to 15 percent occurred at several locations," the study said.

More recently, the Montgomery County Council voted last week to require developers in Silver Spring to take steps to cut down on the number of single-occupant vehicles. County officials said that will probably mean restrictions on the amount of long-term parking that is built in Silver Spring.

Transportation planners acknowledge that persuading public and private employers to voluntarily change their parking policies is difficult.

According to the COG report, the most effective strategy would be to make employer-provided parking a taxable fringe benefit, the same as any other.

"You would make it part of the employe's taxable income," Kirby said. "If we take away the tax benefit, there is no longer any incentive."

A recent study by the House-Senate Committee on Taxation estimates that taxing employer-provided parking could raise $1.5 billion a year in income and employment taxes.

Until that happens, the study suggests, the automobile with a solitary driver will remain a "stubborn competitor" to other modes of transportation.