More and more commuters to downtown are forsaking the solitude of their automobiles and joining car pools or riding mass transit to work each day.

That conclusion jumps from the pages of the local Transportation Planning Board's annual report on how Washington area residents get to work. While the report shows major increases in the number of people using transit or car pools just in the past year, the most dramatic figures come when comparisions are made with the board's first survey five years ago.

Since then, there has been a 34.5 percent jump in the number of morning rush hour commuters riding the bus or subway downtown, a 6.1 percent increase in car pools, and a 9.5 percent decline in automobiles carrying only the driver.

During the same time, the number of people traveling to the central business district has increased more than 37,000 per day (from 359,190 to 396,450 or 10.4 percent) while the number of private automobiles and taxis entering the downtown area has decreased from 180, 900 to 171,780, a drop of 5 percent. The number of people riding in an automobile has increased slightly, from 1.41 in 1975 to 1.49 this year, another indicator that more car pools are being formed.

The last five years have seen the opening of the Metro subway, a substantial realignment of bus routes throughout the region and, significantly, a dramatic increase in the price (and sometimes the availability) of gasoline.

Albert Grant, director of the transportation board, a division of the Metropolitan Washington Council of Governments, predicted in an interview five years ago that the subway would not eliminate or substantially reduce the number of automobiles coming downtown, but would permit growth in the number of people and jobs, without major new highways and parking lots. Metro was needed, in other words, for the city to grow economically. The report indicates that his prediction is coming true.

". . . We have been able to make a dent in the traditional patterns of growth in transportation," Grant said yesterday. "I think as long as Metro continues to grow, communting patterns will continue to show this trend."

The energy situation was undoubtedly a major contributor to increased car-pooling and transit use, Grant said.

"When we started five years ago," he said, "air quality was just beginning to become a big issue, and major energy worries were still on the horizon . . . We were just looking at the traffic congestion problem." But now, he said, the increased use of mass transit and car pools has had "multiple payoffs."

There is another benefit as well, and that is the major redevelopment of Washington's sprawling downtown. Much of the new building has occurred within walking distant of subway lines, and sections of the old downtown enthusiastic competition.

Metro also apparently is attracting retail sales back to the downtown area. Robert Dunphy, who is working on a long-range study of the effects of the subway system on the area, told the planning board yesterday that sales in the metropolitan area last year were up 8 percent, while sales in the District were up 9 percent.

"That is the first time the city has been ahead of the metropolitan area in years," Dunphy said. Further, he said, the trend is continuing. For the first seven months of this year, sales in the metropolitan area were up 3 percent, while sales in the District were up 5 percent.

The transportation planning board's commuter survey counts autos, buses, trucks, trains and people as they enter the central business area -- defined as downtown Washington from Capitol Hill to Georgetown and the close-in Virginia employment centers of Rosslyn, Crystal City, the Pentagon and National Airport.