The Greater Washington Board of Trade warned yesterday that highway construction has taken "a back seat" to expansion of the Metro transit system and called for major steps to improve roads and bridges throughout the Washington area.

"It is clear that the automobile rather than public transit will continue to be the primary mode of travel well into the future," the region's main business alliance said in an "agenda for transportation policies" issued after a more than two-year study of highway, mass transit, airport and taxi problems.

Existing roads "cannot accommodate" continuing increases in traffic, the report said, describing suburban congestion as a "critical concern." The board praised the subway system for relieving congestion in the downtown area, but said that Metro "cannot solve the transportation problems of the region."

The group said its recommendations were designed to stir debate over transportation, which it described as a key factor in the area's ability to retain businesses and attract new employers. A reassessment is needed, the board said, because many current plans date from the 1950s and 1960s.

Arlington County Board Vice Chairman John G. Milliken, chairman of Metro's board of directors, said he agreed with the business group's emphasis on road improvements in outlying suburban areas but described the Metro system as likely to remain a key means of transportation in the downtown area.

"There needs to be a renewed emphasis on highways," Milliken said.

Other transportation officials said that many of the roadway improvements recommended by the Board of Trade currently are scheduled to be carried out by state and local governments. However, several of the board's proposals are embroiled in controversy or have been shelved, officials said.

Two months ago, the board released a survey indicating widespread concern among Washington-area residents about traffic congestion, inadequate road maintenance and other transportation problems.

In yesterday's report, the board said it supports completion of the proposed 101-mile subway system, long advocated by local officials. But it called for efforts to curtail Metro's rising multimillion-dollar deficits, warned against siphoning off highway funds for subway construction and suggested increases in privately run bus and taxi services.

The report traced current highway problems to the late 1960s, when it said major projects were stalled because of neighborhood opposition, environmental disputes, shifts in government financing and subway plans. "Since 1968, the region's attention, time and resources have been devoted, almost exclusively, to the construction of the Metrorail system," it said.

Although the rail system was designed for travel between the suburbs and the downtown area, the report said that these trips "are no longer the primary pattern." Instead, many suburban commuters now travel to work in suburban areas that are not served by the rail system, the report said.

In addition, the study said that Metro has been "less successful" than hoped in providing "a skeleton around which the Washington region could grow and develop." Instead, the Capital Beltway has been a focus of much growth, it said.

The report endorsed 18 major road improvements, including such long-debated projects as the Springfield Bypass in Northern Virginia, the Inter-County Connector in suburban Maryland and renovation of the District's Whitehurst Freeway.

The list also included an Outer Beltway in Prince George's County and a Rockville Facility in Montgomery County, two roads now viewed as unlikely to be built.

The board suggested a reorganization of transportation agencies to coordinate highway and mass transit plans. It urged officials to consider building more toll roads and new bridges across the Potomac River. It recommended eliminating free and low-cost parking for federal employes.

In addition, the board's report urged improvements in parking and access at National Airport along with moves to increase use of Dulles and Baltimore-Washington airports.