The proportion of Washington area residents traveling to work by public transportation fell during the 1970s, even as a massive government investment in Metro rail and bus service was under way, according to a new Census Bureau report.

As subway lines opened and suburban bus service improved, the number of commuters using public transit in the area rose in the decade by almost 38,000 or 18.5 percent, the census report said. But the area's work force increased much more, climbing by 307,000 or 24.6 percent, mostly through an upsurge in suburbanites going by auto to new suburban jobs.

Thus, the share of the Washington area's work force commuting regularly by public transit dropped from 16.3 percent in 1970 to 15.5 percent in 1980, the report said. The proportion using private cars--either alone or in car pools--rose from 73.9 to 76.6 percent.

Nationally, the share of workers using public transit decreased in the decade from 9 percent to 6.4 percent, or by about 487,000 commuters.

"Transit did better in Washington than in most metropolitan areas," said Philip N. Fulton, chief of the journey-to-work and migration statistics branch of the Census Bureau, who wrote the report. "But it still couldn't keep up with the demographic trends."

Fulton added, "There was a tremendous boom in suburban jobs and a tremendous loss of population in the District. So public transportation just wasn't available to more and more people."

Richard S. Page, general manager of the Metro transit system, agreed that the dispersal of people and jobs in the metropolitan area has been a problem for public transit. But without the subway, which so far has cost $5 billion to build, Page said, "the sprawl and its costs would have been much greater."

Since the first Metrorail lines linked the District to the suburbs in 1977, Page said, transit has expanded its share of commuters going to downtown Washington and has held its own for rush-hour trips that cross the Beltway. But Page said Metro "has not had the time or the ability" to serve the growing amount of suburb-to-suburb commuting.

"I'm afraid those are still low-density markets that are not well suited to mass transit," Page said.

According to the 1980 census, about 1.2 million Washington-area workers usually travel to their jobs by private car, compared with 241,074 who ride public transit.

About 122,000 others either work at home or walk to their jobs.

Nationwide, 6.2 million workers regularly use public transportation, the census said, out of a work force of 96.6 million.

In his new report, Fulton contends that the 1980 census data contradicts "a fundamental assumption of planners that mass transit would provide the ultimate remedy to the urban transportation problem by reshaping urban form and modifying consumer behavior.

"On the contrary," he wrote, "the principal lesson . . . from the census is that for transit to retain its public, it must better adapt itself to the changes in urban form and consumer preference that are taking place around it."

Yet doing this "might not be economical," Fulton said in an interview, because of the large subsidies needed to finance transit in low-density suburbs.

According to Metro data, the operating subsidy for the area's transit system already has risen from $17.5 million in 1973-74, the first year after private bus companies were taken over by the transit authority, to $181 million in the current fiscal year.

The census report said that these trends occurred from 1970 to 1980:

* The number of suburban residents using transit to get to work increased by 51,712, with the proportion using transit climbing from 8.5 to 10.3 percent. Even so, the rate was still less than a third of the 38 percent among city residents. Also, it was not high enough to hold steady the area-wide proportion of transit users, which was depressed by the shift of population from city to suburbs.

* The number of jobs in the District rose by 13.9 percent, a far greater gain than the 2 percent rise in the previous 10 years but much less than the 41.4 percent employment increase in the suburbs over the past decade.

* The number of suburban residents working in the District rose by 81,387, a healthy 30 percent gain. Suburbanites became a 53.5 percent majority among area residents commuting by transit, compared with a 38 percent minority of transit users at the start of the decade.

But the largest increase in commuting was from suburb to suburb, which grew by 243,229 or 45.3 percent.

Fulton said most of these workers travel by car, though he said detailed information on transit use for different commuting flows will not be available for several months.

Jon Williams, a transportation engineer for the Washington Metropolitan Council of Governments, noted that the census count was taken in April 1980 when transit use had climbed sharply because of gasoline shortages.

Since then, even though Metro ridership remains well above where it was in the mid-1970s, the number of Metro passengers has dropped about 7 percent.

According to COG data, transit's share of downtown commuting, which is the backbone of the Metro system, fell from 41 to 35 percent--compared with 32 percent in 1977.

The D.C. Department of Employment Services said employment in the District dropped about 2.5 percent from 1980 to 1982 while jobs in the suburbs rose slightly.

"I'm afraid all these signs are not good for Metro," said Ronald F. Kirby, director of transportation studies at the Urban Institute.

"With a great deal of effort transit stayed just about level in its share of commuting in the 1970s. But it's becoming harder and harder. . . . We're building a high-capacity system on relatively few corridors with relatively few stations. The census shows what we need is flexibility--small buses, van pools. There was a lot of wishful thinking about how the system would reshape the area."

In his report, Fulton said the largest decreases in public transit use across the country occurred in metropolitan areas in the Northeast and Midwest, particularly New York, Philadelphia and Chicago.

The only part of the country with transit gains was the West--notably Los Angeles, San Francisco, and Seattle.

Fulton said these metropolitan areas invested major amounts of money and effort "to market their systems and accommodate their ridership."