The population of Americas cities will continue to decline and suburdan sprawl will increase if public transportation is not made dramatically more attractive, a national transportation study finds.

Although the study provides no recommendations, it makes a major contribution in defining the trap that transportation planners have built for themselves.

The fuel-hungry, air-polluting automobile, fed by a federal highway program that makes it possible for individuals to move easily from wherever they want to work, has made questionable the survival of center cities.

Out-going Transportation Secretary William T. Coleman Jr. dropped the study on the table yesterday and devoted a significant number of its 400 pages to metroplitan problems. Two key questions are raised:

Should government policy encourage people to live more closely together in metropolitan areas? If they do, more energy-efficient public transportation can be provided.

Should automobile use be restricted and public transportation favored by governmental action?

If those actions were taken, they would be reversals of the road-building and subdivision expansions that have characterized metropolitan areas since the end of World War II.

The study does not answer the questions. But Coleman expressed his own view at a news converence yesterday.

"We'll be making a great mistake," he said, "if New York, Philadelphia, Boston, Baltimore, Cleveland, the other cities, are not restored and their vitality maintained and increased."

Coleman said that during his tenure more than $6 billion in federal money had gone to improve metropolitan mass transit systems.

He cited contributions to build a Metro system in Washington and rehabilitate the subway in New York. Once a good public transit system is in place, he said, "A local political leader can have the political guts and courage to have restrictive policies" for automobiles.

The study Coleman released yesterday said that about half the dollars spent for transportation in the United State are spent to move people and freight locally. The character of those localities has changed markedly over the years.

In 1970, almost half of the jobs in metro areas were located outside the central cities. The share of metro area retail sales that took place outside the central city grew from half in 1960 to two-thirds today.

That reflects the growing popularity of suburban shopping centers, most of which are located at the intersections of major expressways and interstate highways, most of which were built primarily with federal money.

The physical size of metro areas that has expanded out of proportion to the population. The metro areas including the four old-line cities of New York, Chicago, Philadelphia and Boston expanded by 166 per cent on land area from 1950 to 1975, but the population in those four centers increased only by 81 per cent, according to the study.

It is the automobile that has made that possible and has redefined the American dream for some, at least, as a single-family home in the suburbs. Despite signs of renewed interest by some people in living in the center of town, census studies show that cities are continuing to loose population.

In his press conference yesterday, Coleman described a litany of govermental actions that in his view have contributed to suburban expansion and urban decline - from public housing constrained by zoning policies to core cities to the boost in sprawl abetted by highways and sewer lines.

"If we do nothing," he said, the problem of the cities "will get worse. There has to be an active discussion. I may be wrong. I came from a city. I feel that way. All I want is for the Congress and the President to debate that issue, then based on that debate, make an intelligent choice."