In the realm of transportation planning, few tools are as useful as the U.S. Census. Fed into the mainframe computer at the Metropolitan Washington Council of Governments, the data is used to foretell automobile travel patterns and mass transit demands for decades to come.

But now, that capacity is in jeopardy, transportation planners say, because of an Office of Management and Budget suggestion to reduce the number of questions in the 1990 Census. COG passed a resolution yesterday protesting the suggested change, adding its name to a long list of agencies that have taken similar action.

"Without the kind of detailed information that only the census can provide, we would be at a loss," said George Wickstrom, technical services manager for COG.

Under authority of the Paper Work Reduction Act, OMB proposed cutting the questionnaire to be distributed to most of the nation's households during the 1990 Census.

Under the U.S Census Bureau's tentative plan, five-sixths of the 106 million dwelling units in the United States will get a "short form" with 17 basic questions. The remaining one-sixth will receive a longer form that includes the 17 questions plus about 50 others.

Although no formal proposal has been made, OMB has asked the Census Bureau to look for cheaper ways to get the same information. OMB officials have said they would notify the Census Bureau by Tuesday of the agency's decision on which questions should be stricken.

"They're trying to reduce paper work, and there's also some concern that the questions take too long to answer," said John McClain, director of metropolitan development and information resources for COG. "Those of us who are in the business feel that it's not too much to ask of Americans to give up 30 to 40 minutes of their time once every 10 years."

Their answers are of more than academic interest. On the basis of census questions about the places people live and work and how they travel between them, COG churns out reams of information about commuting patterns in the metropolitan area.

Thanks to census data, for example, transportation planners can predict that 54 percent more people will travel to work by automobile in 2010 than in 1980. That is the kind of information that highway departments find extremely useful when presenting their annual budgets to state legislatures.

COG officials fear that without adequate census data, they may be forced to conduct their own studies at great expense. In 1968, for example, COG spent $1 million on a survey of 30,000 households in the Washington area. The 1980 census data, by contrast, cost about $40,000.