The highway lobby, hurting because about $7.5 billion in federal tax money earmarked for interstate highway construction is sitting in the Treasury while Congress holds out for some favorite projects, started a public offensive yesterday in an attempt to solve the problem.

John A. Clements, new president of the Highway Users Federation, stressed at a briefing yesterday that highway construction is a pay-as-you-go proposition, that the interstate construction money is in the bank and that keeping it there will not reduce the federal deficit.

"Virtually the entire 5 cents per gallon gasoline tax increase passed by Congress late in 1982 is locked up," Clements said.

Before money for interstate construction can flow, Congress must pass and the president must sign something called the Interstate Cost Estimate. The estimate is a listing of how interstate money shall be apportioned to the states. It is drafted by the Federal Highway Administration and usually is noncontroversial.

This time, however, there is a difference of opinion between the highway administration and Massachusetts about whether a proposed $2.2 billion freeway-tunnel project in Boston should qualify for the interstate program.

Massachusetts, and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), thinks it should. The highway administration does not. The Senate keeps trying to pass a simple ratification of the highway apportionment; the House keeps adding the speaker's project, and a few more worthy projects from other congressional districts. Congress adjourned last year without resolving the situation. One of the first things that happened in the new 99th Congress was the introduction in the Senate of a straightforward interstate cost estimate. In the House, a draft bill, not yet introduced, contains more than 20 special-interest projects at last report, including the Boston freeway-tunnel.

"The issue should be resolved by Congress," Clements said.

Of the funds withheld so far, Maryland is out $284.2 million; Virginia $204.2 million; West Virginia $94 million, and the District of Columbia $55.6 million.

Federation officials predicted that several western states may seek legislation to permit them to raise the speed limit above the federally mandated 55 mph standard.