President Carter has approved a legislative proposal from Transportation Secretary Brock Adams that would put public transportation projects on an equal footing with highway programs in competing for federal dollars.

The Adams legislative program was sent to Capitol Hill staff workers yesterday after winning the President's approval, according to authoritative sources.

There is little new money in the program for either highways or mass tranportation, according to the sources, although the bias in favor of highway projects would be removed. The four-year authorization for federal aid to interstate highways, other highways and urban mass transportation projects would be $45.4 billion, about a $3 billion cut from what Adams had sought.

Additionally, there would be a fifth-year authorization of about $5 billion for major mass transportation construction projects. The purpose of the five-year program in that area is to make predictable the level of federal support for a number of years. The lack of such predictability has been a key complaint of metropolitan transit managers nationwide.

The key provision in the legislative proposal is that all funding for surface transportation projects - highway or transit or bridge replacement - would be ast the same ratio of 80 per cent federal to 20 per cent local.

At the moment, interstate highway projects receive 90 per cent federal money, mass transit projects 80 per cent, bridge replacement 75 per cent and secondary highways 70 per cent.

The result has been that local and state governments get more for less if they went for interstate highway programs.

The proposal calls on states to decide by 1982 whether they will complete their portions of the interstate highway system. If they choose to transfer money from uncompleted interstate highways to other projects - such as subways - they could do so at the interstate matching rate: 90 per cent federal funding.

At the moment, such transfers can be made, but the matching rate changes to that of the new project. Washington's Metro, for example, is being constructed largely with interstate transfers, but being matched at the urban transportation project ratio of 80 per cent federal, 20 per cent local.

If that proposal wins legislative approval, it would have the number of local dollars neccessary to build mass transit systems with highway dollars, but also would reduce the total number of dollars available for a project.

Another proposal in the package that would give transit an equal footing with highways is that funds from the federal highway grant program or the urban mass transit grant program could be used for highway and transit programs.

This kind of flexibility does not exist today. It is the forerunner of Adams' stated hope of creating a Surface Transportation Administration in his department that would replace the transit and highway administrations. Such a merger would have been impossible politically two years ago, because both program have powerful allies on Capitol Hill. It remains to be seen whether such a merger is possible now.

The proposal would requie that the 25 largest metropolitan areas complete transportation plans and get those plans approved by the secretary. Metropolitan planning agencies and state houses would have to approve the plans before federal dollars in those urban areas would be available.

The proposal also includes a rural transportation program that would provide funding for highways and small transit systems, and would make available federal money for the replacement of bridges on nonfederal highways. Thousands of older bridges in the country need repair or replacement.

Adams had proposed a program to provide 100 per cent federal funding for transportation programs in selected major cities as part of an urban revitalization program. That suggestion did not come back from the White House.