A three-year-long study of federal transportation policy due out today is expected to conclude that the nation's transportation system is in serious disrepair and that policies and programs are ill-equipped to meet the future demands of a more mobile America.

The report also reportedly found that public and private investment in transportation is insufficient even to preserve the inadequate system that exists, and that an additional $4 trillion will be needed to accomodate transportation growth by the year 2000.

The final report of the National Transportation Policy Study Commission will be made public later today. It is expected to contain some far-reaching proposals comprising a complete overhaul of national transportation policy.

Most dramatic of the report's findings is the conclusion that the federal government's transportation bureaucracy is one of the main impediments to the development of future national transportation policy. The report will recommend that the Interstate Commerce Commission, the Federal Maritime Commission and the Civil Aeronautics Board be abolished.

Instead of the 1,000 separate transportation policies administered by more than 60 federal agencies, the report will recommend the establishment of a single federal transportation commission and a restructuring of the Department of Transportation to include all transportation programs now run out of other Cabinet departments.

The commission was appointed by the 94th Congress. Its final report is the first step toward the development of a national transportation policy. Rep. Bud Shuster (R-Pa.), who headed the commission, said he is "in the process of turning our recommendations into legislative language."

Shuster said he hopes congressional hearings will be held next year on his planned "National Transportation Improvement Act," which will incorporate the commission's findings.

But as the commission report concludes, any future national transportation policy is predicated on the immediate adoption of a national energy policy. The report is based on several assumptions about the nation's future energy needs, including the assumption that 17 percent of the nation's energy will be derived from nuclear power by the year 2000, and up to 20 percent of U.S. domestic crude oil will come from a synthetic fuel industry.

The report also assumes a tripling of domestic coal production and a "substantial use of diesel fuel to power personal automobiles."

Shuster said the study also predicts that for at least the next two decades, the nation still will be 97 percent dependent on petroleum for its energy. Therefore, he said, "a lackluster energy policy, impeding production of oil, coal and other forms of energy, is endangering transportation's ability to keep up with demand."

No accurate record of federal transportation spending exists, because the money comes from various agencies and departments. But the report finds that from 1979 through the year 2000, federal, state and local governments may have to pay up to $1.292 trillion of a staggering total transportation bill of more than $4 trillion.

The report also will recommand that the federal government minimize its regulatory role over the transportation industry. Excessive federal regulations, Shuster said, are often conflicting and impede free market forces.

The report will also suggest that federal assistance to state and local transportation efforts be more "flexible" and less subject to federal restrictions.

The commission explored and rejected the idea of nationalization of any of the nation's transportation industries.