The United States must shift from expanding to improving and mantaining its existing transportation network. Transportation Secretary Brock Adams said yesterday in a new statement of transportation policy.

The 22-page statement, the first comprehensive one in Adams' tenture as secretary, states explicitly for the first time what has been hinted at piecemeal in the past when it says:

"We have a national transportation system that appears to need no major expansion for the next 10 to 15 years." However, the statement says, costs will not diminish because the system requires major maintenance and improvement.

Most of the interstate highway system is completed, although some of it already is deteriorating; the railroads are in place, but in trouble; urban transit systems have been taken over from failing private companies and improved, but much needs to be done.

Themes like these run through the report along with an emphasis Adams has sought to make, with little success, from the day he was confirmed: Transportation "must take the lead in changing wasteful habits and transporting alternative sources of energy."

More than half of the energy that the United States consumes goes for transporation purposes, the report says, and more than three-fourths of that is used by motor vehicles.

The emphasis on conservation and reorientation of priorities is a shift from the policy statement of Adams'immediate predecessor, William T. Coleman Jr.

For example, Coleman said that "the automobile is and will continue to be the most universally accepted form of transportation in America."

Nothing Adams says disagrees with that, but the new statement places much more emphasis on good public transist systems and other alternatives to the automobile as energy-saving necessities.

"We can start right now to conserve fuel and create some alternatives to the automobile," or we can wait for the tidal wave of necessity to hit us - and swim like hell," Adams said in a speech at the National Press Club as he released his statement.

The statement urges a continuing policy of forcing fuel-efficient automobilies off production lines, suggests incentives for such ride-sharing programs as car pools and van pools, and advocates strict adherence to the 55-mph speed limit.

The statement also makes these points many of which have been spelled out in various Carter administration proposals:

Transportation must be environmentally responsible. That means air-craft-noise-abatement programs as well as careful study of new highway programs.

Economic regulation should be reformed so that no one transportation mode has an advantage over another.

Amtrak should be retained "in large part to insure the availability of an energy-efficient alternative to intercity air and automobile travel in the event of a severe fuel shortage."

During his appearance at the press club yesterday, Adams also said he would support the designation of some highways for truck traffic, although that is "not an administration position yet."

"I don't see how we can maintain 42.000 miles of interstate highways if it is all to be used by heavy trucks," he said. Some highways would be designated as "freight corridors" under that concept.

One possible exception to a general policy of little expansion "would be facilities for increased movement of coal." Adams' statement said. Pipelines to carry coal and major road improvements in coal areas both have received attention from the Carter administration.