A Reagan administration advisory group has suggested that most of the remaining 1,547 miles of the interstate highway system should not be finished and that further automobile safety regulation might not be cost effective.

The task force report, a copy of which was obtained by The Washington Post, covers a wide range of transportation issues, ranging from the problems of the air traffic control computer to the question of how much public money should be spent on Amtrak.

Transportation Secretary-designate Drew Lewis was a member of the task force, which was headed by former Republican transportation secretary Claude S. Brinegar. It is known that there were sharp divisions of opinion within the task force on several subjects. It is one of several reports being prepared by advisory groups and transition teams to guide the new administration.

The most recent official Federal Highway Administration report on interstate highways said that 96.36 percent of the planned 42,500-mile system is open to traffic or under construction. A total $78.1 billion has been spent since the system was approved in 1956.

The remaining miles will cost at least that much again to complete, because about one-third of the uncompleted miles are controversial urban freeways and many of them are tied up in environmental lawsuits.

"Most of the remaining 5 percent should not be completed," the task force report said. "Some of the yet-to-be built portions would be extremely expensive; and some of the short sections in urban areas are too expensive and too disruptive to be worth building."

The most controversial and expensive uncompleted segment is the $1 billion 4.2-mile Westway project in New York City, and President-elect Ronald Reagan was applauded by construction workers there in October when he said he would remove federal bureaucratic roadblocks to the project.

The task force endorsed the continued use of the federal highway trust fund to finance a highway program and noted that the tax base for the fund must be increased if it is to do its job. This year, for the first time, the fund has spent more money than it has collected.

The task force delivered what can only be read as strong criticism of Joan Claybrook, the combative former Ralph Nader staffer who has headed the National Highway Traffic Safety Administration (NHTSA) during the Carter years. NHTSA, the report says, "has effectively exhausted its ability to increase automobile safety at reasonable social cost, although there may remain opportunities to improve the competence of drivers."

The report said that NHTSA's rule requiring airbags or other passive devices to protect auto passengers in collisions "should be considered carefully in order to avoid unjustifiable expenses by manufacturers and, in turn, consumers." The rule is scheduled to take effect with the 1983 model year. The report also said "the frequency and magnitude of recalls may have passed way beyond a reasonable cost-effective limit" in the past four years and that the criteria for recalls "should be examined promptly."

More stringent federal fuel-economy standards are probably not needed because of the impact "market forces" have had no automakers, the report said. It waffled on the Republican platform's promise to eliminate the 55-mile-per-hour speed limit, noting there were "good arguments on both sides." "On balance," the report said, "the task force favors returning authority to set limits to the states."

The report said that major candidates for reduction in the transportation budget include subsidies for Amtrak and Amtrak's Northeast Corridor trackbed and station improvement program; subdisides for mass transit operating costs; highway programs other than the interstate and primary system; railroad loan guarantee programs (which have not been used to their capacity in the Carter years), and funds for the U.S. Railway Association, a quasi-governmental agency that oversees the ConRail system.

The report places particular emphasis on the problems facing the Federal Aviation Administration, and said the selection of its administrator "is probably second in importance to the secretary [of transportation]."

It recommended a blue ribbon commission to examine the well-publicized difficulties with the air traffic control computer system. The report also noted the "serious threat" of a nationwide strike by the air traffic controllers when their contract expires in March and said the threat "touches on the larger issues of public employe unions and their activities."

Aid programs for mass transit -- particularly for the construction of new subway and trolley systems -- are in deep trouble if the report's recommendations become policy. Federal support of new rail systems "has been largely a wasted effort" and no new systems should be started, the report said. However, it supported "at a modest level" money to upgrade established rail systems.

The report also opposed operating subsidies for rail transit systems, a recommendation that would hit hardest at the old Northeastern cities of Boston, New York, Philadelphia and Chicago, not to mention Washington, which derives 20 percent of its operating budget from federal aid. A Republican-led effort in the closing hours of the lame-duck Congress killed a multi-year mass transit authorizing bill.

One local note from the report:

"Disposition (and repair?) of Washington's Union Station will be a problem requiring more time than it warrants. The secretary should develop a joint position with the secretary of interior [who owns the buidling] and handle the issue quickly."