THE DARING of President-elect Reagan's advisory group on transportation policy is remarkable. It has actually proposed that the construction of the interstate highway system come to an end. Wait until the highway builders, labor unions and road users sink their teeth into that proposal. It could provide one of the gaudier struggles of the next four years.

The logic behind this proposal is pretty compelling in its own way. The 42,500-mile highway network is more than 96 percent complete. The 1,547-mile remainder, much of it in urban areas, will cost more to build than did the first 41,000. Therefore, the advisers have told Mr. Reagan, most of the additional miles should not be built. They will be, the report said, "too expensive and too disruptive."

If the new president is not yet convinced of the underlying wisdom of this recommendation, he might drive across the river some day and take a look at what should be one of the last great urban freeways -- I66. It has ripped its way through Arlington and will, before too many more years, be pointing its traffic at a set of bridges and city streets barely able to accommodate it.

There may be a few other sections of the interstate highway system that deserve to be completed, either on their merits or as a result of the kind of political horse-trading that brought the last few miles of I66 to completion. But on the whole, the Reagan administration would do the country a great favor by decreeing an end to the quarter-century of building a transportation network so totally devoted to cars and trucks.

Unfortunately, the advisory group doesn't seem to have much in mind to replace the highways it proposes to not build. It recommends cuts in the federal funds available for the railroads and mass transit systems. It brushes off projects like subways and light rail commuter lines as "largely a wasted effort."

An administration pointing at rapid economic growth, as Mr. Reagan's will be, deserves better. The nation's transportation network at any particular time is a major factor in the decisions that lead to economic growth -- or decay. New York City developed in its present conformation because of its subways; Los Angeles would be an entirely different kind of community if less money had been spent on highways.

One of the prerequisites of halting further urban decay in the Northeast and Midwest is better transportation for commuters and for freight. Similarly, good transportation for workers and freight is essential to orderly development in the Sun Belt. It may be possible, although history suggests it is unlikely, to get the necessary transportation network by relying on state and local governments. But the Reagan administration ought to consider that matter carefully before withdrawing at the same time the federal commitments to mass transit, railroads and new highways.