ALTHOUGH THE INTERSTATE highway system - already 20 years in the building - is not yet complete, parts of it are falling apart. Potholes go unfilled, cracks are allowed to widen, and even elementary safety defects are not corrected. In a recent series, Post reporter Douglas B. Feaver spelled out the problems: Repairs are costly. State legislatures and highway departments prefer to spend their funds on local roads. And the federal government is reluctant to commit large numbers of federal dollars to inter-state repairs. Underlying these problems is the question of whether the federal government, which pays 90 percent of the construction costs of this great highway system, should pay for its maintenance, too.

Congress did increase money for repairs in this year's highway bill, and it did strengthen the hand of the Transportation Department in its efforts to require the states to provide adequate maintenance. But since the federal funds are available only when the highways are almost falling apart, it is tempting for the states to neglect routine repairs and wait for the roads to get so bad Washington will pay part of the bill.

This arrangement may keep highways already built from deteriorating too much while the rest of the system is being completed. But in the long run, it seems to us, the federal government is going to have to provide most of the money for maintaining all 42,500 miles of superhighway. The logic for building that network - to meet a national need for better interstate transportation - applies equally to maintaining it. The federal government has a $100 billion investment to protect.

Curiously, the financing device that has provided the funds for building the system - the highway trust fund - is now partially in the way of maintaining it. Unlike most federal revenues, those going into the highway trust fund do not increase each year with inflation: They come primarily from a fixed tax, rather than a percentage tax, on each gallon of fuel. The very existence of that tax gives the states an excuse not to raise their own gasoline taxes to provide more money for maintenance, while the increasing cost of construction means that each year's trust-fund revenues buy fewer miles of concrete.

Someday, perhaps, the entire highway system will be finished; only 8 percent of it remains to be built. Perhaps Congress will then do what it should have done long ago - eliminate the trust fund and put both the taxes that feed it and the projects it funds in the regular legislative process where real needs can be weighed against each other without the artificial restrains the trust fund has imposed for more than two decades.