The Interstate Highway System and the financial device that made it possible are both badly in need of repair on this, their 25th birthday.

While most of the planned 42,500 miles are open to traffic, interstate drivers have all experienced unexpected bumps and jostles, sometimes on relatively new pavement, have concentrated their dwindling maintenance resources on local streets and roads. Almost one of every 10 miles on both the urban and rural interstate systems is officially rated in poor condition (2.5 or less on the Federal Highway Administration's scale of 1 to 5.)

Further, the highway trust fund, which provided $9 of every $10 needed to build the interstate system, is spending more money that it is making.

That's all right, for the short term, because the fund has a balance of about $10 billion. In the long run, however, the fund will disappear unless there is an increase in the gasoline tax that feeds it or a reduction in federal highway spending.

The Reagan administration has chosen to cut highway spending and forced the Federal Highway Administration into a defensive crouch after decades of running the largest public works program in the nation's history. Retrenchment is obvious in the administration's proposed highway legislation that would, among other things:

Deglorify uncompleted interstate highway projects by cutting the number of lanes and eliminating planned interchanges and noise barriers. That is supposed to reduce the cost of building the remaining part of the interstate system from $53.8 billion to $31.5 billion.

Create what whould amount to a new category of highway aid just for maintenance and reconstruction of the interstate system, diverting money from new programs to protect an existing federal investment.

Reduce or eliminate federal aid in most other highway programs. The financial responsibility for urban arterials and other secondary roads would become the exclusive problem of state and local governments, for example. The Carter budget proposed $1.6 billion for urban and secondary roads in 1982; Reagan is poroposing the same amount that year, half as much in 1983 and none in 1984.

The administration would keep at their current levels all the taxes, that support the trust fund, which is the same thing as cutting the highway program. sThat's because two-thirds of the trust fund comes from a 4-cent-a-gallon gasoline tax, which is producing less revenue now because Americans are using less gasoline.

Both the House and Senate are working on their own highway bills, which are likely to come up for debate in the fall. The Senate bill resembles the administration proposal in most respects. The larger debate will be in the House, where the Public Works Committee has been the federal highway program's guardian angel for decades. House Democrats have been pushing a bill covering only one year in an obvious attempt to reopen the complex highway aid issues next year, when, they hope, the Reagan honeymoon will be over.

The impending fight over highway policy will be in sharp contrast to the euphoric mood in 1956 when President Eisenhower signed the law creating the National System of Interstate and Defense Highways. The distinctive feature of the act, one that took Congress two years to develop, was the creation of the highway trust fund to pay for the program.

At that time, it was estimated the system would be completed in 1972 at a cost of $28 billion. The official cost is now estimated at more that $130 billion with a 1990 completion date.

Inflation is blamed as the primary cause of the interstate system's cost overrung. For example, a cubic yard of structural concrete cost $53.74 in 1956. Last year it cost $226.68, an increase of 322 percent.

Interstate highways have been blamed for the suburbanization of America, for the death of the passenger railroads and for the heartless elimination of thousands of units of inner-city housing that met the needs of low-and moderate-income residents much more satisfactorily than the projects built to replace them.

Despite those criticisms, it has given Americans unprecedented personal mobility and what in undoubtedly the finest highway network in the world. One mile out every five that is driven in the United States is driven on an interstate highway.