The once splendid roadway of Interstate 80 across central Iowa is cracked and rough. Uneven joints between sections of concrete bounce a driver as if he were on a washboard, an occasional pothole jars the steering wheel and chunks of concrete have flaked from piers supporting overpasses, laying bare reinforcing steel turned soft and red with rust.

The situation is but one example of the misfortunes that have befallen the interstate highway system, the shining bequest of the Eisenhower administration to the mobility of the American and his motorcar. The planned 42,500-mile system is uncompleted, but many miles of pavement are 20 or more years old and the warranty just expired.

The concrete buckles at the joints on the Capital Beltway around Washington; badly deteriorated paving forces speeds below the 55 mph limit on Interstate 40 east of Memphis, Interstate 64 east of Huntington, W.Va., and Interstate 80 west of Sacramento, to name only a few such places.

It is a sad commentary on what has happened to one of the great change-makers of postwar America. The interstate system has cut a day and half from the driving time coast to coast, has been a major factor in a dramatic shift of freight from railroads to trucks and has been blamed for white flight, decaying cities, unconscionable displacement of poor families, the decline of public transit and suburban sprawl.

The estimated total cost of the completed system is $132.4 billion, with $9 of every $10 coming from the federal government. It has been history's largest public works project, with one of the largest cost overruns: the original estimate to complete the system was $28 billion.

Now, with 40,714 miles, or 95.8 percent of the system, open to traffic, the road builders have turned from creation to salvation. Heavier-than-expected traffic, dramatically increased mileage by heavy trucks, weather and, in a few cases, poor construction, have exacted a heavy toll.

The problems on the showcase highway network are the most obvious clues to the decay and ruin attacking the nation's basic systems: roads and bridges, water mains and sewer lines, locks and dams, buses and trains, docks and runways. All have been assigned a dreadful Washington word, infrastructure.

"We have to do something about the infrastructure of highways, bridges and mass transportation . . . . That's the greatest challenge we're facing," Transportation Secretary Drew Lewis said.

Lewis has been carrying that banner almost from the day he took office and now, as the Reagan administration prepares for its third year, there are signs he is being heard. President Reagan, in his most recent news conference, said the administration has "under discussion" a Lewis plan to increase the federal gasoline tax by 5 cents a gallon and devote the money to highways, bridges and mass transit.

The increase would not violate Reagan's pledge of no new taxes, he said, because it would be a "user fee." Gasoline taxes are a traditional source of road-building revenue at all levels of government, regardless of what they are called.

While the gasoline tax debate continues, attempts to total the cost of fixing things have become a cottage industry. The American Road and Transportation Builders Association says maintaining current standards in transportation alone by 1990 will cost $623 billion. The Associated General Contractors, whose members have been devastated by the recession, add in water and sewer systems and put the bill at $1 trillion.

Pat Choate, an economist who has worked for various congressional committees, the Commerce Department and now TRW Inc., has produced a booklet entitled "America in Ruins" in which he estimates the total cost to repair transportation, water, housing and prisons will be $2.5 trillion to $3 trillion, three times the national debt.

The incomprehensibility of such numbers raises a difficulty best defined by George E. Peterson, director of the public finance center at the Urban Institute. "In my judgment," he told the House Public Works Committee recently, "there is no chance that this much public investment will occur . . . . We as a nation must accept the fact that not all public facilities can be restored to their former condition."

Whether that will be accepted is a growing issue. However, if the trend of reducing federal spending continues, cities and states will be left increasingly to their own resources to repair their problems.

Seven states, including Maryland and Virginia, increased gasoline taxes this year, and six others considered it. New Jersey Gov. Thomas H. Kean has proposed a joint federal-state "Infrastructure Bank" that would make low- or no-interest loans to agencies that build or maintain basic services. The money for the loans would come from federal grants and unused proceeds from state bond issues.

The Highway Users Federation, which monitors road spending, reports that in constant dollars investment in highways by all levels of government dropped from $33.7 billion in 1970 to $24.9 billion in 1981. In cents per mile of vehicle travel, highway expenditures were halved, from 3.2 cents to 1.6 cents, between 1956 and 1981.

The federation is one of many interests in the highway lobby, and the members of that group are remarkably unanimous on what they think is needed to fix the federal-aid highway system: more than $16 billion a year by 1986, double the size of the present federal aid program.

The federal share of that $16 billion figure could be financed with a 4-cent increase in the gasoline tax. Lewis' proposed 5-cent increase would add another penny for transit programs.

There are 3,955,387 miles of highways, streets and byways in America, but only 822,679 are on the federal aid system. Twenty percent of every mile driven in this country is driven on the interstate highways, part of the federal aid system. In total, that system carries 80 percent of all miles driven.

Thus, Lewis' department, in drafting a five-year program for highway spending, is putting a premium on interstate maintenance, frequently regarded as secondary in priority to state or county road repair.

Many miles on the rest of nation's roads must be repaired or replaced. If not, the costs of slower, less economical transportation will become a standard entry on the nation's bill for goods and services.

Iowa State Rte. 87, for example, is a four-mile-long, farm-to-market road that carries about 800 vehicles daily between the community of Elkhart northeast of here and Interstate 35. Rte. 87 was formerly a section line, one of the thousands of unpaved roads that carve the Midwest into one-mile squares, or sections.

As he drives Rte. 87 at reduced speed while dodging bumps, Bob Humphrey's car bucks and rocks to the rhythm of an uncertain roadbed. Elkhart citizens, he said, call him occasionally to complain that their roads are not up to standards. He knows that, because he is district highway engineer.

In the last three years, Humphrey's central Iowa district maintenance budget has dropped from $328,000 to $312,000. "I think we're losing ground," he said. "It's just a matter of time before the public will see that it's not getting any better. Right now, I don't think they think it's serious enough." Of Iowa's 8,900 miles of paved rural primary road, 3,000 were laid before World War II.

Iowa's roads are not the nation's best and not the worst, according to federal highway officials. But they are typical of conditions in many states. Iowa has 112,000 miles of streets and highways: 12,000 miles in cities, 90,000 in counties and 10,000 under state control.

The state is responsible for interstates and other major primary highways, including those within cities and towns. But the counties receive 37 percent of state highway money and the cities 18 percent. That leaves 45 percent for roads carrying 80 percent of the traffic.

Iowa counties argue that good roads are essential because, over the last decade, railroads have abandoned more than 5,000 miles of track in the state and short-haul trucking is now moving much of the state's grain from farm to elevator.

In Ames, 30 miles north of here, Warren B. Dunham considers the challenge of his new job as director of the state transportation department. "We think it takes an average of about 60 years before a road needs complete replacement," he said. "That means we need to replace about 160 miles per year. Our current program averages out to 42 miles per year."

Dunham and his staff have been squeezed by a continuing shortage of money because of a deep recession in agriculture, a leveling of gasoline tax revenues despite two tax-rate increases in five years and exclusion of gasohol from the fuel tax.

Dunham said he expects "flat revenues" between now and 1988, meaning he will be given the same number of dollars to run his program for the next six years regardless of how salary increases or a rise in the price of asphalt affect his costs.

So Dunham's department is considering dropping 10,000 miles of road from the state system and letting them deteriorate. It is a question of priorities.

"We won't landlock anybody," Dunham said. "We're talking about cutting off the second or third access. What we're really addressing is the question of whether we can afford to keep all of that system. That roadway grid was built for the horse-and-buggy days . . . ."

Dunham's engineers are also looking for ways to save money by building cheap, easily replaced bridges in low-use areas and by recycling old asphalt.

But it is difficult to justify to a political constituency the kind of long-range, properly financed maintenance programs that are needed.

"We've developed a quick-fix mentality from the point of view of politics," said Bob Harpster of the Iowa municipal league, an association of cities and towns. "Why should politicians get involved in long-range planning when the next election is only a few months away?"

Because of such political realities, taxes are cut, or tax increases are deferred. Then the highway engineer can choose between repaving a deteriorating road that carries a lot of traffic or building a new road that improves life for an assemblyman's constituents.

The assemblyman, however, is not likely to cut a ribbon on the repaving project, so the new road is built and the maintenance program deferred.

That has been the history of the interstate system. It was envisioned in the 1930s but not until 1956 did Congress authorize the system and create the Highway Trust Fund to make it financially possible.

The system began primarily as an intercity program, but big-city mayors worked hard to have urban freeways included. Only years later did they begin to understand that freeways are not necessarily good for cities.

Nobody anticipated the demand created by the limited-access ribbons of concrete. "If we had," said Deputy Federal Highway Administrator Lester P. Lamm, "we wouldn't have kept adding lanes to the Washington Beltway. We would have made it eight lanes wide the first time."

When the I-80 section north of Des Moines was opened in 1958, engineers predicted with great certainty that traffic would average about 8,700 vehicles daily.

In fact, 21,700 vehicles were driving it daily in 1980, 22 years later. The pavement was designed to last 20 years at the anticipated rate, and the result was inevitable.

Trucks are a major problem. In 1956, they carried 31.9 percent of freight moved between U.S. cities, while railroads carried 39.7 percent.

In 1980, with most of the interstate system in place, the trucks' share had increased to 36.4 percent while railroads' dropped to 28.9 percent. During that time, total freight carried by trucks increased from 1.2 billion tons to more than 2 billion tons.

Railroaders have screamed that the truckers were given a taxpayer-supported right of way but, despite that opposition, truckers have won two substantial victories. All states other than Missouri, Arkansas and Illinois have increased truck weight limits from 72,000 pounds to 80,000 pounds.

More important to those concerned with highway wear, according to federal experts, was an increase in maximum permitted weight per single axle from 18,000 pounds to 20,000 pounds and for a tandem [multiple] axle from 32,000 pounds to 34,000 pounds.

That meant that two key assumptions when much of the interstate highway system was designed were incorrect: many more trucks and cars were using the roads, and trucks were putting unexpected strain on the roadbed with each pass of an axle.

Iowa's Bob Humphrey discussed these things on a tour of his district. It was raining lightly on I-35 just south of Ames, and red, silt-like material was rising to the highway surface along the edges of a concrete patch in the pavement.

"Classic pumping action," Humphrey said. Water filters under the slab, a couple of big trucks roll over it, and the slab starts rocking back and forth, pumping the roadbed to the top, one grain at a time. "It's not just the weight of the trucks, it's the repetition," he said.

At the Iowa DOT, Warren Dunham worried about the future: "We feel that we're falling behind. We're trying to focus on the highest priority needs . . . . But we can't really wait until we see gravel on the road surface to fix it; then it's too late, the road has to be rebuilt."

The federal government, he said, should have a substantial role. "When you have a state like Iowa, with all its grain exports, we have to be able to get our products to market . . . . I would hate to see the federal government back away."