The Interstate Highway System, a multilane ribbon of concrete and asphalt that now carries motor vehicles one of every five miles they are driven in the United States, is inexorably nearing completion.

It is also falling apart.

That uneven status report comes 22 years after Congress established the financial mechanism that made interstate highways possible and signals the beginning of a different emphasis for road builders: reconstruction of existing freeways rather than massive new projects.

A recent study by the Federal Highway Administration identified $18.8 billion in reconstruction that will be needed on existing sections of interstate highway over the next 17 years.

Immediate work estimated to cost $2.6 billion is needed on about 8,000 miles of interstate, the study said, and there will be a continuing annual requirement for $950 million in reconstruction work.

Federal officials generally blame several factors for the breakdown in paving that is, for the most part, less than 20 years old. Those factor are heavier-than-expected traffic, dramatically increased mileage by heavy trucks, weather, and, in a few cases, poor construction. Adding to the problem has been a somewhat less-than-vigorous maintenance program by some states. There is no federal aid for maintenance, but interstate highways are built with $9 of every $10 coming from the federal government.

That which needs repair was built as part of the largest public works project in history. By the time the interstate system is completed, it is estimated, it will have cost more than $110 billion. That price will reflect one of the most gigantic but seldom-discussed cost overruns in history. The original estimate was $28 billion. Metro pales by comparison.

For that money the American public has received a highway network that makes it possible to drive from Washington to Atlanta in one day, or to live in Fredericksburg, work in Suitland and get home in time for dinner.

The Interstate Highway System has cut a day and a half off a coast-to-coast motor trip and has accompanied a dramatic increase in the trucking business at the cost of the railways.

The Interstate Highway System, wooed by the mayors of America's cities before they discovered what they had done, has been blamed for white flight, decaying urban cores unconscionable displacement of poor families, the decline of public transit and suburban sprawl. The evidence however, is equivocal.

The Interstate Highway System has been blamed for the rape of parklands, the destruction of neighborhoods, the needless generation of thousands of miles of automobile trips and, therefore, air pollution.

The Interstate Highway System has made it possible for a husband and wife with two kids, a dog, a moderate income and a suburban Chicago residence to visit the Grand Canyon, Stone Mountain, Disney World, Cape Cod and the Gateway Arch on successive summers at reasonable costs and still get back home in two weeks.

The United States will not see another road-building program like it. For while the system has become enormously popular with the American motorist, each new difficult to buy because of increasing environmental concerns and the high costs of construction.

The FHA, in its most recent quarterly report on the interstate program, states that 91.9 percent of the planned 42,500 mile interstate system is open to traffic.

The estimated cost of completing the entire system is now officially placed at $104 billion, but that is a 1975 estimate. A more realistic current estimate from within the highway administration is $113.9 billion, including contingencies, according to federal highway officials.

About $68 billion has already been spent on the system. It will take another $45.9 billion to build that last 8.1 percent and bring already open sections of interstate up to present-day safety and convenience standards.

Some of those uncompleted segments are among the most controversial in the country. Several inroutes that neighborhoods have fought for years.

One segment of Interstate Route 80 called Westway, will replace four miles of the West Side Highway on Manhattan Island and is estimated to cost $1.4 billion. That's $350 million per mile and will make it the most expensive urban transportation project of any kind anywhere.

Some cities - Atlanta and Houston are examples - got most of urban freeways in place early and thus avoided the enormous environmental battles that swirled elsewhere later.

San Francisco added to its reputation for being a little unusual when, one day in 1965, the city stopped the Embarcadero Freeway in mid-ramp, right over the Ferry Building on the Bay. The ramp is still there, leading to nowhere, a monument to what was to happen in many other cities latger.

Some states - Oklahoma and New Mexico and classics - bowed to the wishes of local businesses and built the interstate segments between towns first, then added the bypasses later. For years, one drove from Oklahoma City to the Texas state line and spent all 155 miles on Interstate 40, except for the main streets of Yukon, El Reno, Weatherford, Clinton, Elk CIty and Sayre.

As the interstate system has progressed, the total mileage permitted has been increased from 38,548 to 42,500. That represents about 1 percent of the road milage in the United States.

The FHA's most recent report shows that 39.064 miles are open to traffic and that 1,387 miles and under construction. Of the remaining 2,049 miles, only 213 are regarded as controversial and all but 10 of the controversial miles are located in urban areas.

Two cities, Washington and Boston, have made radical excisions of the urban freeway networks once planned for them. Other cities have cut back to a lesser extent.

What has been completed is far more important than what has been stopped for the interstate system has permanently changed America and the way it does business.

America's railroads charge, for example, that the interstate highways made trucking what it is today.

Whether that charge is accurate or not, there is no question that trucks have dramatically increased their share of the freight market.

From 1950 to 1975, according to data compiled from the Interstate Commerce Commission by the American Trucking Association (ATA) motor carriers increased their share of freight revenues from 30 percent to 53 percent.

At the same time, the railway share was decreasing from 63.7 percent to 37.7 percent.

However, the shift began long before the interstate program. In 1940, railroads had 75.4 percent and trucks had 17.8 percent.

Regarless of when the trend started, there is no question it was helped by the interstates. "We've been able to reduce the number of relay points and consolidate terminals," said Richard Hinchcliff of the ATA. "We're able to operate at a conistent speed, and thus get better fuel economy and reduced maintenance. Productivity increases."

But the ATA also hastens to point that its members help pay for the interstates. In fiscal year 1977, trucks, trailers and buses paid $708 million in road use taxes directly into the Highway Trust Fund, which is used to finance the federal share of interstate highway and other federal-aid road projects. That represents just slightly more than 10 percent of the $7.3 billion collected by the fund in 1977.

Truckers also buy fuel and tires and pay the 4-cents-per-gallon fuel tax and the excise tax on rubber that everybody else pays and that also goes into the trust fund. It was estimated in a 1975 study that automobiles contributed 57 percent to total trust fund revenue, and trucks and buses the rest.

The interstate highways are charged with destroying public transit in the United States. No doubt they helped, but the big hemorrhage in public transit ridership occurred between 1945 and 1955, when annual when annual transit trips excluding transfers dropped from 18.9 billion a year to 9.2 billion a year. That was one year before the interstate program began.

Transit ridership reached rock bottom in 1972 and has been climbing slightly in the years since to 5.7 billion trips in 1976, according to statistics from the American Public Transit Association.

Interestingly, public transit is now seen in some cities as the salvation of the freeways. Express bus lanes are becoming a part of any urban freeway plan. Houston just voted to tax itself to put more buses on clogged freeways. Los Angeles is in the midst of an aggressive exclusive bus lane program. San Francisco has actually reduced traffic on the Golden Gate Bridge in recent years through a combinatin of car pools and express buses.

Retailers look for freeway-oriented shopping centers in which to put their new stores. "We attempt to locate on or near major arteries," said Ernie Arms of Sears. 'With the abundance of freeways and interstate systems the patterns of trade areas have changed - extended," he said.

The interstate highway system was made possible in 1956 when Congress created the Highway Trust Fund and established the principle that the federal government would pay 90 percent of the cost of building the system. That meant the states had only to come up with 10 percent. Although the interstate highway program had technically been mapped since the late 1940s, serious construction did not begin until that 90-10 formula and the trust fund were established.

The trust fund, more than half of which comes from fuel taxes, has carried the federal share of the interstate program since 1956.

It has been much attacked by economic theorists, who object to earmarking revenues, and by other interests, especially mass transit, who did not have a trust fund of their own.

Despite what its critics think, however, the trust fund has worked. It was originally supposed to end along with the interstate highway program. Nobody thinks that will happen now.

"The end is coming to the interstate program," said Lester Lamm, executive director of the highway administration. "But not the end for the demand on federal dollars." the system once built must now be maintained.

Lamm waved in the direction of a huge wall map of the interstate system. "When I get out there," he said, "I'm very proud of it."