The U.S. transportation system is in trouble.

Highways are crumbling, railroads are shrinking, waterways are clogged at critical points and the last frontier of transportation freedom, the sky, is becoming so crowded that federal officials are talking seriously about "allocating airspace."

Those difficulties will cost billions of dollars to solve, and Americans already spend $1 of every $5 they have to keep their cars, buses, trains, planes, trucks and barges moving.

It is not enough. The full cost of maintaining mobility in the United States is being deferred. Preventive maintenance is delayed to the point that major systems have to be replaced; needed improvements are tied up for years by uneven regulation, congressional inaction and citizens' lawsuits.

Transportation is the sleeper in America's budget.

In economists' terms, 20 percent of the U.S. gross national product goes for transportation, from the steel in automobiles and freight cars to the fuel consumed to run them. Last year, the bill was $480 billion, or $8,648 for every family of four.

Transportation penetrates to every level of the economy. In 1978, according to the Transportation Association of America, one civilian in 10 worked in transportation or a transportation-related industry.

Half of the petroleum, three-fourths of the rubber, one-fourth of the steel, almost one-fourth of the aluminum, two-thirds of the lead, one-third of the zinc, one-sixth of the cement and one-seventh of the copper used in the United States were consumed for transportation purposes.

Tinkering with something that pervasive is a worrisome proposition, yet there is a growing conviction that tinkering needs to be done.

In a speech last November, Transportation Secretary Neil Goldschmidt suggested that improved efficiency and performance by the transportation system will be critical as the nation expands its exports, partially to offset the cost of importing oil.

"Without prompt action," Goldschmidt said, " we may not have the system we need."

The U.S. transportation system is not really a system, but a series of independent systems that have grown over the years with independent histories, independent regulators and competing interests. Only now, in the early years of the nation's third century, are the beginnings of a cohesive, unified approach to transportation on the table.

Two massive studies, one commissioned by the Department of Transportation and the other by Congress, have helped greatly to define the problem. Its solution, however, is still undecided. That may be just as well, because two major rules of the game just changed:

For years, Americans assumed that fuel for transportation (and anything else) would be cheap and plentiful, and they built their systems accordingly.

For years, railroads and airlines and truckers assumed they would be regulated by federal agencies in a way that would protect the status quo.

Fuel is no longer cheap. Deregulation, either de facto or de jure, is changing the status quo in ways that are not perfectly understood. Those two changes pull on each other, and against the past.

Cheap gasoline and the interstate highway system, for example, have clearly contributed over the past two decades to a substantial decentralization of urban populations.

People moved further out, bought bigger homes for less money on larger lots, and drove to work in less than an hour on the interstates.

Most of the population growth in the Washington area in the 1970s, the new census is beginning to confirm, really took place far from Washington, in places as far away as Frederick and Fredericksburg.

Some of those interstate highways are beginning to fall apart. The classic source of highway-repair money has been the gasoline tax, and politicians are reluctant to raise the gasoline tax while the price of fuel is climbing so rapidly.

Thus, in an extreme example, the guy who was encouraged by simple economics to move a long way out may find himself in a few years living miles from his job, paying $2 a gallon for gasoline and driving on a freeway that is as pockmarked and dangerous as the urban streets he left.

Another example. Because of deregulation, the airlines are empowered to adjust schedules, fares and destinations with great freedom and decreasing governmental interference. In such a hotly competitive situation fares should decline, and at first they did.

Then the price of jet fuel took off. In the past year, airline fares have increased about 40 percent, despite all those cut-rate promotions you see, and airlines are carefully paring their schedules to make sure they fly almost-full airplanes.

One effect is that "it's getting more and more difficult to make a one-day business trip," in the words of Neil Effman, vice president for planning at Trans World Airlines.

The business community, having decentralized itself across the country partly because the commercial jetliner made the one-day business trip possible, is creating its own airline of corporate jets to meet the need abandoned by the airlines. Business jet sales are soaring, and so are demands on the air traffic control system.

An increase in jet fuel costs, it can thus be argued, has increased the burden on the air traffic system because a business jet requires every bit as much attention from a controller as a Boeing 747.

Because of its pervasiveness, it is easy to take transportation for granted, and that may be the reason the magnitude of the problems has escaped general notice.

The most recent of the major studies was completed for Congress by the National Transportation Policy Study Commission last year. In the next 20 years, the commission estimated, the United States will need to spend the incomprehensible sum of $1.2 trillion in a combination of private, corporate and tax dollars just for transportation.

A brief status report on the various U.S. systems, drawn from the report and from dozens of interviews with industry and government officials: AVIATION

A nasty war is brewing in aviation over a very old transportation issue: access to the right of way. Federal Aviation Administrator Langhorne M. Bond created a stir in April when he told the New York Society of Security Analysts:

"It is becoming more and more doubtful, given the constant erosion of inflation, that we will be able to meet the demands of aviation over the next decade. We may face the necessity of working out some equitable way of allocating airspace use."

The FAA's air traffic control computer is becoming overloaded. So are many airports and runways.

The present estimate of the cost for the next computer is $1.5 billion, but nobody knows when that is going to be made available or how long it will take to develop and install. The study commission estimated that, between now and the year 2000, a total of $34.6 billion will be needed for airport capital investment and land.

In the meantime, which Solomon decides who is going to be permitted to use the airspace or the runway? Will it be Giant Airlines, or New Commuter, or Fortune 500 Learjet, or Sam's Flying School?

The airlines have another worry. All of a sudden, sooner than they had planned, they need the next generation of jet airplanes for the simple reason that the next generation will be more fuel-efficient than the present models. Fuel was 12 percent of airline operating costs in 1973; now it exceeds 30 percent.

The Air Transport Association of America has worked up some numbers which claim that, if you assume an 18-year life for present airplanes, it will cost the airlines $90 billion to buy the replacement fleet needed between now and 1990.

The best year the airlines have had was 1978, when they posted a record profit of $1.2 billion. They are in a bit of a down cycle now. How, given those numbers, are they going to replace the fleet? HIGHWAYS

There are 3,867,400 miles of road in the United States, and repairs are needed on many of those miles. The nation's highway system is referred to by Department of Transportation officials as "a trillion-dollar investment," and its impact on the way Americans move around or ship things is as important to transportation generally as transportation is to the nation.

The statistics are mind-boggling: passengers traveled 2.7 trillion miles in the United States in 1977, and more than 9 of every 10 passenger miles was traveled on a road. More than 5.5 billion tons of freight were carried intercity in 1977, and two of every five of those tons was carried in a truck.

There has been much national attention on the problems of the deteriorating interstate highway system and the hundreds of substandard bridges, but the problem extends to thousands of miles of other roads as well.

Many primary and secondary highways have been neglected in recent years as state highway departments have put most of their dollars into the interstate system.

The National Transportation Policy Study Commission projected that $900 billion will be needed through the year 2000 to build and maintain highways so that 1975 conditions would remain.

However, if tax revenues devoted to highways are collected from the same sources at the same rates, there will be a minimum $150 billion difference between needs and revenues, the commission said.

Many state legislatures have begun to reexamine their highway repair budgets, and a major reexamination of the federal role is certain in the next session of Congress, according to Rep. James J. Howard (D-N.J.), chairman of the House Public Works surface transportation subcommittee. RAILROADS

Of all the transportation systems, American railroads have had the most visible problems in recent years. There was the collapse of the Penn Central, followed by the Rock Island and the Milwaukee Road. Amtrak, depending on whom you listen to, is either a great national treasure or a great national boondoggle.

Despite all the concern and all the worry, it is the opinion of many interviewed that the railroads, even Amtrak to some extent, are on their way back. Further, if deregulation has the effect of permitting railroads to negotiate their freight rates with individual shippers instead of with the Interstate Commerce Commission, the comeback could be extraordinary. Railroads are still a highly fuel-efficient way to move things, and fuel-efficiency has become the bottom line in transportation economics.

"The decline and fall of the American railroads is overblown," a top railroad executive said in an interview. "Ten railroads carry about 85 percent of the business and all of them are in pretty good physical condition." p

However, those carrying the other 15 percent for the most part are overbuilt, have immense deferred maintenance problems and, in some cases, should be abandoned, in the view of some federal officials.

In the last three or four years, according to the Association of American Railroads, there has been an explosion of capital investment by the roads or their shippers.

For example, 90,000 new freight cars were purchased in 1979, more than in any year since 1968. A total of 1,900 new or rebuilt locomotives were added to the fleet of 28,000, the most in one year since the conversion from steam to diesel was completed in the early 1950s.

According to a Federal Railroad Administration study, the American railroads will have to spend between $13 billion and $16 billion by 1985 to put their plant in good working order. The railroad association says that figure may be too low.

Whatever, the railroads remain the backbone of the long-haul freight operations in the United States, something even truckers are discovering as they put more of their trailers into piggyback service.

The railroads are also shrinking, a process that has been inevitable and painful. Line mileage for Class I railroads decreased about 9 percent between 1967 and 1977 from 209,826 miles to 191,205. TRANSIT

After years of inactivity and decay, transit is making a comeback almost despite itself. Among transit's problems:

A shortage of qualified managers, particularly at the nuts-and-bolts operating level. Those who once knew how are retiring or dead; their replacements are young and inexperienced.

Rundown facilities. The established rail transit systems in the Northeast and Midwest -- New York, Boston, Philadelphia, Chicago and Cleveland -- are, in varying degrees, just beginning to recover from years of deferred maintenance and uncertain local and state tax support. Many smaller cities with once thriving bus or trolley systems have the same problems: tired equipment, rundown garages.

A confused federal program. The Urban Mass Transportation Administration is regarded by professionals familiar with the Department of Transportation as the least effective agency there. Transit managers around the country have stories of grant applications getting lost and of difficulty in finding anybody to make a decision. At least seven top-level staff jobs at UMTA have been vacant for months, and nobody seems to be moving to fill them.

Despite these problems, transit ridership has increased steadily nationwide since bottoming out in 1972. Last year saw a 7 percent increase to 8 billion riders.

If the expansion rate of the last few years continues, the American Public Transit Association estimates, ridership will double in the next decade.

Meeting that demand will require 36,637 more buses and 3,421 new rail cars and $38.9 billion in federal aid, APTA claims. The Carter administration is projecting a lower ridership increase and a federal program of $29.6 billion for the same period. WATERWAYS

There are 25,543 miles of freight-carrying waterways in the United States, and 9.6 percent of the tons of freight shipped domestically moves on them. Great Lakes and oceangoing shipments account for another 5.6 percent of the total tons moved in domestic trade.

Most waterway operations are privately owned and economically successful. One of the reasons is that, for years, locks and dams and navigational aids have been provided free by the federal government, and only 5 percent of waterway freight is subject to federal regulation.

Although a number of projects stand in the way of the waterway system barge operators would like, the biggest constraint on the system is still Lock and Dam 26 at Alton, Ill., on the Mississippi River.

Congress has approved the funds to expand and improve this bottleneck, but it remains tied up in an environmental lawsuit. Between 1976 and 1978, the average waiting time for a tow to move through Lock and Dam 26 increased from 11.3 hours to 18.7 hours. Peat, Marwick, Mitchell & Co. estimated recently that if nothing is done at Lock and Dam 26 by 1989, annual demand would exceed capacity by 16.18 million tons.

The National Transportation Policy Study Commission projected that the capital requirements from all sources for domestic water transportation systems will total $9.2 billion by 2000, and that the cost for equipment will total $24 billion, most of which would be met with private capital.

About the same total of projected need -- $32 billion -- is seen as the U.S. capital expenditure from all sources for international marine transportation by U.S. ships. Of that total about $13 million will be needed for port and harbor facilities.

Despite the problems and the costs, the consensus of many specialists in and out of government is that the U.S. transportation system, on the whole, still works very well.

"In transportation, you will never have a system working perfectly at one time," said Richard F. Walsh, director of economic analysis for the Department of Transportation. "It is foolish to try for a system with no capacity constraints and with zero defects. If you build for maximum capacity, you would be over capacity 95 percent of the time." CAPTION: Picture 1, America's highways: a "trillion-dollar investment." By Larry Morris -- The Washington Post; Picture 2, The FAA's traffic control computer is becoming overloaded, and a new model may cost at least $1.5 million. By Douglas Chevalier -- The Washington Post; Picture 3, Airlines are expected to spend $90 billion by 1990 to replace their fleets. By John McDonnell -- The Washington Post; Picture 4, A national study projects that $900 billion will be needed through the year 2000 to build and repair highways to maintain the standards of five years ago. By Joel Richardson -- The Washington Post