Transportation experts proudly describe the Interstate Highway System as the greatest public works project ever built in the United States.

But in some respects, they admit, it is also a study in poor planning.

Fifty years after its birth, the 46,876-mile network is imperiled by crumbling concrete, decaying steel, insufficient lanes and overstuffed traffic – and federal and state gas tax receipts can’t keep up with the cost of the needed improvements.

The cause of the roadway plight is easy enough to explain. Millions of more cars and far heavier freight-hauling trucks are pounding away at the aging system than engineers anticipated.

Consider these national statistical changes since the interstate system was opened to traffic:

Four-fold increase in vehicles to 237 million.

Three-fold increase in licensed drivers to 210 million.

Three-fold increase in vehicle miles of travel to 3 trillion per year.

Two-fold increase in truck loads to 40 tons for double-trailers.

The U.S. Department of Transportation reports more than one-fourth of the interstate highways, bridges and beltways in America are badly in need of immediate repair, upgrade or expansion. The American Society of Civil Engineers gives the system a “D” rating.

Jam-packed interstates and traffic bottlenecks have become common in most urban areas, taking a grinding toll on both the roads and the drivers, and creating a national crisis of traffic congestion.

Engineers and demographers acknowledge they did not expect the interstate system to remake the country so rapidly into sprawling suburbs and exurbs occupied by big houses and big cars.

Highway historian Tom Lewis, author of a book on how the interstate system transformed everyday life in America, said too many transportation experts and motorists took the highway network for granted as the nation’s population shifted from rural to urban regions.

“The interstate highways are the backbone of this economy,” he said. “But we have created a sense in this country that they just happened. Nothing could be farther from the truth.”

Lewis said they should be considered a national treasure rather than allowed to slowly decay.

“Now we’ve got this wonderful legacy that’s going to hell,” he said. “These highways are in a terrible state. We’re celebrating a skeleton on this 50th anniversary year.”

Fast-growing truck traffic and heavier, longer rigs are a major source of deteriorating interstate roads and bridges, according to the Federal Highway Administration. The agency said one study showed 40 percent of interstate paving damage is caused by commercial trucks, including supersize tankers and double-trailer rigs that haul loads of 40 tons or more.

Bridge collapses are a special problem because they force truck traffic off main routes, delaying the delivery of goods and affecting the nation’s economic health. In 1983, for instance, a 100-foot section of the bridge carrying Interstate 95 over Connecticut’s Mianus River buckled from the weight of a tractor-trailer truck. It took six months to reopen the critical northeast corridor highway to normal traffic.

Still, big trucks and the speed of the interstate system are central to the nation’s freight business. They account for two-thirds of the goods moved about the country – everything from fresh produce and fruit to livestock to grain to gasoline to garbage.

“The trucking industry is a critical part of delivering the American quality of life and is meeting the needs and demands of the U. S. economy,” asserts Bill Graves, president and chief executive officer of the American Trucking Associations.

Graves and others argue that interstate highways need to be rebuilt and repaved to carry more volume. In urban regions, they need extra lanes or wider shoulders to improve safety and eliminate bumper-to-bumper slowdowns during peak traffic times.

It is costly to rebuild highways and replace corroded bridges, creating a funding dilemma for both the federal government and the states. A good example is the longtime plan to expand Interstate 93 by two lanes in each direction in southern New Hampshire. The price tag for the 20-mile project has grown from $35 million when first proposed in 1986 to the current estimate of $700 million due to increased costs for land, concrete and steel.

States and the federal government pay for improvements to the interstate system mostly from fuel taxes, but with the price of gasoline and diesel at record levels, there’s little political will in Washington or state legislatures to increase those fees.

Yet some highway experts say they may have no other option.

Spending on interstate system improvements in 2006 will amount to $17 billion, a sum the federal government estimates is $4 billion less than needed to just keep up with normal wear and tear.

“An annual investment of approximately $35 billion would result in significant improvements in physical conditions and in a reduction of traffic congestion levels,” according to The Road Information Project - TRIP - a Washington research organization.

Highway historian Lewis recently urged a Congressional committee to boost the federal gas tax from 18.4 cents to $3.50 per gallon for three years, with half the money going to rebuild the interstate system and half to the development of fuel-efficient vehicles.

He said motorists could be convinced to “think of it not as a tax but a freedom fee, the sacrifice each of us must make to maintain our liberty and our way of life that depends so much on our efficient mobility.”

Federal Highway Administrator J. Richard Capka opposes higher federal fuel taxes, preferring that the government explore ways to give states more options to fund the interstate system’s future.

“The Department of Transportation strongly supports giving states as many options as possible to select the solutions that best meet their local needs, finance projects and manage congestion,” said Capka.

One of those options is leasing high-volume roads to private companies that then charge tolls to pay for improvements and also make a profit.

Indiana, Illinois, Virginia and Texas have taken this approach.

Indiana Gov. Mitchell Daniels told a Congressional hearing last spring that his state was losing money on Interstate 80, a toll road that runs across the northern edge of Indiana. So, he said, the state signed a $3.8 billion, 75-year lease with an Australian-Spanish transportation company. Lease fees are earmarked for highway maintenance and building projects.

Less than 4,000 miles of interstate highways carry a toll. Most were four-lane roads already in existence when Congress passed the Interstate Highway Act in 1956 and were added to it. But there’s no law to prevent states from adding tollways to the system.

There are other ways, of course, to relieve the pressure on the nation’s interstate highways. They include car-pooling, more commuter trains and greater government spending on mass transit.

But the reality is that Americans love their automobiles, and won’t give them up even when there are other, more convenient ways to get around. The price for that attitude is a battered road system in need of updating.

“We are at a crossroads in our nation’s transportation story where we must decide which path to take,” Sen. Jim Jeffords of Vermont, the senior member of the Senate Environment and Public Works Committee, recently observed.

“We can take the easy path and maintain the status quo … or we can take the more difficult path and change the course of surface infrastructure funding for the future.”

Jeffords, who retires from Congress in January, said standing still will surely create a national traffic crisis because the number of cars and trucks on the interstate system will only increase as the population grows by leaps and bounds.

And worsening congestion, he and other experts say, will cost the nation billions in wasted fuel, pollution to the atmosphere by hydrocarbons from idling engines, and lost productivity.

Matt Milner is a CNHI News Service Elite Reporting Program fellow.