The budget deficit in the US is not massive. The Bush administration this week cut its forecasts for the fiscal shortfall this year by almost a third, saying it will come in at 2.3pc of gross domestic product. This is smaller than most European countries - including the UK - which have deficits north of 3pc of GDP.

Prof Kotlikoff, who teaches at Boston University, says: "The proper way to consider a country's solvency is to examine the lifetime fiscal burdens facing current and future generations. If these burdens exceed the resources of those generations, get close to doing so, or simply get so high as to preclude their full collection, the country's policy will be unsustainable and can constitute or lead to national bankruptcy.

"Does the United States fit this bill? No one knows for sure, but there are strong reasons to believe the United States may be going broke."

Experts have calculated that the country's long-term "fiscal gap" between all future government spending and all future receipts will widen immensely as the Baby Boomer generation retires, and as the amount the state will have to spend on healthcare and pensions soars. The total fiscal gap could be an almost incomprehensible $65.9 trillion, according to a study by Professors Gokhale and Smetters.

The figure is massive because President George W Bush has made major tax cuts in recent years, and because the bill for Medicare, which provides health insurance for the elderly, and Medicaid, which does likewise for the poor, will increase greatly due to demographics.

Prof Kotlikoff said: "This figure is more than five times US GDP and almost twice the size of national wealth. One way to wrap one's head around $65.9 trillion is to ask what fiscal adjustments are needed to eliminate this red hole. The answers are terrifying. One solution is an immediate and permanent doubling of personal and corporate income taxes. Another is an immediate and permanent two-thirds cut in Social Security and Medicare benefits. A third alternative, were it feasible, would be to immediately and permanently cut all federal discretionary spending by 143pc."

The scenario has serious implications for the dollar. If investors lose confidence in the US's future, and suspect the country may at some point allow inflation to erode away its debts, they may reduce their holdings of US Treasury bonds.

Prof Kotlikoff said: "The United States has experienced high rates of inflation in the past and appears to be running the same type of fiscal policies that engendered hyperinflations in 20 countries over the past century."

Paul Ashworth, of Capital Economics, was more sanguine about the coming retirement of the Baby Boomer generation. "For a start, the expected deterioration in the Federal budget owes more to rising per capita spending on health care than to changing demographics," he said.

"This can be contained if the political will is there. Similarly, the expected increase in social security spending can be controlled by reducing the growth rate of benefits. Expecting a fix now is probably asking too much of short-sighted politicians who have no incentives to do so. But a fix, or at least a succession of patches, will come when the problem becomes more pressing."

Some experts welcome the trend. Robert Poole, transportation director for the conservative think tank Reason Foundation, said private investors can raise more money than politicians to build new roads because these kind of owners are willing to raise tolls.

"They depoliticize the tolling decision," Poole said. Besides, he said, foreign companies have purchased infrastructure in Europe for years; only now are U.S. companies beginning to get into the business of buying roads and bridges.

Gas taxes and user fees have fueled the expansion of the nation's highway system. Thousands of miles of roads built since the 1950s changed the landscape, accelerating the growth of suburbia and creating a reliance on motor vehicles to move freight, get to work and take vacations.

In 1956, President Eisenhower pushed to create the interstate highway system for a different: to move troops and tanks and evacuate civilians.

The Bush administration's plan to let a foreign company manage U.S. ports met a storm of protest in February. But plans to sell or lease highways to companies outside the United States have not met such resistance.

John Foote, senior fellow at Harvard's Kennedy School of Government, said the government can take over a highway in an emergency. But he objects to selling roads to raise cash.

But that is just what Chicago has done.

Last year, the city sold a 99-year lease on the eight-mile Chicago Skyway for $1.83 billion. The buyer was the same consortium that leased the Indiana Toll Road - Macquarie Infrastructure Group of Sydney, Australia, and Cintra Concesiones de Infraestructuras de Transporte of Madrid, Spain.

Chicago used the money to pay off debt and fund road projects. Skyway tolls rose 50 cents, to $2.50; By 2017, they will reach $5.

The Indiana Toll Road lease is a better deal, Foote thinks, because the proceeds will pay for urgent projects such as road and bridge improvements.

That need is precisely why cities and states have begun to look to foreign investors.

Between 1980 and 2004, people drove 94 percent more highway miles, according to Federal Highway Administration statistics. But the number of new highway lane miles rose by only 6 percent.

Washington is not likely to produce more money to build roads. The federal highway fund - which will have a balance of about $16 billion by the end of 2006 - will run out in 2009 or 2010, according to White House and congressional estimates.

About half the states now let companies build and operate roads. Many changed their laws recently to do so.

So Illinois lawmakers are examining privatizing the Illinois Tollway, New Jersey lawmakers are considering selling 49 percent of the state's two big toll roads and a gubernatorial candidate in Ohio wants to sell the turnpike.

Indiana Gov. Mitch Daniels, who championed his state's toll road deal, now wants investors to build and operate a toll road from Indianapolis to Evansville.

Patrick Bauer, the Indiana House's Democratic leader, says such deals are taxpayer rip-offs.

Bauer believes Macquarie-Cintra could make $133 billion over the 75-year life of the Indiana Toll Road lease - for which Indiana got $3.8 billion.

"In five, maybe 10 years, all that money is gone, and the tolls keep rising and the money keeps flowing into the foreign coffers," Bauer said.

Orange County, Calif., got burned by a toll-road lease for a different reason.

The road, part of state Route 91, was built and run for $130 million by California Private Transportation Company, partly owned by France-based Compagnie Financiere et Industrielle des Autoroutes. The toll road opened in 1995.

Seven years later, Orange County was looking at gridlock. But it could not build more roads because of a provision in the lease. So it bought back the lease - for $207.5 million.

To encourage more domestic investment in highways, former Transportation Secretary Norman Y. Mineta made a pitch to Wall Street on May 23.

"The time is now for United States investors - including our financial, construction and engineering institutions - to get involved in transportation investments," said Mineta, who left office July 7.

U.S. companies are getting the message.

San Antonio-based Zachry Construction Co., along with Cintra, received approval on June 29 for a 50-year lease to build and run a toll road from Austin to Seguin for $1.3 billion.

That is part of Texas Gov. Rick Perry's vision to attract more than $80 billion in private funds for roads by 2030. He wants a new tollway from Oklahoma to Mexico and the Gulf Coast, and one from Shreveport, La., and Texarkana to Mexico. Cintra-Zachry reached a $7.2 billion deal last year to develop the project's first phase. The announcement of a $1.3 billion deal in June was part of that $7.2 billion agreement, said Perry's spokesman, Robert Black.

"In Texas, our population is going to double in the next 40 years and our current infrastructure can't handle that growth," Black said.

Not everyone in Texas buys the idea. Harris County officials recently voted against selling three toll roads. Also, independent gubernatorial candidate Carole Keeton Strayhorn opposes Perry's toll road plan.

"Texas freeways belong to Texans, not foreign companies," she said.

Pitt was in New Orleans to give an update on a project he's promoting - a competition to choose ecologically sound designs for rebuilding neighborhoods.

"There's a big opportunity here," he said, to rebuild the city using energy-efficient building materials and appliances that would improve quality of life, particularly in low-income communities.

Global Green USA, a national environmental organization, is working with Pitt on the design project. Pitt heads a jury of architects, city residents and others who decided Friday on the top five environmentally friendly designs out of more than 100 entries. The designs were submitted by individuals and architect firms.

He admits the new designs, which use energy-saving materials such as metal roofing and recycled textiles, might not reflect the historic architecture often found in New Orleans. But, he said, it's time to look to the future.

"It's impossible to replicate the past," Pitt said. "The original designs are really good. They're really efficient." But, he added, "we can do better."

Design project finalists are to be announced Monday.

Global Green USA is also providing technical assistance in green standards for 10,000 buildings in New Orleans. It opened a resource center in the city last month to give residents free design advice and information about environmentally friendly building products and strategies.

Pitt was not asked about his girlfriend or their three children. Personal questions were put off limits by a publicist for Global Green who said the news conference would be ended if such questions were asked.

"It wouldn't have ended it. It just wouldn't have been a question that we would have answered," Global Green communications director Ruben Aronin said later.

The jet slammed into a house at 4:28 p.m. and destroyed it, she said. No one was home at the time, she said. The pilot's name was not immediately released.

Another house with people inside sustained "significant damage," but no one was hurt, King said. The attic exterior of a third house was damaged, and there was fire damage in the yard of another, she said.

A firefighter was treated at a local hospital for heat exhaustion and released.

Ed Kerbs, a neighborhood resident, was hosting an air show party on his lawn when the plane went down.

"As it came in, it pitched up its nose and it looked like he was trying to stay afloat," Kerbs said. "I was talking to a buddy of mine and I said 'Hey, he's flying way too low; he's not going to make it.' And then there was a plume of smoke and a bang."

Diana Halvorson, who lives on the street where the plane crashed, said she and her family ran to a neighbor's house when they saw flames.

"It was a noise, a huge, huge, noise," she said. The flames "shot up like a bolt of lightning."

The plane crashed toward the end of the two-day Hillsboro International Air Show, where the plane had been on display, but did not perform, said Steve Callaway, an air show board member.

The Federal Aviation Administration provided a tail number that indicated the plane was registered to Robert Guilford, 73, an aviation attorney from Southern California.

According to information on his law firm's Web site, Guilford has been flying planes since 1961. But authorities would not say if he was piloting the plane Sunday. His law firm, Baum Hedlund, did not return a page Sunday.

Air show organizers canceled the show immediately after the crash. It was the first crash in the show's 19-year history.