No one knows how much it will cost to rebuild the streets, the highways and the bridges devastated by Hurricane Katrina (search). One thing for sure, however: It will cost more than any other post-disaster reconstruction effort in U.S. history.

Estimating the amount to rebuild isn't on anyone's radar screen yet, as most of New Orleans (search) still is under water and transportation officials are concerned primarily about opening roads for emergency access.

"There's no one on earth who can give you an informed opinion," said Brian Taylor, professor of urban planning at UCLA (search). "They have to get flashlights to crawl around and look at the stuff."

Jeffrey Brown, assistant professor of urban planning at Florida State University (search), said the total cost will be staggering.

"There's just nothing that comes close to this," said Brown, who specializes in infrastructure financing. He said the price tag will be many billions of dollars.

Transportation systems in Mississippi and Louisiana sustained the worst damage. State and federal damage assessment teams are being trained and sent into the field.

A major New Orleans artery, Interstate Highway 10, can't be used because the ramps are under water. Giant sections of the 8-mile Twin Span bridge — about 40 percent of the structure that connects New Orleans with Slidell, La. — collapsed into Lake Pontchartrain (search).

U.S. Highway 90, a hundred-mile stretch of road that runs along the Gulf Coast between New Orleans and Pascagoula, Miss., was basically destroyed. The Bay St. Louis-Gulfport and Biloxi-Ocean Springs bridges are gone.

David Luberoff, a transportation expert at Harvard University, said as they embark on the enormous task, transportation officials will seek to identify and fix the most critical links first.

Already, Louisiana officials are preparing to award a contract to fix the Twin Span. Mississippi is trying to allow work to begin on replacing a 300-foot span of the Pascagoula Bridge. Alabama officials say they'll work as quickly as possible to replace five 50-foot concrete spans on the eastbound U.S. Highway 90 Causeway in Mobile.

Once the waters recede in New Orleans, transportation officials will have to assess whether a road is safe enough to drive on, said Tom Malloy, vice president of the Intermodal Association of North America in Calverton, Md., which represents truckers, shipping companies and railroads.

"There are some older buildings compromised, their foundations shot," Malloy said. "A couple major trucks rolling down the street can shake it loose."

Until then, said Malloy, damage to roads and bridges will force railroads and truckers to take circuitous routes around New Orleans, which will drive up the cost of commodities from Central and South America such as coffee and bananas.