Hundreds of thousands have elected to leave, reports Dave Cohen of CBS radio affiliate WWL-AM, and for others, time is running out.
"The window of opportunity to leave the city is quickly closing," Mayor Ray Nagin said.
When the winds hit gale force, authorities will close all the highways.
"I would anticipate between now and noon at the latest, the highways will start to be closed down, and you won't be able to leave," Nagin warned.
Residents streamed inland in bumper-to-bumper traffic in an agonizingly slow exodus Tuesday amid dire warnings that Ivan could overwhelm New Orleans with up to 20 feet of filthy, chemical-polluted water. About three-quarters of a million more people along the coast in Florida, Mississippi and Alabama also were told to evacuate.
One caller to WWL reported it took him 12 hours to drive from New Orleans to Baton Rouge.
"It's been a struggle, a definite hard way to go," he said.
Another caller said it had taken her family 9 hours to drive 38 miles toward Tyler, Texas.
Forecasters said Ivan, blamed for at least 68 deaths in the Caribbean, could reach 160 mph and strengthen to Category 5, the highest level, by the time it blows ashore as early as Thursday somewhere along the Gulf Coast.
"It is expected to turn more toward the north. This will take it closer to the Mississippi-Alabama border, but this is such a big storm, the entire Gulf north coast needs to be watching this," National Hurricane Center forecaster Tricia Wallace told WWL.
Tourists in Fort Walton Beach, Fla., have spent days listening to weather forecasters talk about Ivan's path
"We're just going to take their word for it, and get out," one told CBS News Correspondent Cami McCormick.
But when she searched the Internet for a hotel room north of here, all she saw was "no vacancy, no vacancy," she said.
The beach hotels in Fort Walton Beach also closed their doors. Hotel manager Andrea Allsbrook said the storms this summer have battered this area's economy, and she can only hope reservations pick up soon.
"I'm looking forward to Fall and hurricane season being over," she said.
With hurricane-force wind extending 105 miles from its center — and forecast to continue as much as 150 miles inland — Ivan could cause significant damage no matter where it strikes. Officials ordered or strongly urged an estimated 1.9 million people in four states to flee to higher ground.
Alabama Gov. Bob Riley ordered changes to Interstate 65.
"When this comes on shore, you don't want to be there, so we're going to open up all four lanes going north," he said from an emergency operations center. "We can replace the buildings down there, we can replace the streets — whatever it takes. We can't replace a human life."
"I beg people on the coast: Do not ride this storm out," Mississippi Gov. Haley Barbour said, urging people in other parts of the state to open their homes to relatives, friends and co-workers.
The National Hurricane Center in Miami posted a hurricane warning for about a 300-mile swath from Apalachicola in Florida western Panhandle to New Orleans and Grand Isle in Louisiana. Forecasters said Ivan could bring a coastal storm surge of 10 to 16 feet, topped by large, battering waves.
"If we get the kind of tidal surge they are saying, the fishing boats are all going to be in the trees," said Jamee Lowry, owner of a bar and restaurant in Perdido Key, Florida, near the Alabama border.
New Orleans, the nation's largest city below sea level, is particularly vulnerable to flooding. The city's Louis Armstrong Airport was ordered closed Tuesday night.
"We certainly hope everybody got out who needed to get out," said airport director Roy Williams.
Up to 10 feet below sea level in spots, New Orleans is a bowl-shaped depression that sits between the half-mile-wide Mississippi River and Rhode Island-size Lake Pontchartrain. It relies on a system of levees, canals and huge pumps to keep dry.
The city has not taken a major direct hit from a hurricane since Betsy in 1965, when an 8- to 10-foot storm surge submerged parts of the city in seven feet of water. Betsy, a Category 3 storm, was blamed for 74 deaths in Louisiana, Mississippi and Florida.
Experts said Ivan could be worse, sending water pouring over levees, flooding to the rooftops and turning streets into a toxic brew of raw sewage, gas and chemicals from nearby refineries.
Despite the potential need for emergency housing, no shelters had been opened in the city as of Tuesday night. Nagin said the city was working on setting up a shelter of "last resort" and added that the Superdome might be used, but a spokesman for the stadium said earlier Tuesday that it was not equipped as a shelter.
In the French Quarter, businesses put up plywood and closed their shutters. A few people were still hanging out at Cafe du Monde, a favorite spot for French roast coffee and beignets, and a man playing a trombone outside had a box full of tips.
"They said get out, but I can't change my flight, so I figure I might as well enjoy myself," said George Senton, of Newark, N.J., who listened to the music. "At least I'll have had some good coffee and some good music before it gets me."