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Hurricane Rita (search) was downgraded to a Category 4 storm Thursday afternoon as it swirled toward the Gulf Coast with sustained winds measured at 145 mph.

"It's not worth staying here," said Celia Martinez as she and several relatives finished packing up their homes and pets to head to Houston. "Life is more important than things."

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An estimated 1.8 million residents or more in Texas and Louisiana were under orders to evacuate to avoid a deadly repeat of Hurricane Katrina (search).

Traffic on some of Texas' main arteries was at a virtual standstill Thursday morning as people tried to move inland from the coastal areas. Drivers ran out of gas in 14-hour jams or looked in vain for a place to stay as hotels hundreds of miles in from the coast filled up. Others got tired of waiting in traffic and turned around and went home.

"This is the worst planning I've ever seen," said Julie Anderson, who covered just 45 miles in 12 hours after setting out from her home in the Houston suburb of LaPorte. "They say we've learned a lot from Hurricane Katrina. Well, you couldn't prove it by me."

Texas authorities were to begin airlifting at least 9,000 people from Beaumont and Houston, including nursing home residents, those without transportation and the homeless, to inland Texas cities. Military troops in South Texas also started moving north and schools, businesses and universities were closed.

"This is a big, dangerous storm. It's a massive storm. It covers half of the Gulf of Mexico. It is still very unpredictable," said R. David Paulison, acting director of the Federal Emergency Management Agency (search), in a news conference Thursday afternoon.

But preparations were going smoothly, he added.

"The evacuations are going very well," Paulison said. "We still feel we have plenty of time to get those people out of harm's way before this storm makes landfall sometime tomorrow."

As Texas Gov. Rick Perry (search) urged residents along the state's entire coast to begin evacuating well in advance of Rita's predicted Saturday landfall, outer bands of rain began lashing New Orleans on Thursday, the first rainfall since Hurricane Katrina, raising fears that the patched-up levee system could fail and swamp the below-sea-level city all over again.

"I've never seen a storm like this. This is worse than, it looks like it's worse than even Katrina," House Majority Leader Tom DeLay of Texas, who lives 45 miles in from the coast in Sugarland, told FOX News Thursday morning.

A tidal surge of 15 to 20 feet was expected from Corpus Christi, Texas, to south-central Louisiana.

Engineers say a 10- to 12-foot surge was required to overtake the levees at 17th Street and the London canal in New Orleans. But in neighboring St. Bernard Parish, a surge of 5 to 6 feet was all that was needed to swamp the area again.

"This is a big storm and it's really important for our citizens on the Texas coast to follow the instructions of the local authorities," President Bush said Thursday. "Officials at every level of government are preparing for the worst."

The Category 5 (search) storm weakened slightly Thursday to a Category 4, and forecasters said it could be down to a Category 3 — meaning winds as high as 130 mph — by the time it comes ashore late Friday or early Saturday.

It also made a sharper-than-expected turn to the right late in the afternoon, on a course that could spare Houston and nearby Galveston a direct hit and send it instead toward Port Arthur, Texas, or Lake Charles, La., at least 60 miles up the coast.

But it was still an extremely dangerous storm — and one aimed at a section of coastline with the nation's biggest concentration of oil refineries. Environmentalists warned of the possibility of a toxic spill from the 87 industrial plants and storage installations that represent more than one-fourth of U.S. refining capacity.

"Don't follow the example of Katrina and wait. No one will come and get you during the storm," said Harris County Judge Robert Eckels in Houston.

At 5 p.m. EDT, Rita was centered about 405 miles southeast of Galveston and was moving at near 9 mph. Its winds were near 140 mph, down from 175 mph earlier in the day. Forecasters predicted it would come ashore somewhere along a 350-mile stretch of the Texas and Louisiana coast that includes Port Arthur near the midpoint.

'Today is the Day to Leave'

Galveston, Corpus Christi and surrounding Nueces County, low-lying parts of Houston, and New Orleans were under mandatory evacuation orders as Category 5 Rita drew energy from balmy gulf waters.

It was taking people 10 to 12 hours to navigate Houston's freeways.

To speed the evacuation out of the nation's fourth-largest city, Perry halted all southbound traffic into Houston along Interstate 45 and took the unprecedented stop of opening all eight lanes to northbound traffic out of the city for 125 miles. I-45 is the primary evacuation route north from Houston and Galveston.

Police officers along the highways carried gasoline to help people get out of town.

Texas Attorney General Greg Abbott told FOX News that some businesses are already taking advantage of the emergency situation and participating in price gouging.

"Unfortunately, we have already received reports [of price gouging] on gasoline, hotel rooms, car rentals — a variety of other things people need," Aboott said, noting that whereas a 6-pack of water normally costs less than $2, some businesses are now charging up to $7 for that product.

"It's very important for businesses and people in Texas to understand, there are very stiff penalties for this price gouging," including fines of up to $20,000, he added.

Galveston Mayor Lyda Ann Thomas told NBC's "Today" show Thursday that her city is "fairly well emptied, but we're sending our police forces ... with their loudspeakers reminding people that today is the day to leave."

In Houston, Mayor Bill White said residents in low-lying areas and in mobile homes should leave immediately.

"We're the best prepared city in the country but nothing of this magnitude is welcome," he told ABC's "Good Morning America." "There's not much you can do if you have 150-mph winds."

Forecasters said Rita could be the strongest hurricane on record to ever hit Texas. Only three Category 5 hurricanes, the highest on the scale, are known to have hit the U.S. mainland — most recently, Andrew, which smashed South Florida in 1992.

A spokesman with the Homeland Security Department told FOX News Thursday morning that Texas local and state authorities had a good idea of what the storm may bring and noted that, in comparison with Katrina preparation, mass evacuations in Texas were called for three days from when landfall was expected, whereas mandatory evacuations in Louisiana were only announced 24 hours beforehand.

"When the evacuation is called has a significant bearing on what happens after the storm makes landfall in terms of human life and the federal government's role," the spokesman said. "When people don't evacuate, the federal government has to go into rescue mode and that can jeopardize the lives of crews. "

The spokesman also said the federal government is well prepared to deal with two huge storms and their aftermath at once. "We have the capacity to focus on both, Katrina and Rita, as well as any other man made security threats. The resources are there," he said.

Bush declared a state of emergency in Texas and Louisiana. Homeland Security Secretary Michael Chertoff has designated Coast Guard Rear Adm. Larry Hereth as the government's pointman for Rita response and recovery in Texas. Hereth will be based in Austin and will coordinate federal government activities with state and local officials.

Hereth has a reputation "for getting things done" and has run national disaster programs before, the DHS spokesman told FOX News.

At Houston's Johnson Space Center, NASA evacuated its staff, powered down the computers at Mission Control and turned the international space station over to the Russian space agency.

Along the coast, petrochemical plants began shutting down and hundreds of workers were evacuated from offshore oil rigs. Environmentalists warned of a worst-case scenario in which a storm surge pushed spilled oil or chemicals from the bayous into the city of Houston itself, inundating mostly poor, Hispanic neighborhoods on its south side.

Perry said state officials had been in contact with plants that are "taking appropriate procedures to safeguard their facilities."

In New Orleans, Rita's steady rains Thursday were the first measurable precipitation since Katrina. The forecast was for three to five inches in the coming days — dangerously close to the amount engineers said could send floodwaters pouring back into neighborhoods that have been dry for less than a week.

"Right now, it's a wait-and-see and hope-for-the-best," said Mitch Frazier, a spokesman for the Army Corps of Engineers, which added sandbags to shore up levees and installed 60-foot sections of metal across some of the city's canals to protect against storm surges.

But as the rain fell, there were ominous signs it might not be enough. In the city's lower Ninth Ward, where water broke through a levee earlier this month and caused some of the worst flooding, there was standing water a foot deep in areas that were dry a day earlier.

Katrina's death toll in Louisiana rose to 832 on Thursday, pushing the body count to at least 1,069 across the Gulf Coast. But workers under contract to the state to collect the bodies were taken off the streets of New Orleans because of the approaching storm.

In southwestern Louisiana, anywhere from 300,000 to 500,000 residents along the state's southwest coast were urged to evacuate and state officials planned to send in buses to take refugees, some of whom had already fled Katrina.

"Rita has Louisiana in her sights," Louisiana Gov. Kathleen Blanco said. "Head north. You cannot go east, you cannot go west. If you know the local roads that go north, take those."

As for those who refuse to leave, she said: "Perhaps they should write their Social Security numbers on their arms with indelible ink."

National Guard and medical units were put on standby. Helicopters were being positioned, and search-and-rescue boats from the state wildlife department were staged on high ground. Blanco said she also asked for 15,000 more federal troops.

'Be Ready for the Worst'

Hundreds of buses were dispatched Wednesday to evacuate the poor and move out hospital and nursing home patients, and truckloads of water, ice and ready-made meals, and rescue and medical teams were on standby in an effort to show the lessons learned in Katrina.

"We hope and pray that Hurricane Rita will not be a devastating storm, but we got to be ready for the worst," President Bush said in Washington.

"Now is not a time for warnings. Now is a time for action," said Mayor White in Houston.

He added: "There is no good place to put a shelter that could take a direct hit from a Category 5 hurricane. I don't want anybody out there watching this and thinking that somebody is bound to open a local school for me on Friday, not with a hurricane packing these kinds of winds."

The U.S. mainland has never been hit by both a Category 4 and a Category 5 in the same season. Katrina at one point became a Category 5 storm, but weakened slightly to a Category 4 just before coming ashore.

In the Galveston-Houston-Corpus Christi area, about 1.3 million people were under orders to get out, in addition to 20,000 or more along with the Louisiana coast. Special attention was given to hospitals and nursing homes, three weeks after scores of sick and elderly patients in the New Orleans area drowned in Katrina's floodwaters or died in the stifling heat while waiting to be rescued.

Galveston was already a virtual ghost town. The city's lone hospital was evacuated along with residents of a six-story retirement home.

The coastal city of 58,000 on an island 8 feet above sea level was nearly wiped off the map in 1900 when an unnamed hurricane killed between 6,000 and 12,000. It remains the nation's worst natural disaster.

City Manager Steve LeBlanc (search) said the storm surge could reach 50 feet. Galveston is protected by a seawall that is only 17 feet tall.

"Not a good picture for us," LeBlanc said.

In Houston, the state's largest city and home to the highest concentration of Katrina refugees, geography makes evacuation particularly tricky.

While many hurricane-prone cities are right on the coast, Houston is 60 miles inland, so a coastal suburban area of 2 million people must evacuate through a metropolitan area of 4 million people where the freeways are often clogged under the best of circumstances.

Officials in Corpus Christi were loading up about 100 buses Thursday morning to evacuate people who have no other way to get out.

Crude oil prices rose again on fears that Rita would destroy key oil installations in Texas and the gulf. Texas, the heart of U.S. crude production, accounts for 25 percent of the nation's total oil output.

Rita is the 17th named storm of the Atlantic hurricane season, making this the fourth-busiest season since record-keeping started in 1851. The record is 21 tropical storms in 1933. The hurricane season is not over until Nov. 30.

Jennifer McDonald in Galveston planned to ride Rita out. She and her husband have enough food and water to last 10 days in their wooden house. If it gets really bad, the couple will take to the roof.

"If it goes, it goes," the 42-year-old nurse said of the house. "We're completely prepared."

FOX News' Catherine Herridge and The Associated Press contributed to this report.