Gulf Coast residents in Texas and Louisiana took to the roads Thursday in a massive exodus from the anticipated onslaught of Hurricane Rita, creating huge traffic jams as they heeded evacuation orders and official warnings.

Cars clogged highways north of Houston, causing backups that reportedly stretched for 100 miles. In neighboring Louisiana, still reeling from the impact of Hurricane Katrina late last month, residents of the state's southwestern coast were told to flee inland as fast as they could. The storm turned slightly to the northwest late Thursday afternoon, and the National Hurricane Center announced at 5 p.m. EDT that a tropical storm warning has been extended along the Louisiana border to the Mississippi River, including New Orleans and Lake Pontchartrain. The area could get three to five inches of rain, the National Weather Service said.

In Washington, President Bush also urged people to follow evacuation orders as Rita, now a Category 4 hurricane, barreled toward the Gulf Coast.

"This is a big storm, and it's really important for our citizens there on the Texas coast to follow the instructions of the local authorities," Bush said. "Officials at every level of government are preparing for the worst." He said U.S. troops have been moved into position to help provide a swift response.

The acting director of the Federal Emergency Management Agency, R. David Paulison, said the huge storm, covering half the Gulf of Mexico, is still unpredictable, and he urged all Gulf Coast residents to pay attention to its track.

"I don't think anyone in the Gulf Coast is out of harm's way," he said. "If you're in the evacuation zone, please, you need to leave now."

He said a top priority is moving fuel to the area, in part so that people have enough gasoline to get out of the path of the storm. Asked at a briefing about frustrated evacuees who are giving up and turning around rather than sit in massive traffic jams, Paulison said, "I would advise them not to turn around and go back home."

Although Rita weakened slightly Thursday afternoon to a Category 4 storm, the National Weather Service said it remains an "extremely dangerous hurricane" as it heads toward the Gulf Coast.

Evacuation orders have been issued for more than 1.1 million people, including residents of Galveston, Houston and flood-ravaged New Orleans, where the relatively few remaining holdouts from Katrina were told to leave because of the new hurricane.

Thousands of citizens streamed out of the Houston area Thursday, creating miles of traffic back-ups in the country's fourth largest city. In an unprecedented move, city officials announced in the morning that they would reverse the flow of the city's freeways to clear the horrendous traffic jams that had developed, but they complained that state officials took hours to approve the decision.

Houston Mayor Bill White (D), interviewed on CNN while the cable channel showed bumper-to-bumper traffic stalled in the outgoing Interstate highway lanes while incoming lanes were completely clear, said the state move was "unacceptably late."

Long lines formed at gas stations in the area.

Galveston, a coastal city of 60,000, seemed like a ghost town Thursday morning, with much of it boarded up with big sheets of plywood.

Louisiana Gov. Kathleen Babineaux Blanco (D) urged people living in coastal areas to secure their property, pack provisions as if for a camping trip and "head north." In a news briefing, she said buses are being mobilized to evacuate people without transportation. Blanco also said she asked the federal government Thursday morning to provide an additional 15,000 troops to Louisiana to help with anticipated search-and-rescue efforts and relief operations.

"We can expect serious consequences from this hurricane," Blanco said. "I'm urging Louisiana citizens to take this storm very seriously."

The National Hurricane Center in Miami, in its latest advisory , said Rita's winds had weakened slightly to 145 mph with higher gusts, down from 165 mph earlier in the day. Hurricane force winds extended outward up to 60 miles from the massive hurricane's eye, and tropical storm winds extended up to 205 miles outward, the advisory said.

The Hurricane Center said Rita's center was about 405 miles southeast of Galveston and was moving west-northwest near 9 mph. It said coastal storm surge flooding of 15 to 20 feet above normal tide levels can be expected near and to the right of where the center makes landfall.

The latest advisory said a hurricane warning was in effect from Port O'Connor, Tex., to Morgan City, La.

"Some slight weakening is forecast during the next 24 hours, but Rita is expected to remain an extremely dangerous hurricane," the bulletin said.

Earlier, Houston's Mayor White urged residents of low-lying areas to quickly evacuate.

"If your neighborhood has flooded before, you should evacuate this city," White said in a televised news conference. "What you do not want to do is wait until Friday afternoon when the storm surge wind comes. Don't wait. The time for warnings is over."

The storm is approaching the southwest Louisiana and upper Texas coast and is expected to make landfall late Friday or early Saturday. Its path shifted a bit north overnight and is expected to turn more to the northwest during the next 24 to 36 hours, the Weather Service said. Rainfall inland from the storm could total eight to 12 inches.

Rita's atmospheric pressure at its center is dangerously low, at 913 millibars in the latest advisory, according to the Hurricane Center. The lower the pressure in the center of a hurricane, the stronger the storm.

Katrina, which was once a Category 5 storm but came ashore as a Category 4 with 140-mph winds, had a pressure of 923 millibars when it made landfall.

In Texas, authorities ordered mandatory evacuations in Galveston and parts of Houston, which is about 50 miles from the mouth of Galveston Bay. Galveston was the scene of the deadliest hurricane in U.S. history when an unnamed storm killed 8,000 to 10,000 people in 1900.

New Orleans was also under a mandatory evacuation order because of Rita.

Authorities in New Orleans warned that even as little as a few inches of rain could cause a breach in the city's damaged levees, flooding neighborhoods that remain coated in a crusty layer of muck. A tropical storm watch is in effect for the southeastern coast of Louisiana to the mouth of the Mississippi River near New Orleans, meaning tropical storm conditions are expected in the area during the next 24 hours.

Valero Energy Corp., the largest U.S. oil refiner, and other oil producers were closing refineries in anticipation of the storm. The Texas Gulf Coast -- the heart of U.S. oil production -- has seven of the 13 largest U.S. refineries, accounting for about one-quarter of the nation's total crude oil production. After suffering sizable losses in Katrina, several oil refineries shut down Wednesday, pulling hundreds of workers from rigs off the Texas shore.

Painful memories of Katrina -- with a death toll that reached 1,000 Wednesday -- drove thousands of private citizens and elected officials to act fast rather than take the wait-and-see approach that greeted some coastal storms in recent years.

"If Katrina did anything, it woke people up to the power of Mother Nature," said Anthony Griffin, 51, who spent Wednesday boarding up his Galveston law office before heading to his brother's home in Fort Worth. "When Katrina hit this country, it was in a city that everyone knew and those folks looking at the TV camera looked like folks we knew."

The Department of Defense, taking lessons from Katrina, intends to send surveillance aircraft soon after Rita strikes land to "determine the magnitude of the relief required and, secondly, where it would be required," Paul McHale, assistant secretary of defense for homeland defense, said Wednesday. "We want to ensure as a matter of policy we have better eyes on target."

Already, about 5,000 Texas National Guard troops have been mobilized and another 1,300 who had been assisting in New Orleans are returning from Louisiana. The Pentagon is drawing up plans to assist local law enforcement "in the event that the first responders become the first victims," as happened in Katrina, he said. "The National Guard MP [military police] response to Katrina was nothing short of extraordinary, but it was a response that was formulated on the fly as we recognized an emerging law enforcement requirement," he said.

FEMA positioned 45 truckloads of water and ice and 25 truckloads of Meals Ready to Eat at Fort Sam Houston in San Antonio. More than 400 medical workers and 14 urban search-and-rescue teams, comprising 744 people, have been stationed in Houston, San Antonio and Fort Worth.

FEMA also asked the Pentagon to provide 26 helicopters to ferry people and supplies, five two-person communications teams for first responders, temporary hospital beds for 2,500 patients and field kitchens capable of serving 500,000 meals a day.

Moreno and Harden reported from Galveston. Staff writers Daniela Deane, in Washington, and Ceci Connolly, in New Orleans, contributed to this report.