But a sharper than expected turn to the right has set Rita on a course that could spare Houston and nearby Galveston a direct hit.
Meteorologists instead believe the hurricane may push towards Port Arthur, Texas, or Lake Charles, Louisiana, at least 100km up the coast, by late Friday or early Saturday.
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 chemical plants and petroleum installations that represent more than one-fourth of US refining capacity.
New Orleans revisited
Rita also brought rain to already battered New Orleans, raising fears that the city's Katrina-damaged levees would fail and flood the city all over again.
The storm's march toward land on Thursday sent hundreds of thousands of people fleeing the nation's fourth-largest city in a frustratingly slow, bumper-to-bumper exodus.
"This is the worst planning I've ever seen," said Judie Anderson, who covered just 72km 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."
In all, nearly 2 million people along the Texas and Louisiana coasts were urged to get out of the way of Rita, a 640km-wide storm that weakened on Thursday from a top-of-the-scale Category 5 hurricane to a Category 4 as it swirled across the Gulf of Mexico.
Late on Thursday, the storm shifted slightly to the west, toward Texas, but the National Hurricane Centre still predicted it would make landfall somewhere along a 565km stretch of the Texas and Louisiana coastline.
Environmentalists warned of a
possible toxic chemical spill
Meteorologist Chris Sisko said hurricane specialists were carefully monitoring the hurricane's "wobble" to determine whether it could indicate a change in direction.
At 11pm EDT on Thursday (0300 GMT Friday), Rita was centred about 565km southeast of Galveston and was moving at near 16kph. Its winds were 225kph, down from 282kph earlier in the day.
The evacuation was a traffic nightmare, with red brake lights streaming out of Houston and its low-lying suburbs as far as the eye could see.
Highways leading inland out of Houston, a metropolitan area of 4 million people about an hour's drive from the shore, were clogged for up to 160km north of the city.
Drivers ran out of gas in 14-hour traffic jams or looked in vain for a place to stay as hotels filled up all the way to the Oklahoma and Arkansas line. Others got tired of waiting in traffic and turned around and went home.
Nasir al-Husaini, Aljazeera's correspondent in the US reported in preparation for the hurricane, Huston International Airport will close on Saturday at 12am.
State officials hoped to transport more than 200,000 gallons (757,000 litres) of gas to service stations that reported running out of fuel. Police officers and National Guard trucks carried gas to motorists whose tanks were on empty.
By late Thursday, the traffic bottlenecks were improving, with congestion easing on many major arteries leaving Houston, said Robert Black, spokesman for Governor Rick Perry.
Evacuees piled into buses to
escape the hurricane's wrath
The traffic jam extended well into Louisiana, with Interstate 10 jammed from Lake Charles through Baton Rouge. State police said the biggest backups were at exits where cars stacked up in long lines of motorists trying to get gasoline.
Rather than sit in traffic, some people walked their dogs, got out to stretch or switch drivers, or lounged in the beds of pickup trucks. Fathers and sons played catch on freeway medians. Some walked from car to car, chatting with others.
Galveston's disaster history
In Galveston, a city rebuilt after an unnamed 1900 hurricane killed between 6000 and 12,000 residents in what is still the deadliest natural disaster in US history, the once-bustling tourist island was all but abandoned, with at least 90% off its 58,000 residents cleared out.
The city pinned its hopes on its 5m-high granite seawall to protect it from the storm surge, and a skeleton crew of police and fire-fighters to ward off potential looters.
"Whatever happens is going to happen and we are going to have a monumental task ahead of us once the storm passes," said City Manager Steve LeBlanc. "Galveston is going to suffer and we are going to need to get it back in order as soon as possible."
In south-western 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"
"Rita has Louisiana in her sights," Louisiana Governor 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."
The US mainland has not been hit by two Category 4 storms in the same year since 1915. Katrina came ashore 29 August as a Category 4.