Traffic jams, fuel shortages and full flights face residents trying to run from Hurricane Rita.
One way only: thousands are leaving Houston by car
Roads are clogged and airports crammed with people trying to leave Texan towns and cities such as flood-prone Houston that lie in her path.
At airports, people waited for hours to get on flights and crowds hoping to leave gathered at stations.
"Its frustrating bumper to bumper," said Willie Bayer, 70, from the wheel of her car on the road out of Houston.
"It can't get much worse, 100 yards an hour," she told the Associated Press news agency.
The hurricane is due to make landfall late on Friday local time.
To the north of Houston, a traffic queue more than 150km (93 miles) long was reported as people headed inland in high temperatures.
"I don't think they would have made this big deal about it before but Katrina has made everybody want to get out," Karen Mclinjoy told Reuters from a Houston traffic jam.
"This is not going to be fun," a resident of the island city of Galveston told the BBC.
"I went through Carla back in 1961 and this is going to take pretty much the same path. And Carla did a lot of damage to the island."
National Guard trucks are taking badly-needed fuel to petrol stations and stranded motorists around Houston.
But one eyewitness told of how people were abandoning their cars and possessions when their vehicles ran out of petrol on the motorway.
Plea for patience
Another woman described her painfully slow progress.
"Right now I am 20 miles (32km) from my house. In the past hour I have onIy moved five miles (eight kilometres)," she said, "There is a shortage of gas ... the cars are not moving because they are stalled."
Fourth largest city in the US
Population of 2m in the City of Houston (2003 estimate)
Second largest port in the US (in total tonnage)
Two major airports serving more than 38m people a year
Home to the world's largest rodeo event
Founded in 1836
She later chose to turn back, deciding to weather out the storm from her home rather that risk facing it from an endless traffic jam.
Airlines said that all flights out of Houston were fully booked.
"It's been very, very busy," David Messing, a spokesman for Continental Airlines, told AP.
"If you don't have a ticket, you're better off making other arrangements," he said.
Texas Governor Rick Perry has appealed to people to continue to leave - however difficult conditions become.
"Stay calm, stay patient," he said, addressing all those stuck in traffic in what he described as the biggest evacuation in the history of Texas.
"You have done the right thing by leaving two days before Hurricane Rita makes landfull. We are on schedule with this evacuation."