More than 1.3 million residents in Texas and Louisiana were under orders to get out to avoid a deadly repeat of Katrina.
The Category 5 storm weakened slightly Thursday morning, and forecasters said it could be down to a Category 3 by the time it comes ashore late Friday or early Saturday. It could still be a dangerous storm.
"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 said.
Highways leading inland out of Houston were gridlocked, with traffic bumper-to-bumper for up to 100 miles north of the city. Gas stations were reported to be running out of gas. Shoppers emptied grocery store shelves of spaghetti, tuna and other nonperishable items.
To speed the evacuation out of the nation's fourth-largest city, Gov. Rick 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.
At 8 a.m. EDT, Rita was centered about 490 miles southeast of Galveston and was moving at near 9 mph. It winds were 170 mph, down slightly from 175 mph earlier in the day. Forecasters predicted it would come ashore along the central Texas coast between Galveston and Corpus Christi, with up to 15 inches of rain in places.
Hurricane-force winds extended up to 70 miles from the center of the storm, and even a slight rightward turn could prove devastating to the fractured levees protecting New Orleans.
"Now is not a time for warnings. Now is a time for action," Houston Mayor Bill White said.
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."