Texas highways were clogged Thursday with a mass exodus of people moving inland to escape Hurricane Rita, which is roaring through the Gulf of Mexico and closing in on Houston.

The National Hurricane Center said Thursday that Rita's path has shifted northward and appeared to be headed just east of Galveston and Houston, where it's expected to hit in the early hours of Saturday.

But officials with the Federal Emergency Management Agency are warning that Rita remained unpredictable.

"I don't think anyone in the Gulf Coast is out of harm's way," said David Paulison, FEMA's acting director.

An estimated 1.8 million residents in Texas and Louisiana were under orders Thursday to evacuate to avoid a deadly repeat of hurricane Katrina.

"Clearly, people have taken the warning very seriously," said CTV's Lisa LaFlamme, reporting from Houston.

Texas Governor Rick Perry said the exodus is the largest in his state's history. And the million plus people trying to leave the city at the same time left freeways from southern Texas into Louisiana looking like parking lots.

In an unprecedented move, Perry ordered southbound traffic on Interstate 45 shut down and all eight lanes redirected north out of the city for over 200 kilometres.

Local officials warned residents to get out, and told them they would not be rescued if they stayed.

"I'd rather you sit in traffic for 8 to 10 hours" than remain in the city, said Perry. "These are people's lives here."

Dan McTeague, parliamentary secretary for Canadians abroad, said there are about 30,000 Canadians in Texas at any given time.

He said efforts were underway to contact 175 permanent residents who have registered with consular officials in Dallas.

As Rita continues to edge closer to shore, the U.S. National Hurricane Center has downgraded the still-massive storm to a Category 4.

Even though that's a slight retreat from the scale-topping Category 5 Rita had been pegged at some 24 hours earlier, the storm's sustained winds of more than 241 kilometres per hour still threaten to smash into the U.S. Gulf Coast with devastating force.

"It's certainly going to be one of the major storms of all time," Steve Miller of the Canadian Hurricane Centre told CTV.

Tracking the storm from Dartmouth, Nova Scotia, Miller said Rita is already much larger than the hurricane that devastated the Louisiana coastline last month.

"The central pressure is stronger than what I saw with Katrina when it was in the middle of the Gulf of Mexico."

Plotting the storm's course through the weekend, Miller added, is considered a tricky long-term forecast.

"Tracking an area of 1,000 kilometres on either side around the storm, and trying to figure out the mean steering flow" is difficult, he said, but the fact the storm is now speeding across the Gulf means landfall is inevitable.

In addition to Dallas, shelters for at least 250,000 evacuees were being established in Huntsville, College Station, San Antonio.

Some 5,000 Texas National Guard troops are also on standby, as another 1,000 Department of Public Safety officers await Rita's aftermath.

New Orleans

Rita's ominous drift up the coast towards Louisiana spells big trouble for the already ravaged city of New Orleans, which is under a tropical storm warning.

Outer bands of steady rain from the storm were already falling on New Orleans beginning early Thursday afternoon.

It's feared much more rain and a storm surge could overwhelm fragile levees and inundate the city all over again.

Louisiana Gov. Kathleen Blanco is urging the 500,000 coastal residents to leave immediately, that hinting those who don't leave won't live to regret it.

"If some people insist on staying and believe that they can
weather this kind of storm," she said Thursday, "then perhaps they should write their social security numbers on their arms with indelible ink."

The Mexican government, meanwhile, has issued its own tropical storm watch across its northeast coast from Rio San Fernando northward.

Only three scale-topping Category 5 hurricanes -- including Andrew in 1992 -- are known to have hit the U.S. mainland. There is no record of both Category 4 and 5 hurricanes striking the U.S. mainland in the same season.