HOUSTON—Cars and trucks streamed inland and chemical companies buttoned up their plants Thursday as a gigantic Hurricane Ike took aim at the heart of the U.S. refining industry and threatened to send a wall of water crashing toward Houston.

Nearly 1 million people along the Texas coast were ordered to evacuate ahead of the storm, which was expected to strike late Friday or early Saturday. But in a calculated risk aimed at avoiding total gridlock, authorities told most people in the nation’s fourth-largest city to just hunker down.

Ike was steering almost directly for Houston, where gleaming skyscrapers, the nation’s biggest refinery and NASA’s Johnson Space Center lie in areas vulnerable to wind and floodwaters. Forecasters said the storm was likely to come ashore as a Category 3, with winds up to 130 mph.

But the storm was so big, it could inflict a punishing blow even in those areas that do not get a direct hit. Forecasters warned that because of Ike’s size and the state’s shallow coastal waters, it could produce a surge, or wall of water, 20 feet high, and waves of perhaps 50 feet. It could also dump 10 inches or more of rain.

“It’s a big storm,” Texas Gov. Rick Perry said. “I cannot overemphasize the danger that is facing us. It’s going to do some substantial damage. It’s going to knock out power. It’s going to cause massive flooding.”

Hurricane warnings were in effect over a 400-mile stretch of coastline from south of Corpus Christi to Morgan City, La. Tropical storm warnings extended south almost to the Mexican border and east to the Mississippi-Alabama line, including New Orleans.

In Surfside Beach, a coastal community about 40 miles south of Galveston, the police chief was so worried that the entire force planned to ride out the storm inland.

“I don’t have a crystal ball, but if I did, I think it would tell me a sad story. And that story would be that were faced with devastation of a catastrophic range,” said Chief Randy Smith. “I think we’re going to see a storm like most of us haven’t seen.”

Most of the evacuations were limited to sections of Harris County outside Houston, as well as nearby bayous and Galveston Bay. But the 2 million residents of the city itself and 1 million in other areas of the county were asked to remain at home.

“We are still saying: Please shelter in place, or to use the Texas expression, hunker down,” said Harris County Judge Ed Emmett, the county’s chief administrator. “For the vast majority of people who live in our area, stay where you are. The winds will blow and they’ll howl and we’ll get a lot of rain, but if you lose power and need to leave, you can do that later.”

Authorities hoped to avoid the panic of three years ago, when evacuations ordered in advance of Hurricane Rita sent millions scurrying in fright and caused a monumental traffic jam so big that cars ran out of gas or overheated. Ultimately, the evacuation proved deadlier than the storm itself. A total of 110 people died during the exodus, including 23 nursing home patients whose bus burst into flames while stuck in traffic.

This time, traffic was bumper-to-bumper on the freeway leading away from Galveston immediately after the evacuation order, but by late afternoon, many evacuees had made it past Houston, to the north. And just in time: Waves were already inundating the beach on one end of Galveston Island.

Some gas stations began running out of fuel, but fuel trucks were called in to replenish them.

Houston Mayor Bill White said one of the lessons of Rita mess was that too many people fled who didn’t need to. Instead, he asked residents to protect their homes.

“Think how your barbecue could become a flying object,” he said.

At 5 p.m. EDT, the storm was centered about 400 miles east of Galveston, moving to the west at 10 mph.