In a calculated risk aimed at avoiding total gridlock, authorities told most people in Houston, fourth-largest US city, to stay put.
As cars and trucks streamed inland waves were already inundating the beach on one end of Galveston Island. The storm was expected to strike the Texas coast late Friday or early Saturday.
Ike was steering almost directly for Galveston and, beyond that, Houston, where gleaming skyscrapers, the US' biggest refinery and NASA's Johnson Space Centre 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 210kmh.
The storm is 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, 6m high, and waves of perhaps 15m.
It could also dump 25cm or more of rain.
"It's a big storm," Texas Gov. Rick Perry said.
"I cannot overemphasise 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 650km stretch of coastline from south of Corpus Christi to Morgan City, Louisiana. Tropical storm warnings extended south almost to the Mexican border and east to the Mississippi-Alabama line, including New Orleans.
Most of the evacuations were limited to sections of Harris County outside Houston, as well as nearby bayous and Galveston Bay. But the two million residents of the city itself and one 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.
At 8pm EDT (midday Friday NZT), the storm was centred about 595km southeast of Galveston, moving to the west-northwest at 19kmh. Top sustained winds were 160kmh.
The oil and gas industry was closely watching the storm because it was headed straight for the nation's biggest complex of refineries and petrochemical plants. The upper Texas coast accounts for one-fifth of US refining capacity.
Wholesale gasoline prices spiked 30 percent Thursday out of fear of what Ike might do.
Exxon Mobil Corp., Valero Energy Corp., ConocoPhillips and Marathon Oil Co. began halting operations as Ike closed in. Dow Chemical Co. started closing up its enormous Freeport complex, home to 75 plants producing some 12.24 billion kilograms of chemical products each year.
BASF, the world's largest chemical company with 14 manufacturing sites in the Gulf Coast region, also began shutting down some operations. Spokesman Daniel Pepitone said each site has a hurricane plan that outlines detailed steps for securing plants, and precautions such as tying down hoses and taking down scaffolding began days ago.
Industry officials said their refineries and chemical plants are designed to withstand high winds. But power outages could still knock them out of service.
Ike would be the first major hurricane to hit a US metropolitan area since Katrina devastated New Orleans three years ago. For Houston, it would be the first major hurricane since Alicia in August 1983 came ashore on Galveston Island, killing 21 people and causing $US2 billion ($NZ3.1 billion) in damage.
Ike is huge, taking up nearly 40 percent of the Gulf. The National Hurricane Centre said tropical storm-force winds of at least 63kmh extended across more than 820km, and hurricane-force winds of at least 119kmh stretched for 355km. A typical storm has tropical storm-force winds stretching only 485km.
Because of its great size, storm surge and gigantic waves are the biggest risk, said Hugh Willoughby, former director of the federal government's hurricane research division. The larger the storm, the longer it hits and the higher waves can build.
And because the water is so shallow along the Texas coast, the waves pile up, creating a big storm surge, he said.
"We're not talking about gently rising water," Harris County's Emmett said.
"We're talking about a surge that will come into your homes." Authorities put the frail and elderly on buses headed for shelters. And thousands of Texas prison inmates were also moved out of the storm's path.
