Highways in US East Coast areas braced for Hurricane Florence are congested with motorists fleeing “the storm of a lifetime”.
Up to 1.7 million people have been ordered to evacuate across South Carolina, North Carolina and Virginia.
South Carolina authorities have turned four motorways into one-way routes away from the coast to speed the exodus.
The category four storm with 130mph (215km/h) winds is forecast to make landfall early on Friday.
Hurricane Florence could wreak more than $170bn (£130bn) of havoc, according to analytics firm CoreLogic.
Its projection suggested the storm could damage nearly 759,000 homes and businesses from Charleston, South Carolina, to Virginia Beach, Virginia.
A National Weather Service forecaster in Wilmington, North Carolina, said: “This will likely be the storm of a lifetime for portions of the Carolina coast.
“And that’s saying a lot given the impacts we’ve seen from Hurricanes Diana, Hugo, Fran, Bonnie, Floyd and Matthew.
“I can’t emphasise enough the potential for unbelievable damage from wind, storm surge and inland flooding with this storm.”
As well as in the Carolinas and Virginia, states of emergency have been declared in Maryland and Washington DC amid concern over flooding.
Jeff Byard, of the Federal Emergency Management Agency, urged people to flee before the hurricane hits.
“This is not going to be a glancing blow,” he said.
“This is going to be a Mike Tyson punch to the Carolina coast.”
But while many coastal residents have complied with mandatory evacuation orders, others are boarding up their homes and vowing to ride out the storm.
In a video posted to his Twitter account on Wednesday, US President Donald Trump warned residents in Florence’s bullseye to heed official warnings.
“Get out of its way, don’t play games with it, it’s a big one, maybe as big as they’ve seen, and tremendous amounts of water,” said Mr Trump.
“Bad things can happen when you are talking about a storm this size. It’s called Mother Nature. You never know, but we know. We love you all, we want you safe.”
What makes Florence so dangerous?
Forecasters say the storm poses such a threat because it is expected to slow down and hover for nearly three days over the Carolina coast.
It is forecast to bring 20-40in (50-100cm) of rain and life-threatening storm surges of up to 13ft (4m).
Hurricane force winds will emanate up to 70 miles from the centre of the storm, say meteorologists, meaning the impact may be felt on shore well before Florence makes landfall early on Friday.
National Hurricane Center Director Ken Graham warned that rivers up to 40 miles inland may flood.
Mr Graham said on Wednesday morning the Pamlico and Neuse rivers in North Carolina will see their flows “reversed” as storm surges push water back inland.
He added that half of fatalities during hurricanes are caused by storm surges, and another quarter of deaths are due to inland rains and flooding.
Is global warming to blame?
The relationship between climate change and hurricanes is a complex one.
Warmer seas power hurricanes. So as the temperature of ocean water goes up, we might expect the intensity of hurricanes to increase in future.
A hotter atmosphere can also hold more water, so this should allow hurricanes to dump more water on affected areas.
But there are so many factors that contribute to these rare events, it has been difficult to tease out clear trends from the data.