The A40 has reopened westbound after multiple crashes forced its closure today.
A woman driver was killed and as many as 15 people were injured on the Oxfordshire road before 10am.
Six vehicles were seriously damaged and a further 10 to 15 were also involved in collisions that stretched for more than half a mile.
A severe weather warning for fog has been reissued after multiple traffic collisions on the A40.
Motorists are being warned that freezing fog patches will become more widespread and locally dense Wednesday evening until Thursday lunchtime.
The Met Office said: "Driving conditions will be difficult with journeys likely to take longer than usual whilst some delays to air travel are possible."
The yellow warning, which means "beware", was issued for the whole of England, with the exception of the most western and northern points.
Debris is pictured strewn across the A40 which has been closed in both directions after the multi-vehicle crash.
The collisions stretched for more than half a mile near Witney in Oxfordshire.
Another eyewitness described the "carnage" as multiple cars swerved and crashed on the busy road.
The woman, who asked not to be named, said she "luckily" saw hazard lights ahead as she approached on the slip road from Carterton, and moved on to a grass verge.
"I saw cars going zooming past me and trying to swerve out the way of the stationary cars but crashing into them," she said.
"I saw one car try swerve in between a Mini and another car, ending up crashing into the Mini and (flying) up in the air.
"I saw two cars swerve as they braked so hard and go flying into the barrier in the middle of the road ... it was carnage."
The South Central Ambulance Service spokesman said: "There was a collision towards the front of the queue and the more serious one about half-way down."
He added three ambulances, three rapid response vehicles and two ambulance officers and an air ambulance car had attended the scene.
Six vehicles in the middle were said to be badly damaged, while a further 10 to 15 were also involved.
Pregnant mother-of-two Ami Gaston, from Carterton, saw the first vehicle crash.
She said: "A man on his own had skidded and crashed into middle barrier. My partner was driving - we had to swerve out of the right lane to avoid hitting him. His door was open and air bags were released."
She added that the heavy fog meant the situation was "really dangerous", as it was difficult to see any other car until you were "on top of it".
A woman has died after around 20 vehicles were involved in collisions on the A40.
The woman driver was declared dead at the scene, while another person was taken to the major trauma unit at the nearby John Radcliffe Hospital in Oxford.
A further 10 to 15 casualties were treated at the scene at around 8.25am, a South Central Ambulance Service spokesman said.
The road has been closed in both directions after the multi-vehicle crashes which stretched for more than half a mile near Witney in Oxfordshire, the ambulance service said.
A woman has died after a major crash involving around 20 vehicles on the A40 near Witney in Oxfordshire, Thames Valley Police said.
South Central Ambulance Service spokesman David Gallagher told the Oxford Times: "We’re dealing with two patients, one female and one male, with serious or life-threatening injuries who will be going to the John Radcliffe Hospital Major Trauma Unit.
"We have a further 10 to 15 patients with minor injuries currently being assessed by our staff at the scene.
"The road conditions are reported as being very icy by our staff at the scene."
A Thames Valley Police spokesman said: "Thames Valley Police officers were called today at about 8.25am following reports of a collision involving multiple vehicles on the A40, Witney.
"Officers are at the scene along with the fire and rescue services. A number of people are believed to have been injured.
"Road closures are currently in place, and drivers are advised to avoid the area."