A woman died and several other people were injured in four separate crashes involving more than 20 cars on the A40 in Oxfordshire.
The crashes happened within a mile-and-a-half-stretch between the B4047 and the A415 at Witney, according to South Central Ambulance Service.
Four other people were taken to hospital – one with life-threatening injuries.
About 15 people were treated at the scene, the ambulance service said.
Another of those taken to hospital had suffered serious injuries, while the other two had minor injuries.
Thames Valley Police confirmed the collision via its TVP Roads Policing account.
It said: “The A40, Witney is closed following a multi-vehicle collision. Emergency services are on scene and drivers are advised to avoid the area.”
From-the-scene: BBC Oxford reporter Rahil Sheikh
It is a cold and foggy scene here on the A40 heading towards Witney.
Walls of thick fog are completely blocking the view of the fields to the right and left of the A40.
On the road, cars disappear from view a few hundred metres away.
As I approached the Ducklington turn off, the blue police lights cut through the fog. The road is closed and traffic is being directed towards the nearby service station.
Nikki Cowan was involved in the crash but was uninjured.
“As I tried to apply my brakes, I could feel that the car was just gliding across and realised there must have been black ice,” she told the BBC.
“As much as I tried to either steer or brake, I just didn’t stop.
“It felt like driving in slow motion. I tried to avoid the line of cars and aim for the central reservation, but it was very icy.
“I just careered into a VW that had a child and two adults in it.
“I remember seeking the shock on the driver’s face, and then more cars hitting each other behind me.”
HGV driver Ben Starkey, who was at the scene but not involved in the crashes, said the road conditions were foggy and the road was icy at the time of the collisions at about 08:30 GMT.
“[The crashes] happened ahead of me and then behind, lucky enough I managed to stop my HGV,” he said.
“I saw the emergency services performing CPR on a lady. The person who was with her in the car was walking around saying ‘I can’t believe this’.
“There was also another woman in front of me who was severely injured when a car collided with her when she stepped outside her vehicle.”
Mr Starkey praised the emergency services, who he said had done an “excellent job” having arrived at the scene in about 20 minutes.
Another eyewitness, who did not want to be named, described “carnage” as she approached on the slip road from Carterton.
“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 to swerve between a Mini and another car and end up crashing into the Mini and (flying) up in the air.”
Mother-of-two Ami Gaston, from Carterton, was in the car with her partner when she saw the first vehicle crash about half a mile from the Witney exit, blocking the right lane.
“A man on his own had skidded and crashed into middle barrier,” she said.
“My partner was driving – we had to swerve out of the right lane to avoid hitting him.”
She said heavy fog meant the situation was “really dangerous”, as it was difficult to see any other car until you were “on top of it”.
One lane of the westbound carriageway of the A40 has now reopened and the second lane is due to reopen once repairs have been completed to the central reservation, Thames Valley Police said.
The eastbound carriageway is due to remain closed until about 19:15 to allow the last vehicles involved in the crashes to be recovered and the road surface to be cleared of debris, the force added.
All vehicles not involved in the collision have since been released from the site.
Oxfordshire County Council said the road had been gritted overnight.
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