A stretch of the M20 was temporarily closed and two people were seriously injured after more than 30 vehicles were involved in a crash.
Kent Police said it happened on the London-bound carriageway near Hollingbourne, Maidstone, around 6.41pm on Saturday.
There was "thick fog" in the area at the time, according to National Highways, and the crash forced the closure of the motorway in both directions between junctions 8 and 9 for several hours.
Three fire engines and a technical rescue unit were sent to the scene, said Kent Fire and Rescue Service.
Eleven people were also treated by paramedics and police said two of them were seriously injured and taken to hospital.
Drivers posting on the Kent Traffic and Travel Facebook page said conditions around the time of the crash were hazardous.
"I drove through about half 6," said Rob Rattenbury.
"The fog just appeared and was very thick. All cars were slowing down to about 30mph. Be careful out there."
Ema Louise posted: "I was on the other side of the road and heard a massive bang but didn't see anything as it was so foggy."
Some users suggested that smoke from a nearby fireworks display may have made the visibility worse.
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At around 5am on Sunday, National Highways said both carriageways had reopened.
Another Met Office warning for fog is in place until 10am on Sunday and covers a large part of southern England and the Midlands.
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