Part of the northbound M11 motorway has been shut and a man arrested after six separate crashes in early morning fog.
The motorway would not reopen until the middle of the afternoon, police said.
An eight-mile stretch of carriageway was closed between junction eight at Stansted Airport and junction nine at Saffron Walden in Essex.
Police said a seriously injured woman and a man had been taken to Addenbrooke's Hospital in Cambridge.
Essex Fire and Rescue said 12 cars and three lorries were involved in crashes and they had sent seven appliances to deal with the incidents.