A horsebox was involved in a "very serious crash" with a lorry on the M11 in Essex which caused traffic chaos on Thursday afternoon.
Three cars were also involved in the smash which caused "multiple casualties" and sent thick black smoke billowing into the air.
An air ambulance rushed to the scene, along with two road ambulances and other emergency vehicles, near Stansted airport between junctions seven and eight.
The M11 motorway was shut in both directions to allow the helicopter to land on the tarmac. Motorists spent two hours stuck in queues stretching back two miles.
A spokesman for East of England Ambulance Service said: "We have sent two ambulances, a rapid response vehicle, our hazardous area response team and Hertfordshire Air Ambulance to the scene of a several vehicle collision between junctions nine and eight of the M11 northbound.
"Police closed the motorway in order to allow the air ambulance to land."
Essex police advised motorists to avoid the Bishops Stortford/Stansted Airport area and seek alternative routes. The northbound carriageway was expected to remain shut for several hours