Four people have died and a woman and two children are seriously ill in hospital after a lorry smashed through the central reservation on the M5.
The lorry collided with oncoming traffic heading southbound towards Bristol after it careered across the northbound carriageway between junctions 16 and 14 in south Gloucestershire on Saturday.
Avon and Somerset Police confirmed four people had died, with three others in a life-threatening condition in hospital.
The lorry driver was also taken to hospital to be checked over as a precaution.
The motorway remains closed in both directions.
Cardiologist Amer Hamed, who was in a car travelling "10 to 20 seconds behind" the crash, told the BBC: "There was a lorry involved and at least two other cars. One was absolutely destroyed and another had flipped over."
Along with other medically-trained motorists, he stopped to help an injured woman.
Dr Hamed added: "People are helping as much as they can. Several people offered us water and one man came out of his car to give food. We're going to be stuck here for a few hours yet."
Superintendent Simon Ellis said: "Emergency service personnel have been working tirelessly at the scene in harrowing conditions and I'd like to praise them for the work they've done and are still doing.
"I'd also like to thank the members of the public who courageously went to the aid of those involved in this collision and in some cases rescued them from their vehicles.
"Our priority is to ensure the victims' next of kin are notified and supported, as well as the ongoing management of the scene, and this important work is ongoing.
"The M5 remains closed in both directions between J16 and 14."
He said three miles of traffic trapped between the junctions was still being cleared via one lane of the northbound carriageway.
He added: "We know this has been extremely disruptive to many people but we've had to prioritise the rescue and recovery operation and ensure we carry out a thorough investigation at the scene. This is painstaking work."