Five people were killed in two crashes within an hour of each other on the M6 in Staffordshire on Sunday evening.
Four of the victims, from Birmingham and Stoke-on-Trent, were in two cars involved in a 10 vehicle pile-up.
The accident happened on the northbound carriageway near junction 11A at Great Saredon. A man, 51, has been arrested on suspicion of dangerous driving.
Earlier, a woman was killed when a van overturned on the southbound carriageway near Keele.
The accident took place shortly after 1900 BST between junction 16 and the Keele services.
The female passenger was pronounced dead at the scene and all three carriageways were closed for several hours.
In the multiple pile-up, which took place about 1940 BST, a number of people had to be cut free from their vehicles with two taken to hospital suffering from suspected spinal injuries.
The vehicles involved were a Ford Mondeo, a Renault Clio and three lorries.
Two men, 52 and 30 and from Birmingham, travelling inside the Mondeo and a man and woman from Stoke-on-Trent inside the Clio were killed. The woman was 57 and the man believed to be of a similar age.
The arrested man is a German national and is currently in police custody.
Many motorists stuck on the southbound carriageway for up to five hours abandoned their vehicles and walked to the nearby motorway services.
The Highways Agency later said that although some traffic was moving, the operation was being hampered by the number of vehicles abandoned by their drivers. Paramedics were at the scene in less than 10 minutes and five emergency doctors were also called.
A lorry driver was cut free from his vehicle and received treatment for whiplash.
The spokesman said: "Extensive damage was caused to the vehicles involved. It's quite a phenomenal tragedy.
"From our viewpoint, despite being at the scene within eight minutes, sadly it was to no avail for those four people."
The control centre spokesman said: "We don't know why the collisions happened. We would ask anyone who knows anything to come forward."