A police officer was seriously injured when he was hit by a van while dealing with an earlier crash.
A Mini Cooper came off the eastbound A27 at Cosham, near the Portsmouth junction, just before 1000 BST.
Pc Nick Oliver, 30, attended the scene and was struck by a white Vauxhall Corsa van and taken to hospital with serious head injuries.
A 52-year-old man from Havant was arrested on suspicion of driving without due care and attention.
He has been released on bail pending further inquiries.
The A27 was partially shut for several hours, causing major traffic delays.
Pc Oliver, a roads policing officer, was taken to Southampton General Hospital by ambulance with a police escort and remains in a serious condition, a force spokeswoman said.
The male driver of the Mini was taken to Queen Alexandra Hospital for treatment.