The road was closed following the collision, near the Southwick tunnel, and was only re-opened fully this morning, after work to repair the cntral reservation was completed overnight.
Two men were injured when a lorry travelling eastbound hit a slow-moving people carrier, turned over and went through the central reservation.
The 49-year-old male driver of the people carrier suffered suspected leg and chest injuries and was taken to Royal Sussex County Hospital in Brighton for treatment.
The 79-year-old male passenger of the car suffered serious head injuries and was flown to Southampton General Hospital
The 44-year-old lorry driver from Yorkshire was treated for minor injuries, before being arrested for careless or dangerous driving. He remains in custody this morning (Friday, July 26).