A car driven by a 27-year-old man from Milton Keynes was involved in a five car crash on the A23 north of Brighton on Friday.
Officers are trying to trace the driver of a small beige vehicle which failed to stop after the crash on the southbound carriageway in the early evening.
Several vehicles were forced to swerve and collided with each other.
The black Renault Clio driven by a 27-year-old man from Milton Keynes overturned.
He and his front seat passenger, a 23-year-old woman from Northampton, sustained minor injuries and were treated at the Royal Sussex County Hospital, Brighton.
Anyone with information is asked to email email@example.com uk r or phone 101, quoting serial 1222 of 07/07.