The collision between a van and a motorcycle occurred around 4.30pm, causing the road to be closed for several hours.
Following the collision, the 73-year-old motorcycle rider was airlifted to Royal Sussex County Hospital with serious, potentially life-threatening, or life-changing injuries.
Investigators are particularly interested in speaking with the rider of a moped with learner plates who was ahead of the collision and may have information that can help their investigations.
A 27-year-old man was arrested on suspicion of causing serious injury by reckless driving and has been released on bail until April 30 while investigations continue.
Anyone with information or dash cam footage should contact police by emailing [email protected] and referencing Operation Pence.
