Police are appealing for witnesses following the collision on the A24 Southwater bypass yesterday (Friday August 30).
Emergency services were called at 10.10am to reports that two vans on the southbound carriageway had collided.
The driver of a white Transit van, a 57-year old man from Havant, was taken by air ambulance to Kings College Hospital in South London with serious head injuries.
His passenger, a 55-year old Chichester man, was treated at the scene for minor injuries.
The 30-year old Worthing man driving the other van, a white Mercedes Sprinter, was taken to Worthing Hospital to be treated for serious but not life threatening injuries.
The northbound carriageway was closed while the air ambulance attended, but was open again shortly afterwards.
The southbound carriageway was re-opened early in the afternoon.
Sergeant Stewart Goodwin of the Sussex Road Policing Unit said; “The Transit van was stationary at the time of the collision and the Sprinter van collided with the rear of the Transit.
“Anyone who saw what happened is asked to call us on 101 quoting Operation Bodmin. You can also email us at ‘email@example.com’
“Emergency services and the Highway Authority appreciate the patience and understanding of drivers who will have had their journeys disrupted while the casualties were attended to, the scene was examined and the vehicles removed.”