It happened on the A27 Shoreham bypass Westbound this morning.
Police and emergency services were called following a collision involving two vehicles on the A27 at Hangleton this morning.
At 7.56am it was reported that a BMW had overturned on the westbound carriageway during a collision with a Mercedes Sprinter van.
A road closure was put in place before the Southwick Tunnel, at the junction with the A293 Hangleton Link. The carriageway has since reopened and officers have thanked motorists for their patience while the incident was being dealt with.
A driver has been arrested on suspicion of causing serious injury by dangerous driving and drug driving.
Sussex Police is appealing for witnesses. Anyone with information or dashcam footage should email: collision.appeal@sussex.pnn.police.uk quoting incident 204 of 09/08.