The A23 near Handcross has been closed southbound this afternoon (Thursday October 4) after a lorry left the carriageway.
The driver of the lorry is believed to have suffered serious injuries.
Both carriageways were closed for a short time to allow the air ambulance to land.
The northbound carriageway has now reopened.
A Sussex Police spokesperson said: “We thank drivers for their understanding and patience while emergency services treat the driver and investigate the cause of the incident.”
Anyone who witnessed the incident is asked to contact Sussex Police quoting Operation Sherton.