The A27 is blocked near Hammerpot due to a collision due to a driver crashing into a tree, police said.
One lane in the westbound carriageway is blocked with queueing traffic due to an accident near the Woodman Arms pub. It is affecting traffic heading towards Arundel.
Sussex Police said: "At 3.40pm on Tuesday (5 March) emergency services went to the westbound A27 near the junction with the B2225 Arundel Road after a car had left the road and hit a tree.
"The man driving the car was the sole occupant and no other vehicle was reported to have been involved.
"The man driving the car was being treated at the scene. No assessment of his condition and no further information are available at this time."