An air ambulance has been called to a collision on the southbound A23 near the junction with the A272.
Traffic is being held in both directions to allow emergency services to assist with the single-vehicle collision on the near A272 Cowfold Road (Bolney Cross).
Motorists are being diverted via the Handcross turn off.
Fire crews from Crawley, Horsham and Burgess Hill were called to the scene at 5.30pm and a heavy rescue unit was also called to assist, a fire service spokesman said.
The fire service has also requested the assistance of the electricity board, but the spokesman could not give further detail as to why.