The road is partially blocked westbound on the Shoreham-by-pass, at the junction with the A283 Steyning Road.
Traffic has backed up along the A27 and along Steyning Road.
It has added to an already congested day, with queues all through Shoreham along the A259.
A heat wave today (July 31) has brought sun-seekers flocking to the coast to enjoy the nice weather.