There is queueing traffic on A259 Roundstone Bypass both ways between B2140 Station Road and A280 Angmering Bypass (Blue Star Roundabout), the AA said. The delay are understood to be in the construction area.
Southern Rail and Thameslink warned on August 7 and 8 trains to and from Brighton will be busier than usual.
A spokesman said: “Please allow extra time if travelling.
“Plan your journey. Wear a face covering in crowded spaces if you are not exempt. Be prepared to queue.”