According to traffic reports, A27 Arundel Road remains blocked westbound towards A280 (Angmering / Clapham junction) after the crash around 8.50am.
Traffic sensors suggest that vehicles could not get past the scene.
Pictures from the scene show police directing traffic away from the road, where a Next delivery lorry can be seen.
Sussex Police has now issued an appeal for witnesses.
A spokesperson said: "A man has been taken to hospital with serious injuries.
"Anyone who saw what happened or captured the incident on dash cam is urged to email [email protected] quoting Operation Ilfracombe."
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