Sussex police were called after a blue Ford KA reportedly collided with a hedge around 8.48am.
A section of the westbound A27 at Tangmere is currently closed as emergency services respond to the incident and a man is said to be in a serious condition.
Police have said that a diversion has been set up in the area in the meantime.
Officers are urging anyone who saw what happened, or anyone with dash cam footage, to email [email protected] quoting Operation Laker.
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