Marine Parade in Worthing has reopened hours after it was closed.
The road was closed when a woman in her 70s collided with a cyclist, Sussex Police said.
The woman was taken by road ambulance to the Royal Sussex County Hospital with a serious head injury, police said, while the cyclist was not reported to have been injured.
The road has now reopened, according to the AA.
Anyone who was what happened can contact the police online at https://www.sussex.police.uk/contact/af/contact-us/ or by calling 101, quoting Operation Garston.