A man is fighting for his life after falling while possibly "surfing" on the roof of a van, police said.
The 26-year-old is in a critical condition in hospital following the fall from the moving Volkswagen Transporter.
The van driver, 27, was arrested following the incident in Charmandean Road, Worthing, West Sussex, just after 11pm on Sunday.
A Sussex Police spokesman said: "The casualty may have been on the roof of the vehicle, possibly 'surfing', when he fell to the ground and was badly hurt, but police are still trying to establish the exact circumstances.
"The Kent, Surrey and Sussex air ambulance helicopter flew the man, who is from Lancing, to Southampton General Hospital with potentially life-threatening injuries."
Anyone with information is asked to call Sussex Police on 101, quoting Operation Paisley.