Police are appealing for information following an 'incident' on the M4 near Cardiff at around 3am this morning.
Police said that a man is understood to have fallen from the Cherry Orchard Road bridge, which runs over the M4 at Thornhill, and collided with a vehicle travelling westbound along the motorway.
The 26-year-old was taken to the University Hospital of Wales Cardiff with serious injuries.
The westbound carriageway of the M4 was closed for a short time between junctions 30 and 32 while officers carried out investigations at the scene of the collision.
Anyone with information should call 101.