A driver has been taken to hospital after the van he was in hit a barrier and flipped onto its side.
The air ambulance was called to the crash on the eastbound carriageway of the A40 near Raglan, in Monmouthshire .
The slip road to the A449 southbound is currently closed.
In a statement, Gwent Police said: "At approximately 2.40pm this afternoon, we received reports of a road traffic collision involving one vehicle, on the eastbound exit slip road of the A40, as it meets the A449 southbound in Raglan.
"A van struck a barrier and flipped onto its side.
"A male passenger was able to get out of the vehicle having suffered minor injuries, but the male driver was temporarily trapped inside with serious injuries."
The police spokesman said the driver was taken by air ambulance to Morriston Hospital , in Swansea and that a number of roads in the area are closed while investigation work is carried out.
A spokeswoman for the Welsh Ambulance Service said they were called to the crash at 2.54pm.
South Wales Fire and Rescue Service sent three fire engines to the scene from Usk, Abergavenny and Malpas and a spokeswoman said one person was cut out of the vehicle.
