A motorist has been arrested on suspicion of drug-driving after a crash on the A470.
Two vehicles, a Subaru Impreza and a Honda, were involved in the crash between Nantgarw and Upper Boat, shortly before 12:50 BST.
The 25-year-old driver of the Impreza was arrested.
A 58-year-old man from Splott, Cardiff, who was in the Honda, was taken to hospital with serious but non-life threatening injuries.
The southbound carriageway was closed briefly to allow the Wales Air Ambulance to land but has since reopened along with the main road north out of Cardiff.