The Oxford southern bypass was closed in both directions on Monday morning after a lorry struck a pedestrian.
Police were called at 0643 BST to the Kennington roundabout. A 46-year-old man was taken to the John Radcliffe Hospital with serious injuries.
The road was closed as officers began their investigation and for the vehicle to be recovered. It is understood the road will reopen later in the morning.
Traffic was tailing back to the A34 Western bypass at Peartree.