A man was airlifted to hospital after a driver saw him lying seriously injured in the central reservation of a main Northampton road.
A force spokeswoman said a member of the public was concerned about the welfare of a man who appeared to be injured in the crash barrier area between the two carriageways.
The Warwickshire and Northamptonshire Air Ambulance was called to the scene, which landed on the eastbound carriageway and caused the road to be shut for about an hour.
Queues of more than a mile developed before traffic was allowed to move again at around 11.15am.
The police spokeswoman described the man's condition as 'serious' but said it is not currently thought to be life-threatening.
He was airlifted to University Hospitals Coventry for treatment.