The A413 was closed for two hours heading into Aylesbury from Weedon after a one car smash left an elderly man with serious injuries.
It happened close to the Buckingham Park roundabout and traffic was diverted towards the Berryfields estate.
The incident left the car in bushes at the side of the road and a fire crew had to cut the driver from the car.
South Central Ambulance said the man, who may have become unwell at the wheel, was flown to John Radcliffe Hospital with head and neck injuries.
A spokesman for Thames Valley Police said that the force was called at 10.11 to reports that a car had left the carriageway.
Police shut the road but it has since reopened.