A woman was taken to hospital after her red Mini Cooper car overturned into a ditch along the A41.
The accident happened on the bypass just before the Bourne End junction at around 7.40pm as the driver headed towards Aylesbury.
Police had to enforce a rolling road block to slow traffic down while the driver was treated by paramedics who took her to Watford General Hospital for treatment to minor injuries.
No other cars were involved.
Initial police reports of the incident indicated that the driver of the vehicle was a man.