The incident happened at around 5.20pm on Monday when, according to police, a 29-year-old man and a white Ford transit van were involved in a collision on Portway Road, in the direction of Haddenham.
Investigating officer PC Ashley Jordon said: “I would like to speak to anyone who saw the collision or who saw the car travelling prior to the collision. If you have any information which could help the investigation please contact police.”
If you have any details which would assist officers contact the 24-hour Thames Valley Police enquiry centre on 101 quoting URN 1141 (14/3).
If you don’t want to speak directly to the police, you can contact the independent charity Crimestoppers anonymously on 0800 555 111.