A motorcyclist remains in hospital with serious leg injuries after a crash on the A413 Wendover bypass on Friday evening.
The crash happened at around 7.10pm on March 29 when a black Aprilla Tuono V4 1100 motorbike was travelling on the bypass towards Amersham.
The bike was in collision with a red Vauxhall Corsa that was travelling in the opposite direction.
Police believe the Aprilla was part of a larger group of riders all travelling the same way.
During the crash, the motorcyclist suffered serious injuries to his right leg and was taken to the John Radcliffe Hospital in Oxford for treatment.
The road was closed for several hours while collision investigation work was conducted.
Nobody in the Vauxhall Corsa was injured.
Investigating officer PC Russell Irwin said: “I am appealing to anybody who witnessed the collision, or who may have dash-cam footage from around 7-7.30pm on Saturday to contact us on the 24-hour non-emergency number 101 quoting reference number 43190096198.
“Alternatively, you can call Crimestoppers anonymously on 0800 555 111 or make a report online.”