Officers believe motorists heading towards Aylesbury could hold vital clues about how a woman came to die in a car crash on the A413 earlier this week.
The woman, who was driving a Honda Jazz, collided with a Volkswagen Golf on the road near Little Missenden at around 10.30am on Tuesday.
She was pronounced dead at the scene.
Four other people were taken to Stoke Mandeville Hospital for treatment.
PC Andy Martin, who is investigating the incident said: “Witnesses who were heading towards Amersham have come forward but we are wondering if people going towards Aylesbury saw anything.”
Call police on 101 with information.