A man was taken to hospital in a critical condition after his van hit a tree near Ferndown.
Overnight, between Wednesday January 2 and Thursday January 3 - a white Citroen Berlingo collided with a tree on Stapehill Road, near to a sharp bend.
Stapehill Road remained shut this morning while officers carried out investigations.
A 52-year-old local man sustained life-threatening injuries. He remains in Southampton General Hospital in a critical condition. His next of kin have been informed.
Police Constable Lynn Parsons of Dorset Police's Traffic unit said: "At the moment we are not aware of any other vehicle being involved.
"We are appealing for witnesses to contact us to help determine exactly what happened.
"In particular we are keen to hear from anyone who may have passed the van after it had collided with the tree."
Any witnesses or anyone with information should contact Dorset Police on 101, quoting incident number 03:39.