Officers today asked for witnesses to come forward following the incident on March 9 around 3pm on Brigg Road between Barton and Bonby in North Lincolnshire.
A blue Toyota Prius was travelling along the road away from Barton when it hit a tree.
The man driving the Prius was taken to hospital but has since died from his injuries.
Police are "particularly keen" to hear from the driver of a car that was travelling in the opposite direction around the time of the crash.
Call 101 quoting log 343 of March 19.