Daniel McClellan, 31, died two days after the white Ford Transit van lost control at 11pm on Tuesday, June 7.
The vehicle was travelling west along London Road, Hemel Hempstead, when it hit a telegraph pole at the junction with Stratford Way.
Due to the impact, the van rolled over and the two men inside were injured. No other vehicles are thought to have been involved.
The first man suffered a head injury and was taken to Watford General Hospital before being discharged, while the second – Mr McClellan – suffered a very serious head injury.
He was rushed to St Mary’s Hospital in London for further treatment but died two days later on Thursday, June 9.
PC Martin Longley, from the Beds, Cambs and Herts forensic collision investigation unit, said: “I would like to speak to anyone who saw the collision itself or who believes they saw the white van in the moments before the collision occurred.
“Your information could help us understand exactly how this fatal collision occurred.”
Anyone with information should call PC Longley on non-emergency number 101.