An 18-year-old passenger died, and a 19-year-old driver was taken to hospital with serious injuries, after their car came off the road and collided with a tree on Friday in Hemel Hempstead.
The incident happened shortly before 2.15pm along the eastbound carriageway of the A414 between Hemel Hempstead and St Albans.
It is currently not known why the red Vauxhall Corsa came off the road.
PC Jackie Lister said: “We are doing all we can to fully investigate this incident in order to establish exactly what happened prior to the collision.
“I’m appealing to anyone who witnessed the collision or the events leading up to it, to please contact me.
“I’m particularly keen to speak to the driver of a blue Luton box van, who we believe may have witnessed the collision and may have information that could assist the investigation.
“I’m conscious that many drivers have dash cams fitted to their vehicles and that the incident may have been captured.”
If you have any footage or information that could prove useful to the investigation, call Herts Police’s non-emergency number 101 or report information online at www.contacthertspolice.uk/Report
Alternatively you can contact the independent crimefighting charity Crimestoppers on 0800 555 111 or online at www.crimestoppers-uk.org No personal details are taken, information cannot be traced or recorded and you will not go to court.