A man's suffered serious head injuries after coming out of a car while it was travelling along the A1M near Stevenage.
Herts Police say they don't know what led to the incident, between 7 and 7.45 on Saturday evening (April 13th).
It happened on the northbound carriageway, just before Junction 7.
A spokesperson for the Constabulary said: "For reasons unknown at this time, a man aged in his late 20s exited a white BMW while it was travelling along the carriageway."
The man, in his late-20s, came to rest on the hard shoulder and is said to be in a stable condition in hospital.
Sergeant AJ Boddington, from the Bedfordshire, Cambridgeshire and Hertfordshire Road Policing Unit, said: "We are currently working to establish the circumstances surrounding the incident and are appealing for anyone who saw what happened, or events leading up to it, to please come forward.
"We are particularly keen to hear from anyone who may have captured dash cam footage. If you have any information, please contact us."
Anyone with information should Hertfordshire Constabulary on the non-emergency number 101, quoting ISR 662 of April 13, 2019. You can also report information online at www.herts.police.uk/Report.