Five men and a woman who were travelling along the M6 in a minibus have been arrested on suspicion of assault after a fellow passenger fell on to the carriageway.
The male victim suffered a serious head injury after the fall and was taken to Royal Preston Hospital where he remains in a stable condition.
Officers said the incident took place at around 1.50pm on Sunday on the northbound carriageway between Junctions 33 and 34, near to Lancaster University.
Detective Inspector Bev Mason, of Lancaster CID, said: "We are at the very early stages of our investigation and want to establish what has caused this man to fall from the vehicle.
"While we have made a number of arrests, we are appealing to anyone who saw what happened, or who has dashcam footage of the incident, to contact us."
Anyone with information is asked to call police on 101 quoting log reference 0736 of September 3.