A man from Somerset has been released under investigation after an accident on Saturday near Sherborne.
A silver Peugeot Boxer van left the carriageway at Tiley Knap in Middlemarsh - leaving the passenger trapped.
A 37 year old man from Sherborne was later airlifted to Southampton General Hospital, where he remains in a critical condition.
The driver of the van, a 43-year-old man from Somerset, was arrested on suspicion of driving offences and taken to Dorset County Hospital by ambulance as a precaution.
He has been released under investigation as officers continue to make enquiries.
Sergeant Phil Lee, from the Dorset Police the traffic unit, said: "We are carrying out a detailed investigation to establish the circumstances of the collision.
"I am appealing for any witnesses to the collision, or the manner of driving of the vehicle involved prior to the incident, to contact us.
"I would also like to hear from anyone who may have any relevant dashcam footage.
"Finally, I would like to thank anyone affected by the road closures for their patience and understanding while these were in place to allow the essential examination of the scene to take place to assist in ascertaining the full circumstances."
Anyone with information is asked to contact Dorset Police at www.dorset.police.uk, via email SCIT@dorset.pnn.police.uk or by calling 101, quoting incident number 26:320. Alternatively, contact Crimestoppers anonymously on 0800 555111 or via www.crimestoppers-uk.org