The incident occurred at around 10.10am on Monday 21 August on the southbound carriageway between Hemel Hempstead and Kings Langley.
A 44-year-old man from Ilford, Greater London, was arrested on suspicion of causing serious injury by dangerous driving and remains in custody at this time.
Police are appealing for witnesses, information and dash cam footage following a serious road traffic collision on the A41.
It is believed up to six vehicles were involved in the incident, including a white Mercedes tipper HGV and a black Peugeot.
The three occupants of the Peugeot were taken to hospital. A man and a child, whose injuries are not described as life-changing or life-threatening, were taken to hospital by road.
A woman, meanwhile, was airlifted to hospital with potentially life-changing injuries.
The road was closed for several hours while emergency services dealt with the incident.
Herts Highways are currently completing resurfacing the southbound stretch near Kings Langley, while the northbound carriageway is now open.
The full circumstances of the collision are still being investigated.
Sergeant Alex Ward, from the Road Policing Unit, said: “Our enquiries are continuing, and we are appealing for any witnesses to please come forward.
“If you were driving in the area and have a dash-cam fitted, please check it as you may have recorded some crucial footage to assist our investigation.”
Anyone with information is asked to contact Sgt Ward via email. ISR 207 of 21 August 2023.