Police investigating an incident, in which a woman fell from a car on the busy A69 in Northumberland, say the vehicle was travelling at 60 mph when it happened.
At around 4.35pm on Saturday police received a report that a front seat passenger had come out of a car on the eastbound carriageway near to the Bridge End roundabout.
Emergency services attended and the woman was taken to hospital by paramedics with serious but non-life threatening injuries.
She remains in hospital in a critical condition.
Police say driver of the vehicle involved in the incident allegedly drove off without checking how she was.
A man, aged 23, was later arrested.
Detectives still need anyone who saw the vehicle - believed to be a black Ford Fiesta - in the area around the time of the incident to get in touch.
Officer particularly want to speak to anyone who saw the vehicle travelling from Bardon Mill towards Hexham at around 4.30pm on Saturday.