The car in which the woman was travelling landed upside down amid the traffic at around 10.35am on Friday. She fell from the bridge at the Bowburn interchange on the A1(M), in Durham.
The crash is believed to have involved a red Vauxhall Astra.
A car travelling on the bridge, the A177 flyover, came off the bridge and landed wheels-up on the central reservation of the A1.
The driver managed to extricate themselves from the upside-down car, but the passenger was less fortunate.
The woman, who is in her mid-40s, needed to be cut out of the vehicle wreckage.
She was then airlifted to hospital for attention for “serious injuries”.
Durham Police confirmed a man in his early 30s was arrested on suspicion of causing serious injury by dangerous driving.
He remains in custody.
No other injuries were reported.
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Major delays to traffic have been reported since the incident.
Investigations are continuing, and drivers have been advised to avoid the area.