Early inquiries indicate that she was carried on the bonnet of the other car and fell from the bonnet and was in collision with a Belisha beacon. The vehicle drove away.
Police subsequently attended an address in Barnet and arrested a woman in connection with the inquiry. She was taken to a north west London police station and was subsequently bailed pending further enquiries to a date in late April.
Witnesses or anyone who has information can contact the incident room on 020 8247 7821 or Crimestoppers on 0800 555 111.
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Cloud and outbreaks of rain will ease away eastwards during the morning, clearing the east coast around midday with sunny spells developing.
Police and emergency services are attending the scene.