A 16-year-old cyclist fell unconscious after being hit by the driver of a vehicle and riding home.
The female driver collided with the teenager and she apparently spoke to the boy before driving off.
The boy then rode home, where he fell unconscious before being taken to hospital.
The teen had broken his collar bone in the incident, and police are now appealing for the driver who left the scene to come forward.
The collision occurred in Baldock Way, Borehamwood, Hertfordshire behind the Mace convenience store, at around 3.20pm on Friday 27 August.
PC Tristan Webb, from the Bedfordshire, Cambridgeshire and Hertfordshire Road Policing Unit, said: “We have carried out CCTV enquiries and spoken to witnesses who were in the area at the time.
"We believe the vehicle involved was a grey Nissan Juke, being driven by a woman with blonde hair.
“So far we have been unable to trace the driver and are appealing for her to get in touch with us.
“We would also like to hear from anyone else who may have seen what happened, captured dash cam footage of the incident or events prior to it, or has any other information which could assist us.”