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19 July 2019, 06:08 | Updated: 19 July 2019, 06:10
Reports on social media suggest it happened during a car meet event and that a group of people standing at the side of the road were hit.
12 people have been taken to three hospitals and the emergency services say thyey're expected to remain at the scene for some time.
Hertfordshire Police say the cars were involved in a collision on Monkswood Way at about 9.45pm on Thursday.
A police spokeswoman said: "There were a number of people in the area at the time and 14 people are presently being treated for injuries, some of which are serious."
The East of England Ambulance Service said it had taken 12 people to three hospitals - Lister Hospital in Stevenage, Watford General Hospital and The Princess Alexandra Hospital in Harlow.
An air ambulance also attended the incident.
Emergency services are expected to remain at the scene for some time.
Police have urged any witnesses to get in touch and have asked anyone with footage of the incident to retain it.