The A414 Rush Green roundabout has been closed after a vehicle travelling in the wrong direction collided with other vehicles.
Police received reports of a vehicle believed to be travelling on the wrong direction on the southbound carriageway of the A414 between Hertford and Rush Green roundabout.
It is believed that a dark-coloured Kia Sportage collided with one vehicle on the carriageway and then two further vehicles at Rush Green roundabout.
A number of people have received serious injuries and been taken to the Princess Alexandra Hospital in Harlow.
The vehicle is also believed to have been seen driving erratically in the town centre.
Sergeant Pete Scholes, from the Bedfordshire, Cambridgeshire and Hertfordshire Road Policing Unit, said: "We are appealing to anyone who may have witnessed what happened but who has not yet spoken to us. In particular, did you see any vehicles driving erratically or on the wrong side of the road in the area around the time or just before the collision took place? If so, your information will help us understand what has happened."
Anyone with information is asked to contact the police on the non-emergency number 101, quoting incident reference ISR 598 of 18/11/18, or report information online at www.herts.police.uk/report.
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