A van, a car and a motorbike crashed in Potters Bar, with the motorcyclist taken to hospital with serious injuries.
The incident occurred at around 4.25pm on Barnet Road on Monday (May 16).
Hertfordshire Police, Hertfordshire Fire and Rescue Service and the East of England Ambulance Service attended the scene.
Road closures were also put in place, to allow access for emergency vehicles.
The motorcyclist was taken to hospital with serious injuries, where he remains.
The van and car driver were also taken to hospital, but with injuries that were not believed to be life-changing or life-threatening.
Police constable Stewart Robertson, of the Bedfordshire, Cambridgeshire and Hertfordshire Road Policing Unit, said: “This was a serious collision on a busy road, which took place right before peak rush hour times.
"We believe that several people would have seen this collision taking place, or witnessed the events leading up to it or immediately after.
“If you did, and have not yet been in contact with police, please get in touch as soon as possible. Any information, even something that may seem relatively minor or insignificant, could be key to progressing our investigation.
“I would also like to ask anyone who was driving in the vicinity of Barnet Way around the time specified who has a dash cam fitted to their vehicle, to review the footage and send us anything you think may be of note.”
PC Robertson can be contacted via the following email address: Stewart.Robertson@herts.police.uk
Information can also be reported online, or by non-emergency number 101 and quoting reference ISR 464 of 17 May.
Anonymous information can be given by contacting the Crimestoppers charity on 0800 555 111, or via their website.
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