Emergency services shut a road to treat a person suffering a medical incident as the air ambulance was called out.
Herts Police, the East of England Ambulance Service, and Herts Fire and Rescue were seen in Leeming Road, Borehamwood, from around 12.30pm.
Firefighters provided screens to protect the privacy of the person being treated by the ambulance service.
The road remained closed as traffic was diverted around the scene until around 2.20pm.
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Members of the public reported that the air ambulance was present.
Herts Police and the East of England Ambulance Service have been contacted for more information. Update to follow.
There were further delays in the town during the afternoon as a bus broke down in nearby Shenley Road, with both incidents contributing to "standstill" traffic according to motorists.
As of 4pm there was still knock-on congestion, particularly along Shenley Road.
An East of England Ambulance Service spokesperson said: "We were called just after 12.20pm to a medical emergency on Leeming Road in Borehamwood. Two ambulances, an ambulance officer vehicle and the East Anglian Air Ambulance were sent to the scene. An adult man was transported by road to Harefield Hospital for further treatment."