A man remains in critical condition after falling ill on the A414.
The incident happened at 10am, when a man was taken ill in his car near Longabout, Colney Heath.
Police said they are unsure whether there was a road traffic accident or if the man fell ill whilst driving.
The A414 was closed in both directions after the accident. The eastbound carriageway has now reopened.
Emergency services, including an air ambulance, attended the scene.
A land ambulance took the man, who is in his 50s, to Lister Hospital.
Police are still at the scene and the westbound carriageway remains closed between London Colney roundabout and the A1.
Motorists are being advised to avoid the area.
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