A man was taken to hospital by air ambulance yesterday, following a road traffic collision on the A41 northbound, just before the Tring exit.
Police were called to the scene at 8.23am after reports that a green Skoda Fabia had left the carriageway and hit a tree.
Both sides of the A41 were temporarily closed to allow the air ambulance to land, although the helicopter then landed in a neighbouring field.
The southbound carriageway was reopened just after 9.10am and the northbound carriageway by 10.45am.
Anyone who witnessed the incident should call Herts Police on 101.