Uxbridge Road in Rickmansworth has been flooded by a burst water main.
The A412 is closed between Denham Way and Grove Road.
The water is at least a a foot deep, and is gushing from a large hole in the tarmac by the Mill End pumping station, next to Drayton Ford.
The leak was discovered by police officers who were passing in their car. The officers reported the fault to Hertfordshire Highways and closed the road, at 12.56pm.
The closed section