Motorists face rush-hour delays as main roads around Watford, Rickmansworth and Kings Langley remain closed due to severe flooding.
Hertfordshire County Council said Lower High Street and Water Lane in Watford were still closed as is part of the Stephenson Way link road onto the M1.
In Rickmansworth, Park Road remains closed by the roundabout with High Street as emergency services battle to prevent flood water reaching homes.
The fire service said the road could be closed all night.
In Kings Langley parts of Primrose Hill and Railway Terrace are also still affected by flooding.