Grove Road is closed both ways near Portland Prison while a crane is being operated between Shepherds Croft and Incline Road.
The diversion is not thought suitable for larger vehicles.
The road is due to re-open at 6pm.
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Some 20% of carriageways in England and Wales have less than five years of life remaining before they become unusable, researchers said.
Icy conditions expected after a freezing night. The far west will enjoy sunny spells while the rest of the region sees increasing cloud.