The road outside the QEII hospital in Welwyn Garden City is closed as contractors repair the surface.
Ringway, working on behalf of Hertfordshire County Council, have closed the section of Howlands between Ascots Lane and Beehive Lane.
The work, which began on Monday, is expected to last until Friday.
A diversion is in place along Cole Green Lane and Hollybush Lane.
A spokeswoman for Ringway said: “The work is to do subsidence caused by the poor surface of the road when it was originally laid.”
Initial reports had suggested there was a sinkhole.
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