The Department of Transport is justifying the work currently ongoing at Governor's Bridge.
The ?1.25 million project will see a wider carriageway from the Nook and the creation of a full-sized roundabout.
That section of the A18 Mountain Road is closed, which means motorists are being diverted in and out of Douglas.
The major work started on Tuesday and is expected to take five weeks.
There have been calls for the DOT to operate a shift system throughout the night to ensure it's completed earlier, but that has been deemed inappropriate.
Some have also questioned the decision for this project to co-incide with the Braddan Bridge works, but Network Planning Manager Derek Sewell says they have been deliberately co-ordinated:
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