Motorists have been warned that travelling the wrong way on the one-way system at White Hoe, Old Castletown Road, is an offence punishable by a £120 fine or a court appearance.
The warning comes after reports of drivers ignoring the rules following the section’s reopening to one-way traffic on Thursday.
The road has been closed since March 9 to allow refurbishment work at the White Hoe Wastewater Pumping Station.
In a statement released on Wednesday lunchtime, Manx Utilities confirmed that works had progressed ahead of schedule.
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Subject to successful concrete curing and routine safety checks, the road is expected to fully reopen on March 25.
The closure of this key arterial route into Douglas from the south of the island has caused significant delays for commuters, with some reporting journeys taking two to three times longer than usual.
Concerns over traffic management were raised in Tynwald this week by Arbory, Castletown and Malew MHK Jason Moorhouse, who questioned why vehicle flow data had not been collected prior to the closure.
Responding, Manx Utilities chairman John Wannenburgh said that traffic management approval fell under the Department of Infrastructure.
He added that Manx Utilities had provided the technical scope and timing for the essential works and had proceeded in line with arrangements authorised by the department.
Mr Wannenburgh also addressed concerns about data collection, explaining that no traffic counters were installed at White Hoe as the route was not on the proposed path for transporting wind turbines to the Earystane site.
Mr Moorhouse additionally questioned why the road was closed early on Monday, March 9, before work appeared to begin, describing the situation as ‘absolute chaos.’
Mr Wannenburgh confirmed the closure was implemented at 6.30am, with work commencing from 8am, and emphasised that the closure had been planned months in advance.
He stressed the scale of the infrastructure involved, noting that the valve chamber cover sits above a major underground pipeline linking Douglas and the west of the island.
The closure was unavoidable to ensure the safety of both workers and road users, he said, and efforts are continuing to reopen the road as soon as possible.
Police have reiterated that motorists must obey the one-way system when the road reopens, warning that offenders face fines or court action.