'Inaccurate data' caused M4 relief road inquiry delay
The M4 relief road public inquiry was delayed by almost four months because of inaccurate traffic data predictions.
A pre-inquiry meeting in Newport on Friday confirmed revised traffic forecasts were less than initially predicted, stalling the process.
The Welsh Government want a 15-mile, £1.1bn motorway at Newport to relieve congestion at the Brynglas tunnels.
They hope the relief road will open by the autumn of 2021.
The new road is proposed to be built to the south of Newport, and will connect the M4's current junction 23 at Magor and junction 29 near Castleton.
"The department of transport issued revised guidance," Inspector William Wadrup told the pre-inquiry.
"So they (the Welsh Government) were less confident of the traffic case and the precision of it. So effectively they froze the process."
Mr Wadrup said the Department of Transport had used the planning data "in error."
"This was pointed out to the department, who accepted this," added Mr Wadrup.
The M4 relief road project has been on the agenda since it was first proposed by the Welsh Office in 1991.
Up to 13 alternatives to the Welsh Government-backed route are scheduled to be suggested at the inquiry when it begins next month.
The Welsh Government are also due to propose the remodelling of junctions for the current Magor Services, and the new junction serving Newport docks.
The inquiry, which is expected to last five months, will also look into the Welsh Government's proposed compulsory purchase of land at Newport docks for the construction of the new road.
Owners Associated British Ports have objected to the relief road scheme because of the potential impact on operations at Newport docks.
Mr Wadrup said the docks are "sacrosanct", and should be exempt from compulsory purchase orders.
Secretary of State for transport Chris Grayling, with support from the Welsh Government, has asked the inquiry to investigate the purchase of the land at Newport docks.
A concurrent inquiry will also look into the proposed demolition of the Magor Vicarage as it is a listed building.