New Forth bridge decision was made after corrosion discovery

The Queensferry Crossing. Picture: Big Partnership

They confirmed that corrosion was so bad that lorries could have to be banned from the bridge in eight years.

The shock finding of a cable investigation was to trigger 12 years of frenetic activity that will culminate in today’s opening of the new bridge.

The first signs of the damage had caught everyone by surprise when it was uncovered in a routine inspection a year before, ordered as a precaution after cases in the US.

It finally concentrated minds after the debate over a second road bridge between Edinburgh and Fife had raged for decades – and a crossing came close to being built 25 years ago.

The Conservative Government’s 1992 plans for another bridge at Queensferry were well advanced, and had even reached the construction tendering stage, when Labour came to power in 1997.

Fife councillors backed the scheme with lurid illustrations of jams on the traffic-choked Forth Road Bridge as a tightening noose throttling the region’s economy.

When the corrosion problem threw the issue into sharp focus seven years later, bridge operator the Forth Estuary Transport Authority (Feta) said traffic levels had already exceeded the 1992 worst-case scenario estimates, when Fife had depicted vehicles falling off the end of graphs to back its campaign.

Months before the watershed moment in 2005 when the extent of the corrosion was revealed, Feta had included a new bridge in its plans, albeit with proposals to curb traffic growth by hiking the then £1 toll for cars at peak hours.

Feta’s scheme was for a new “multi-modal” crossing, with the new lanes for buses and drivers carrying passengers.Aware of the pressure from Feta for a new bridge, ministers initially saw the corrosion report as the authority trying to bounce them into agreeing to build it. The Scotsman also revealed at the time that the bridge would have to close to all traffic in 2019 if the corrosion wasn’t halted.

The Scottish Executive was sceptical about the dire conclusions and ordered an independent assessment.

However, the threat to the bridge quickly changed the debate over building a second one, with ministers quickly emphasising it would be a “Forth replacement crossing” that would add no extra vehicle capacity.

By March 2006, the Labour-Liberal Democrat coalition agreed a new crossing was the only option because of uncertainty over whether the corrosion could be halted, and the huge disruption if the main cable had to be replaced.

Detailed studies were ordered, including whether it should be a bridge or tunnel, and where it should be built.

Those favouring a tunnel argued that traffic would not be disrupted by bad weather and it would be more robust than a bridge in the long term.

However, supporters of a bridge said it would be cheaper, shorter, require fewer new road links, and not have restrictions such as barring lorries with hazardous or flammable loads.

The Scotsman launched a campaign in November 2006 for a new crossing to be built by 2013 – the expected date of the bridge lorry ban.

Despite ministers repeatedly insisting no final decision would be made until the studies were completed in 2007, days before Christmas they made a surprise pre-election U-turn by signalling it would be approved.

A total of 65 options were narrowed down to six – a tunnel or bridge in one of three corridors, one to the east and two to the west of the Forth Road Bridge.

In December 2007, with the SNP in power, finance secretary John Swinney pressed the button on the project, which he described as the “largest construction project in a generation in Scotland”.

It was estimated at between £3.25 and £4.22 billion.

This was later reduced, in the first of a series of cost reductions, to £1.72-£2.34bn thanks to a revised design that removed an extra lane for buses or trams.