For some time, ministers in this and the previous Government have considered critical issues in relation to the condition of the existing Forth road bridge. Ministers have recognised the pivotal significance of the crossing to the connectivity and the future of the Scottish economy. A number of reports were compiled by the previous Administration, and this Government has continued that work.

In the transport statement in June, we announced a programme of public information exhibitions to present the full facts that have informed the reports that have been received from Transport Scotland on the form and location of the new crossing of the River Forth. In parallel with that public engagement, further work has been undertaken to assess the viability of tunnel options, and that work has now concluded. That was essential to ensure that all options were properly considered, and the Government is able to recommend a clear and fully assessed proposal.

The Government's purpose is to increase sustainable economic growth, and we recognise the continuity of the Forth replacement crossing as a key contributor to achieving that purpose. The existing Forth road bridge has served Scotland well, carrying far more traffic than was ever envisaged. The effects of that traffic and the impact of the Scottish climate have taken their toll on the structure, and the bridge, despite being strengthened and maintained during its life, faces an uncertain future.

The Forth Estuary Transport Authority is working to protect the integrity of the bridge by putting in place measures to dry out the main cables. We will not know whether those measures have been successful until 2012, which, when set against previous announcements by FETA that the crossing may have to close to heavy goods vehicles in 2013 and all vehicles by 2019, is impossibly late to begin thinking about the problem. Doing nothing is not an option. Work is required now to protect this crucial link in Scotland's transport network and to minimise the risk from the existing bridge not being available— either altogether, or while it is closed for considerable periods of time to allow the cables to be replaced if that proves possible.

In appraising each option, consideration has been given to six assessment criteria: the impact on the environment; operating restrictions; operational risk; cost; cost risk; and time taken to construct. Four options have been identified as possible replacement crossing types and have been fully considered and appraised: a suspension bridge; a cable-stayed bridge; a bored tunnel; and an immersed-tube tunnel. I will set out the consideration that has been given to each option and then explain how each option has performed against the six assessment criteria.

A cable-stayed bridge would run from the northern shore just west of the existing road bridge to a point west of South Queensferry. It would consist of a dual two-lane carriageway with hard shoulders to provide an area for breakdowns to pull off. The traffic consequences of breakdowns on the existing bridge are well known, and hard shoulders would improve reliability. A cable-stayed bridge would be open to all classes of traffic and would include provision for pedestrians and cyclists to cross the bridge.

That option would take approximately five and a half years to construct since it extends incrementally from the three pylons that support the deck, allowing work to be carried out on a number of locations at once. A cable-stayed bridge would cost between £2.79 billion and £3.63 billion in outturn prices including VAT, with a benefit to cost ratio of 4.57. Increasing the width of the bridge deck to accommodate two-lane multimodal systems for public transport would add between £450 million and £580 million to the cost of that option, again in outturn prices.

A suspension bridge would run along essentially the same route as the cable-stayed option, and would consist of a dual two-lane carriageway with hard shoulders. The suspension bridge has a benefit to cost ratio of 4.06 and the estimated cost of that option, excluding multimodal provision, is between £3.17 billion and £4.11 billion in outturn prices including VAT. It would be constructed in approximately six years, due to the sequence of constructing a suspension bridge in an essentially linear fashion.

The cross-section of bored tunnel that would be suitable for a crossing of the Forth is limited by the ground conditions in the area. The proposed tunnel upstream of Rosyth would be a twin-bore tunnel approximately 8.5km long and, due to its location, would require some additional 5km to be travelled by the vast Majority of vehicles compared with the existing bridge. It would require ventilation shafts on both banks of the Forth, which would need to be located to avoid the environmentally sensitive sites of the Forth, including special protection areas. The tunnel would generate approximately 4 million tonnes of spoil, which would require disposal.

The ground conditions of the Forth would affect the size of the tunnel-boring machine that could be used. To incorporate multimodal options into a bored tunnel would therefore require an additional tunnel. There are restrictions on the goods that may be transported through tunnels, including on flammable goods such as whisky and oil. A bored tunnel would take approximately seven and a half years to deliver and it would not accommodate pedestrian or cycle access because of the safety issues of having those vulnerable users in a tunnel. A bored tunnel has an estimated cost of between £4.08 billion and £5.27 billion in outturn prices and a benefit to cost ratio of 2.61. Providing an additional tunnel for multimodal use would add more than £1 billion to the cost.

Immersed-tube tunnel technology takes advantage of the ability to construct the tunnel sections in a dry dock before floating them into position and lowering them into a trench on the bed of the river. The tunnel must then be protected from accidental damage by shipping and dredging. Such a tunnel in the Forth would have to be below the level of the river bed.

An alignment for an immersed-tube tunnel has been considered. It would be located immediately upstream of Rosyth. Although it would connect to the wider transport network—especially the proposed Rosyth bypass—the indicative alignment would impact directly on the Rosyth dockyard. The ITT would comprise some 2.3km of the 8.2km that would make up the crossing. The remainder would be provided by a combination of traditionally mined sections and cut-and-cover excavation.

An ITT would require a large trench to be excavated in the bed of an environmentally sensitive area of the Forth. It would have the same operating restrictions as a bored tunnel, but it could be constructed in approximately five and a half years at a cost of between £3.51 billion and £4.53 billion, with a benefit to cost ratio of 2.85. Multimodal options could be incorporated by providing a widened tunnel section at a similar additional cost to that of a bored tunnel.

I turn to the comparative analysis of the options and their performance against the assessment criteria. Because of the environmental importance of the Forth, we had to work closely during the summer with the statutory consultation authorities Scottish Natural Heritage, Historic Scotland and the Scottish Environment Protection Agency to produce a strategic environmental assessment. The SEA assesses the predicted effects of each option against a range of objectives that relate to environmental quality, communities, health, the natural environment, and cultural heritage.

Based on that assessment, and the additional assessment that considered the sites in the Forth that are designated under the European Union habitats directive, a bored tunnel performs best. The impacts that are associated with a bridge can be mitigated, however, through careful design and working methodologies. The assessment also found that the immersed-tube tunnel option has the greatest risk of impact to the protected environment of the Forth because of its method of construction. To determine the level of that risk would require a major investigation of the Forth, which could take a further year—time that can ill be afforded given the state of the existing bridge.

The options for the crossing improve transport connections and the reliability of travel times. That will reduce the carbon dioxide emissions compared with the base case of continuing to rely on the existing bridge.

I have set out the restrictions that apply to the transportation of goods through tunnels. They stem from the international carriage of dangerous goods by road requirements, which were put in place by a European agreement in 2005. They class a series of hazardous goods as generally prohibited from road tunnels. Although bridges have restrictions, which relate to their design, the existing bridge carries some 200 special loads per annum and there are far fewer day-to-day restrictions than with a tunnel. All traffic can use a bridge, from pedestrians and cyclists to the largest loads, whereas not all traffic can use a tunnel. Assessing the options against the operating restrictions and operating risks criteria therefore means that bridges perform better than tunnels.

The Government takes the view that decisions must be made now to provide the flexibility for multimodal public transport measures to be incorporated into the crossing. The Government has therefore taken the decision in principle to incorporate multimodal public transport in its chosen option.

I have set out the cost estimates for the options already but, to recap, the estimates for each option in outturn prices including VAT and multimodal public transport are: £3.25 billion to £4.22 billion for a cable-stayed bridge; £3.62 billion to £4.7 billion for a suspension bridge; £5.12 billion to £6.6 billion for a bored tunnel; and £4.77 billion to £6.19 billion for an immersed-tube tunnel. That clearly shows that, against the cost criterion, the bored tunnel is the most expensive option and a cable-stayed bridge the least. The benefit cost ratio and the value-for-money tests all indicate the cable-stayed bridge as the best performing option.

The cost risks associated with the options are related to the uncertainty associated with each. That has been reflected in the use of optimism bias in line with Treasury guidance. It is evident from taking market soundings that there is greater concern that the costs of a tunnel may increase than there is about those for a bridge. That concern is greater still where the tunnel is bored, as ground conditions can be fully understood only as the tunnel progresses. There is more cost certainty for an ITT, because the tunnel units are manufactured in a controlled environment before being floated and lowered into place. Furthermore, a cable-stayed bridge or an immersed-tube tunnel would take the least time to construct and a bored tunnel the most.

The consideration of each option against the assessment criteria has been endorsed by both the Scottish Government's Cabinet and an independent peer review, which was carried out by a group of international procurement and construction experts specifically recruited for the purpose. Having assessed all the factors, the Government has come to the view that the Forth replacement crossing should be a cable-stayed bridge with multimodal capacity on a route slightly to the west of the existing road bridge.

The replacement crossing is about more than just the crossing itself: the connections at either side are equally important. Providing a link to the M9 from the new Forth crossing will allow greater choices and opportunities in West Lothian, while the construction of improved junctions to the north will protect and promote access to the development areas of Fife. Including dedicated public transport will provide opportunities for those who travel into and around Edinburgh and offer improved opportunities for links more widely between Fife, Edinburgh and the Lothians.

The Forth crossing is a crucial part of the road network, connecting communities on a local, regional and national scale, and it is overloaded due to single-occupant cars during peak periods. That is predicted only to worsen in the future. Our key objectives of a wealthier, fairer and greener Scotland are well supported by the decision to protect cross-Forth travel while ensuring the flexibility and capacity to provide for other modes of transport, including measures to ensure the reliability of the crossing.

The new bridge will be the single largest transport project for a generation, and we are determined that strong, clear governance is put in place to ensure that the costs and risks are effectively managed and that the project is delivered on time.

By the time that it opens in around nine years' time, the new bridge with a segregated public transport corridor will cost between £3.25 billion and £4.22 billion. Now that we have taken the decision to build a bridge, work can move forward on the legislative and procurement options for delivery. Further announcements on the details of the bridge as well as the authorisation and procurement processes will be made during 2008. Work is continuing on the procurement options, and that will include consideration of the appropriate transfer of risk to the private sector, in line with current Government policy on the development of the Scottish futures trust. The Government is against tolling.

The programme for the development of the project envisages a submission for authorisation in 2009 and a procurement competition in 2010, leading to an appointment of a constructor in 2011. Transport Scotland will take that decision forward and has been procuring the services of a world-class consultant to develop the design of the new crossing. An announcement on the preferred bidder arising from the competition will be made shortly, and we look forward to confirming their appointment early in the new year to deliver the programme that I have set out.

The new Forth crossing is a hugely ambitious project. It will be the largest construction project in a generation in Scotland. It will be an iconic structure. It will maintain a fundamental link across the River Forth and incorporate the opportunity for a real change through multimodal public transport provision. It will create a new and better connection to our transport infrastructure in west and east central Scotland, and it will be delivered through effective and comprehensive care for our natural environment.

The Forth rail bridge created an image of global significance when it was constructed in the 19^th century. In the 21^st century, our vision of a new crossing of equal stature and significance is the Government's promise today.