4 DMRB Stage 2 Corridor Assessment (May to August 2008)
The DMRB Stage 2 Corridor Report was prepared in accordance with TD37/93, Scheme Assessment Reporting, of the Design Manual for Roads and Bridges (DMRB). The purpose of the report was to document the factors that have been taken into account in the provision of alternative route corridor options, considering the scheme objectives and the engineering, environmental, traffic and economic advantages/disadvantages and constraints associated with each.
For the purposes of DMRB Stage 2 Corridor Assessment, North Corridor Options 1 and 2 and South Corridor Options 1 and 2 were considered over the full extent of the corridors. It was noted that the preferred corridor identified need not be implemented over these full lengths.
4.2 Stage 2 Assessment: Report Conclusions
The recommendation made in the DMRB Stage 2 Corridor Report was based upon the requirements of the scheme objectives and the assessment work undertaken from an engineering, environmental, traffic, economic and sustainability perspective.
4.2.1 Engineering Conclusion
North Corridor Option 1 and South Corridor Option 1 were deemed to be the preferable corridor options in terms of engineering assessment.
The preference was primarily based upon the maximum use made within these corridors of existing roads infrastructure which provides better value for money.
4.2.2 Environmental Conclusion
Overall, North Corridor Option 1 and South Corridor Option 1 were also preferred in environmental terms, largely due to the majority of the corridors being online.
North Corridor Option 1 which is largely online would affect the fewest land interests, cross fewer pedestrian/cyclist routes, have less ecological impact and lower potential for impacts on sites of geological importance. It is likely to have less potential for flood risk and water quality impacts. Noise and air quality impacts would be similar for either northern route corridor option but, on balance, North Corridor Option 1 was considered to be preferable. The predominantly online alignment of North Corridor Option 1 would also result in lower landscape and visual change and was therefore found to be preferable in terms of view from the road and driver stress.
South Corridor Option 1 requires less new infrastructure than its alternative and passes through less sensitive areas. It captures very similar comparative benefits to Northern Option 1 as a result.
South Corridor Option 2 was assessed to be better in terms of overall noise effects as it would divert traffic away from the A90 south of South Queensferry.
South Corridor Option 1 was preferred in terms of landscape and visual impacts as it is much more contained and in contrast to South Corridor Option 2 does not cut through open, rural landscape. South Corridor Option 2 would also increase the isolation of the designed landscape of Dundas Estate.
4.2.3 Traffic and Economic Conclusion
In comparing the economic evaluation of Corridor Option combinations, under South Corridor Option 2 it is noted that a proportion of Edinburgh bound traffic would assign to the A904 as a more direct route from the Forth Replacement Crossing to Scotstoun Junction and Edinburgh via the A90, leaking from the new strategic network linking to the M9 and M9 Spur. The attributed traffic cost benefits, including the benefits attributed to the traffic from Fife using the A904, results in a higher Net Present Value (NPV). However, South Corridor Option 2 also comes with a substantial additional cost and therefore a broadly equivalent Benefit to Cost Ratio (BCR) in comparison to those option combinations containing South Corridor Option 1.
Given the additional cost associated with South Corridor Option 2 and its similarity with South Corridor Option 1 in terms of BCR, it was concluded that North Corridor Option 1 paired with South Corridor Option 1 would offer, overall, the preferred solution.
4.2.4 Sustainability Conclusion
The high level evaluation of the northern and southern route corridor options against the key DMRB Stage 2 sustainability objectives showed that North Corridor Option 1 and South Corridor Option 1 are preferred.
4.2.5 Sensitivity Check: Corridor Option 4B
Following completion of the assessment, it was agreed that it was necessary to validate the finding by carrying out a sensitivity check against an additional option, South Corridor Option 4B, an option identified at the Forth Replacement Crossing Route Corridor Workshop held with Transport Scotland on 25 June 2008, with the objectives of providing a direct link from the Forth Replacement Crossing to the M9 and a direct link to the A90.
Similar to that of Option 4A, but to a reduced standard of alignment and junction design, this option would relieve some of the traffic pressures which may build up on the existing road network through the implementation of South Corridor Option 2 in isolation.
South Option 4B would require the closure of the recently completed M9 Spur and the removal of the Scotstoun Junction and M9 Junction 1a, each of which would become redundant. A new all movements junction would be required on the M9 northeast of Winchburgh providing direct access to the proposed replacement bridge.
It was assessed that whilst South Corridor Option 4B is capable of providing direct access to the A90 and the M9, the land take associated with the implementation of such a scheme would be far higher than that of South Corridor Option 1 or South Corridor Option 2 in isolation. This corridor would have higher overall environmental impact.
Further to this, the anticipated cost associated with the implementation of this option is greater than that associated with South Corridor Option 1 and of a comparable order with South Corridor Option 2. Considering the environmental impacts, the cost associated with this options implementation and the amount of existing roads infrastructure made redundant through its provision, South Corridor Option 4B was removed from further assessment in advance of the detailed assessment undertaken for the DMRB Stage 2 Corridor Report.
4.3 Stage 2 Recommendation
Based on the above conclusions, the overall recommendation of the DMRB Stage 2 Corridor Report was that North Corridor Option 1 and South Corridor Option 1 be taken forward as the preferred corridors.
The improvement over the full extent of North Corridor Option 1 and South Corridor Option 1 in the Stage 2 Corridor Report is referred to in this report as the Full Corridor Scheme. The Stage 2 Corridor Report noted that this improvement need not be implemented in full. Further work would be required to define the road improvement within the corridor that would provide best value for money. The project planning work was therefore progressed to allow further detailed consideration to be given to the form and function of the junctions required and the extent of the road infrastructure improvements that should be provided.