20. Effects of Route Corridor Options
20.1.1 Following the route corridor option sifting discussed in Chapter 3 (Description of Route Corridor Options), four route corridor options (two on each side of the Forth) were selected for further assessment as summarised below.
- North Corridor Option 1 is an online upgrade of the A90 / M90 between the northern bridgehead and the A92 at Halbeath Interchange.
- North Corridor Option 2 is an offline upgrade of the A90 / M90 between the northern bridgehead and the A92 at Halbeath Interchange.
- South Corridor Option 1 is a new dual three lane carriageway between the proposed southern bridgehead and the A90 west of Scotstoun Junction with connections to Echline Junction and the A904. The existing Scotstoun Junction is retained with additional links providing interaction between the M9 Spur, A90 and the local road network. No links are provided between the M9 Spur and the A90 to the east. M9 Junction 1a is to be reconstructed to incorporate new west facing links within a free-flow junction arrangement.
- South Corridor Option 2 is a new dual three lane motorway between the M9 west of M9 Junction 1a and the A904. A new free-flow junction is to be provided to M9 providing full connectivity between the proposed mainline, the M9 and the M9 Spur. A new junction is also proposed between the mainline and the A904 with north facing slip roads only. This junction shall provide local and non-motorway access to the proposed replacement bridge, motorway restrictions on the mainline being terminated through the junction area. At Scotstoun, the existing junction arrangement is to be reconstructed, priority being given to the new M9 Spur to A90 eastbound connection.
20.1.2 For traffic and economic appraisal, it is necessary to combine the corridor option alternatives that are subject to more detailed appraisal to create four do-something scenarios. It is assumed that the existing Forth Road Bridge is not open to traffic, under each scenario. The do-something scenarios that are discussed in this section of the report are defined below (with their TMfS:05A test identifiers):
- North Corridor Option 1 and South Corridor Option 1
- North Corridor Option 2 and South Corridor Option 1
- North Corridor Option 1 and South Corridor Option 2 and
- South Corridor Option 2 and North Corridor Option 2
20.2 Northern Route Corridor Options
20.2.1 Figures 20.1 and 20.2 (Volume 2) indicate forecast Annual Average Daily Traffic (AADT) flows on key links in the network north of the Forth Road Bridge in both the opening year (2017) and the consequent design year (2032). Two sets of flows are presented for each year reflecting the forecasts for each option paired with both options at the southern bridgehead. Assignments are based on a fixed demand matrix for each highway model assignment.
20.2.2 Figure 20.1 indicates the forecast AADT flows for North Corridor Option 1 in 2017 and 2032. The forecast flows vary slightly depending on the connecting infrastructure to the south of the Firth of Forth, with flows on the M90 forecast to be slightly higher when this option is paired with South Corridor Option 2.
20.2.3 Figure 20.2 indicates the forecast AADT flows for North Corridor Option 2 in 2017 and 2032. The forecast flows again vary slightly depending on the connecting infrastructure to the south of the Firth of Forth, with flows on the M90 forecast to be slightly higher when this option is paired with South Corridor Option 2.
20.2.4 Comparing the forecast flows on the road network surrounding the proposed new infrastructure indicates little difference between the two northern route corridor options. Forecast flows on the offline section in North Corridor Option 2 are lower than the equivalent on-line sections because the existing road infrastructure remains to cater for local traffic, resulting in greater segregation between longer and shorter distance flows. Both options appear to be capable of accommodating anticipated traffic volumes.
20.3 Southern Route Corridor Options
20.3.1 Figures 20.3 and 20.4 (Volume 2) indicate forecast Annual Average Daily Traffic (AADT) flows on key links in the network south of the Forth Road Bridge in both the opening year (2017) and the consequent design year (2032). Two sets of flows are presented for each year reflecting the forecasts for each option paired with both options at the northern bridgehead.
20.3.2 Figure 20.3 indicates the forecast AADT flows for South Corridor Option 1 in 2017 and 2032. The forecast flows vary slightly depending on the connecting infrastructure to the north of the Firth of Forth.
20.3.3 Figure 20.4 indicates the forecast AADT flows for South Corridor Option 2 in 2017 and 2032. The AADT forecasts for South Corridor Option 2 are largely independent of the northern route corridor option selected.
20.3.4 Comparison of the forecast flows on the road network surrounding the scheme indicates a number of small differences between the options. South Corridor Option 2 (irrespective of which option is selected to the north of the Firth of Forth) is forecast to result in more traffic on the proposed replacement bridge and the M9, both west of M9 Junction 1a towards Linlithgow and south of M9 Junction 1a towards Newbridge Roundabout and Claylands Junction. Traffic flows on the M8 east of Claylands Junction and to a lesser extent the A8 east of Newbridge Roundabout are also forecast to be higher under South Corridor Option 2. However, the higher traffic flows forecast under South Corridor Option 2 are countered by lower flows on the A90 east of Scotstoun Junction, the M9 Spur, A8000 / B800 between South Queensferry and Kirkliston and on the A904 west of South Queensferry. The lower flows forecast on the A904 and A8000 indicate that South Corridor Option 2 is a more attractive alternative for the cross-Forth movements to and from the west of the southern bridgehead due to the slightly shorter travel distance for such trips when compared to South Corridor Option 1.
20.3.5 South Corridor Option 2 assigns significantly more traffic along the A904 to the east of the proposed replacement bridge, compared with South Corridor Option 1. This traffic is effectively rat running between the bridge and the A90 east of Echline Junction. As a strategic model, TMfS05a does not fully replicate the extensive congestion and deterioration of traffic conditions that this level of traffic volume is likely to cause. This routing of strategic traffic is inappropriate and unacceptable on a local road.
20.4.1 This chapter and its accompanying figures, has set out the forecast traffic flows using each element of the proposed northern and southern route corridor options.
20.4.2 South Corridor Option 2 provides better and more direct access to the M9. However, connection to the A90, east of Echline junction is not as good as with South Corridor Option 1. Less connectivity to the A90 results in a propensity for traffic to use the A904 rather than the mainline, which in turn would be likely to result in unacceptable traffic conditions and high levels of congestion.