Transport and Economic Assessment
Transport Modelling Approach
The traffic and economic assessments have been undertaken using the Paramics A9 Dualling Traffic Model (A9DTM). The model years used in the assessment are 2026 and 2041, representing the first year of full programme operation and 15 years thereafter. The forecasting of traffic was undertaken by the Lead Traffic and Economic Advisor (LTEA) (Aecom).
The Paramics model has been used to compare the route options in terms of performance indicators, such as changes in travel behaviour and route choice due to the introduction of each route option to the future year models. The outputs from these models have been used as the basis for an economic assessment using Transport Users Benefit Appraisal (TUBA, v1.9.9) software to determine the economic benefits of each option compared to the Do-Minimum scenario (the conditions as they were in 2015).
It is noted that from a traffic modelling perspective, the proposed route options are similar, aside from the variations in layout at Murthly/Birnam Junction and Dunkeld Junction, and the differences in proposed speed limit on the proposed A9 dual carriageway. All options have the same junction layout at Dalguise Junction and The Hermitage, and all options provide access to Dunkeld & Birnam Station via Station Road in Birnam. The differences in proposed A9 geometry, including changes to the vertical alignment to accommodate a cut and cover tunnel or underpass (Options ST2A and ST2B) is not anticipated to have an impact on traffic flows or journey times on the A9.
Effects of Route Options
Differences in the design of the proposed route options influence journey times for strategic and local traffic, as well as traffic flows on the local transport network on Perth Road through Birnam. Table A.3 shows modelled journey times for the A9 between the project extents in the base year (2015) and forecast values for 2026 and 2041 under the Do-Minimum and Do-Something options. It should be noted that for traffic assessment purposes, Options ST2B and ST2D are considered the same.
| Scenario | Existing | Do-Minimum | Do-Minimum | Do-Something Option ST2A |
Do-Something Option ST2A |
Do-Something Options ST2B & ST2D |
Do-Something Options ST2B & ST2D |
Do-Something Option ST2C |
Do-Something Option ST2C |
|---|---|---|---|---|---|---|---|---|---|
| Year | 2015 | 2026 | 2041 | 2026 | 2041 | 2026 | 2041 | 2026 | 2041 |
| Northbound Journey Time | 05:57 | 06:03 | 06:07 | 05:48 | 05:51 | 05:11 | 05:16 | 04:56 | 04:59 |
| Southbound Journey Time | 06:15 | 06:21 | 06:24 | 05:47 | 05:50 | 05:11 | 05:15 | 04:54 | 04:59 |
Table A.4 shows the existing Annual Average Daily Traffic (AADT) flows at four locations along Perth Road in Birnam that have been taken from the A9DTM. The table also shows the forecast AADT on Perth Road in 2026 and 2041 in the Do-Minimum scenario. The following tables, Tables A.5, A.6 and A.7 show the forecast AADT flows on Perth Road for the route options for 2026 and 2041 under the Do-Something scenario.
| Section | Section 1 | Section 2 | Section 3 | Section 4 |
|---|---|---|---|---|
| Perth Road Traffic Flows (2-way AADT) | 1,800 | 1,300 | 1,800 | 2,600 |
| Section | Section 1 | Section 2 | Section 3 | Section 4 |
|---|---|---|---|---|
| Perth Road Traffic Flows (2-way AADT) |
2,200 (2,400) |
1,600 (1,700) |
1,900 (1,800) |
2,800 (2,800) |
| Section | Section 1 | Section 2 | Section 3 | Section 4 |
|---|---|---|---|---|
| Perth Road Traffic Flows (2-way AADT) |
1,400 (1,600) |
1,500 (1,600) |
2,800 (2,900) |
3,800 (4,000) |
| Section | Section 1 | Section 2 | Section 3 | Section 4 |
|---|---|---|---|---|
| Perth Road Traffic Flows (2-way AADT) |
2,000 (2,200) |
2,000 (2,100) |
3,000 (3,100) |
4,000 (4,100) |
| Section | Section 1 | Section 2 | Section 3 | Section 4 |
|---|---|---|---|---|
| Perth Road Traffic Flows (2-way AADT) |
2,400 (2,700) |
2,400 (2,600) |
2,600 (2,600) |
3,600 (3,600) |
Table Notes
- Section 1 refers to Perth Road between A9 Murthly/Birnam Junction and Woodville.
- Section 2 refers to Perth Road between Woodville and Station Road.
- Section 3 refers to Perth Road between Station Road and Stell Park Road.
- Section 4 refers to Perth Road between Stell Park Road and the A923.
Accidents
The impact on accidents has been assessed for the various options using default rates/costs from the DMRB (Volume 15: Economic Assessment of Road Schemes in Scotland, Section 1 (The NESA Manual)) in both the Do-Something and Do-Minimum scenarios. NESA recommends that, where possible, accident rates derived from local data should be used, however, this data must be taken from periods when conditions on the road have been broadly unchanged. Average Speed Cameras were introduced on the A9 in October 2014 to improve safety. Construction work on the Kincraig to Dalraddy section of the A9 began in autumn 2015 and the section opened to traffic in 2017. Construction work on the Luncarty to Birnam section of A9 dualling began in autumn 2018 and opened to traffic in summer 2021. In addition, various on-line Ground Investigation (GI) contracts for other A9 dualling projects have also resulted in constantly changing travel conditions on the A9. As such, conditions have not been broadly unchanged in recent years. Guidance states that for the derivation of local accident rates, data should cover the five years prior to the NESA assessment, and for local severity splits, data within a minimum of five years must be supplied. Since the A9 has been evolving over the last five years, the assessment has used national default rates.
Table A.8 shows the expected accident savings for each route option. The table indicates that the average number of accidents forecast per year would be expected to reduce under all options, compared to the Do-Minimum scenario. All options are expected to result in a reduction in the number of personal injury accidents. This is expected due to the upgrade of 8.4 kilometres of single carriageway to dual carriageway standards, which prevents right-turn manoeuvres across the carriageway, along with the provision of improved junctions. It is noted that within Table A.8, slight accidents for Option ST2A will nominally increase. This is a result of the Murthly Junction, which is further south that the existing junction and results in an increased volume of traffic utilising the B867, which is to a lesser standard than the A9.
| Accident Severity | Option ST2A | Option ST2B | Option ST2C | Option ST2D |
|---|---|---|---|---|
| Fatal | 0.2 | 0.2 | 0.2 | 0.2 |
| Serious | 0.9 | 0.9 | 1.0 | 0.9 |
| Slight | -0.1 | 0.0 | 1.0 | 0.0 |
Option ST2C would provide a fully compliant D2AP Road (sub-category c) with grade separated junctions at Birnam, Dunkeld and Dalguise to access/egress the A9 and connect to the existing road network. This would provide a safety benefit over the existing layout. Options ST2A, ST2B and ST2D incorporate an at-grade roundabout at Little Dunkeld, which is a Deviation from recommendations, and presents a greater risk of an accident occurring at this location, primarily as traffic decelerates and accelerates to navigate the roundabout. Accidents at the proposed at-grade roundabout are most likely to be either rear-end shunts on the A9 approaches, or sideswipe type incidents as traffic enters the circulatory carriageway. Many of these accidents are anticipated to result in only vehicle damage, and therefore are not included in the personal injury accident reductions. However, it should be noted that if an accident occurs at the at-grade roundabout for Options ST2A, ST2B and ST2D, there is potential for northbound traffic to queue on approach. For Option ST2A, this may result in traffic queuing within the cut and cover tunnel, introducing a potential safety issue.
Economic Performance of Route Options
The purpose of the economics section of the DMRB Stage 2 Scheme Assessment Report is to identify the differences between the options in terms of economic performance and in doing so identify the option(s) that are anticipated to present the best value for money. It is noted that the level of benefits from individual sections of the route will be less than for the entire A9 Dualling Programme (Perth to Inverness). The accrued benefits, for the entire A9 Dualling Programme, are not presented.
To assist with identification of the option(s) likely to produce the greatest value for money, the costs and benefits have been indexed such that the lowest cost option has an index value of 100 and the option with the greatest benefits has an index value of 100. The costs and benefits of the other options are presented relative to this index value. This means that the cost index for all options is greater than or equal to 100 and the benefits for all options is less than or equal to 100.
A comparison of the Economic Performance for each option is shown in Table A.9. The cost of each route option is shown in Table A.1.
| Option | Option ST2A | Option ST2B | Option ST2C | Option ST2D |
|---|---|---|---|---|
| Indexed Total Present Value of Benefits (PVB) | 32 | 78 | 100 | 78 |
| Indexed Present Value of Costs (PVC) | 374 | 135 | 133 | 100 |
| Indexed Net Present Value (NPV) | 21 | 68 | 74 | 100 |
| Indexed Benefit to Cost Ratio (BCR) | 11 | 74 | 96 | 100 |
Table A.9 indicates that Option ST2C would provide the greatest benefits for road users, as it includes grade separated throughout. The economic benefits of Options ST2B and ST2D would be broadly similar, but less than for Option ST2C, largely due to the reduction in vehicle speeds that would be necessary to negotiate the proposed at-grade roundabout at Dunkeld Junction. However, the travel time benefits of Option ST2A would be appreciably less than the other options, due to the 50mph speed limit on the A9 between the southern tie-in and the proposed Dunkeld Junction. This speed restriction, which is less than the posted speed limit on the existing A9, would result in increased journey times on this section of the A9, compared to other options.
As Option ST2A would provide the lowest level of economic benefits at the highest cost, this is clearly the worst performing option in economic terms. Options ST2B and ST2D would provide comparable benefits, but Option ST2D would produce these benefits at significantly lower cost and would therefore perform better than Option ST2B in economic terms. While Option ST2C would likely generate greater travel time benefits and accident savings that Option ST2D, the significantly higher capital cost of Option ST2C means that the NPV and BCR of Option ST2D is likely to be greater.
Consequently, in economic terms, Option ST2D is likely to be the best option.