Scottish Trunk Road Infrastructure Project Evaluation - Evaluation Report for Trunk Road Projects Opened between April 09 and March 10
2 Methodology and Data Sources
2.1 Overview
The projects presented in this report have been evaluated against their objectives and the following criteria, where applicable, to support the evaluation:
- Environment;
- Safety;
- Economy;
- Integration;
- Accessibility & Social Inclusion;
- Costs to Government; and
- Value for Money.
As the evaluations focus on impacts relating to the project's objectives, evaluations against all of the above criteria may not be undertaken for all projects. The evaluations are supported by the consideration of network traffic indicators, including traffic volumes, overtaking opportunities and travel times, as presented in the following section.
2.2 Network Traffic Indicators
Traffic Volumes
Comparison Between Pre and Post Opening Traffic Flows
A comparison of traffic flows pre and post opening has been undertaken for all projects to provide an indication of the impact that the project has had on traffic volumes. The amount of traffic data presented is dependent upon the complexity of the project. The comparison can also serve as a proxy for the effect that the project has had on noise and air quality.
Comparison Between Predicted and Actual Traffic Flows
A comparison of predicted and actual opening year traffic flows has been undertaken for all projects to confirm the accuracy of predictions during the project's preparation. The comparison can also serve as a proxy for whether the predicted benefits of the project are likely to be realised.
Depending on the nature of the traffic modelling undertaken to assess the project, the predicted traffic flow is either derived by:
- factoring the base year or the predicted opening year, design network flows to the actual opening year using National Road Traffic Forecast (NRTF) growth factors; or
- extrapolating from, or interpolating between, the modelled assessment year, design network flows.
The difference between the actual traffic flow and the predictions has been calculated and expressed as a percentage of the actual flow. A threshold of +/‑20% is generally accepted by Transport Scotland as being a reasonable range for future year forecast traffic flow comparisons.
The amount of traffic data presented is dependent upon the complexity of the project. The comparison can also serve as a proxy for the likely impact of the project on noise and air quality.
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Predicted Traffic Flows |
Obtained/derived from the traffic/economic modelling undertaken to support the pre-tender economic assessment. |
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Actual Traffic Flows |
Obtained from automatic traffic counters in the vicinity of the project/study area. |
Carriageway Standard Assessment
A carriageway standard assessment has been carried out for all projects (excluding junction improvements) using DMRB, Volume 5, TA 46/97 - Economic Assessment and Recommended Flow Ranges for New Rural Road Links, which applied at the time of the project's design, to determine the appropriateness of the carriageway standard constructed based on the opening year flow.
Overtaking Opportunities
Post Opening Overtaking Opportunities
Where no overtaking information is available, the impact of providing increased overtaking opportunities has been based on the evaluation of other projects with a comparable standard of carriageway in the same geographic region for which overtaking surveys have been carried out.
Anecdotal, qualitative evidence from stakeholders has also been gathered, where available.
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Post Opening Overtaking Conditions |
Judged from post opening survey information for other projects. |
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Stakeholder Feedback |
Obtained from Dumfries and Galloway Council. |
Travel Times
Change in Travel Times
Based on the evaluation of other projects with a comparable standard of carriageway for which pre and post opening journey time data is available, supported by anecdotal evidence where available.
Comparison Between Pre and Post Opening Travel Times
A comparison between pre and post opening travel times has been carried out for projects where the change in travel times cannot be judged based on other projects of a similar nature for which an evaluation has been undertaken.
Comparison Between Predicted and Actual Travel Times
A comparison between predicted and actual opening travel times has been carried out for projects where predicted and post opening travel time information is readily available.
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Pre Opening Travel Times |
Confirmed through pre opening survey information collected to support the project's economic assessment. |
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Post Opening Travel Times |
Confirmed through post opening survey information. |
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Predicted Travel Times |
Obtained from the pre-tender economic assessment undertaken during the project's preparation. |
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Stakeholder Feedback |
Obtained from Shiel Buses and Dumfries and Galloway Council. |
2.3 Environmental
Mitigation Measures
A review of the environmental mitigation measures implemented during construction has been undertaken for all projects to establish whether or not the measures proposed during the project's preparation have been introduced and to provide comment on their success. The mitigation measures implemented were confirmed through site visits.
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Proposed Mitigation Measures |
Presented in the Environmental Statement produced during the project's preparation. |
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Implemented Mitigation Measures |
Confirmed through site visit. |
Noise and Air Quality
A review of noise and air quality has not been undertaken for the projects that opened between April 09 and March 10 as no significant impacts on noise and air quality were expected.
2.4 Safety
Accidents
Comparison Between Pre and Post Opening Personal Injury Accident Numbers
A comparison of the personal injury accident numbers pre and post opening has been undertaken for all projects to provide an early indication of whether the project is operating safely.
The number of personal injury accidents for the 3 years within the vicinity of the project prior to opening has been compared with the observed number of personal injury accidents for the project in its first year of operation. The comparison shall be updated to include the observed number of accidents in the three year period after opening when the accident data is available.
It is important to realise that road infrastructure projects normally take a minimum of 5 to 7 years to plan prior to the commencement of construction. Many proposed road projects are derived from safety concerns such as fatal and serious accidents and often, these are treated in terms of Accident Investigation and Prevention work prior to planning the permanent solution. The comparison between 3 year pre and post opening accidents, therefore, only demonstrate the minimum road safety improvement derived from the project.
Where the influence of a trunk road improvement project has a significant impact on the local road network, it may be appropriate to extend the scope of the accident analysis.
Road Safety Audits
Road Safety Audit (RSA) reports have been reviewed for all projects, where available, to confirm whether there is any evidence that the project is not operating safely and where recommendations have been made for ameliorative measures, if appropriate.
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Personal Injury Accident Numbers |
Obtained from the STATS19 data collection system. |
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Safety Issues |
Detailed within RSA reports produced following audits carried out 1 year after project opening. |
2.5 Economy
Transport Economic Efficiency
A comparison between predicted and actual traffic flows and/or travel times has been undertaken for all projects as a proxy for whether the predicted benefits of the project are likely to be realised.
A comparison which returns a positive traffic flow difference in an uncongested situation indicates that the economic benefits of the project may have been over predicted as fewer vehicles will actually accrue journey time savings than predicted. Similarly, the economic benefits of a project may also be over predicted where actual travel times are greater (i.e. speeds lower) than predicted.
Conversely, where the comparison returns a negative traffic flow difference or actual travel times are less (i.e. speeds higher) than predicted, the economic benefits of the project may have been under predicted.
Commentary on the impact of the project on local economic development has been provided where anecdotal feedback is available.
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Stakeholder Feedback |
Obtained from Highlands and Islands Enterprise. |
2.6 Integration
Commentary on Transport Integration and Policy Integration has been provided for projects that have specific objectives relating to the Integration criterion. In addition, anecdotal evidence from stakeholders has also been gathered, where available.
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Ferry Patronage |
Annual Carrying Statistics, CalMac Ferries Ltd. |
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Ferry Usage |
Scottish Ferry Services: Draft Plan for Consultation (2011). |
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Stakeholder Feedback |
Obtained from Shiel Buses and Highlands and Islands Enterprise. |
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Local Government Policies |
Outlined within Structure and Local Plans. |
2.7 Accessibility & Social Inclusion
Commentary on Community Accessibility has been provided for projects that have specific objectives relating to the Accessibility & Social Inclusion criterion, supported by anecdotal evidence where available.
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Provision for Non-motorised Users |
Confirmed through site visits. |
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Cycling Provisions |
Detailed within the Cycle Audit report produced during the project's preparation. |
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Stakeholder Feedback |
Obtained from Shiel Buses and Highlands and Islands Enterprise. |
2.8 Costs to Government
Investment Costs
Comparison Between Predicted and Out-turn Costs
A comparison between predicted and out-turn costs has been undertaken for all projects to confirm the accuracy of predictions during the pre-tender stage and support the evaluation of value for money.
The project cost predicted during the pre-tender stage has been used in the evaluation as it is at this stage that the decision is taken on whether or not to proceed with the project. All project costs include 25% optimism bias, unless stated otherwise within the evaluation.
One of the features of the progressive analysis of projects is that the economic assessment is undertaken at each stage based on the return on future investment. This means that project costs incurred prior to the pre-tender economic assessment, which are already spent and cannot be recovered (whether or not the project goes ahead) are excluded from the overall project costs input to the economic assessment. As such, only out-turn costs incurred after the pre-tender economic assessment have been included in the comparison.
Adjustments for Retail Price Indices and discount rates to both the predicted and out-turn costs have been made, taking expenditure by year into account, to convert the figures to a common 'present value year' for prices and values - either 1998 or 2002 depending on the 'present value year' used in the pre‑tender economic assessment.
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Predicted Project Costs |
Obtained from the pre-tender economic assessment undertaken during the project's preparation. |
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Out-turn Costs |
Obtained from out-turn cost records. |
2.9 Value for Money
Initial Indications
Based on the evaluation of economic benefits and project costs outlined in sections 2.5 and 2.8 respectively, a judgement in terms of the potential impact on the projects' value for money has been made.
The value for money of a project is considered to be greater than predicted where the economic benefits have been under predicted and the project costs over predicted. Conversely, the value for money of a project is considered to be lower than predicted where the economic benefits have been over predicted and the project costs under predicted.
Where both the economic benefits and project cost have been under predicted or over predicted, a judgement has been made with regards to the likely overall impact on value for money.
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Predicted NPV and BCR |
Obtained from the pre-tender economic assessment undertaken during the project's preparation. |
2.10 Achievement of Objectives
Initial Indications
The evaluation includes an indication of how the projects that opened between April 09 and March 10 are progressing towards achieving their objectives. Where specific indicators to measure the project's performance against its objectives have not been developed, an indication of how the project is progressing towards achieving its objectives is based on the pre opening data available, supplemented by post opening data collected as part of the evaluation.
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Objectives |
Confirmed from reported Environmental Statements or Route Action Plan, where applicable. |