Transport Scotland Scottish Trunk Road Infrastructure Project Evaluation Evaluation Report for Trunk Road Projects Opened between April 07 and March 09

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.

Road infrastructure projects normally take a minimum of 5 to 7 years to plan prior to the commencement of construction and it is not possible to know exactly what will happen when a project is opened, nor what would have happened had the project not been built, particularly when the project is opened a number of years after its assessment.

2.2 Network Traffic Indicators

Traffic Volumes

The amount of traffic data presented is dependent upon the complexity of the project.

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 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 the whether the predicted impacts 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% was established by the National Audit Office and is accepted as being a reasonable range for future year forecast traffic flow comparisons.

Data Sources

Predicted Traffic Flows

Obtained/derived from the traffic/economic modelling undertaken to support the pre-tender economic assessment.

Actual Traffic Flows

Obtained from automatic traffic counters in the 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

Comparison Between Pre and Post Opening Overtaking Opportunities

A comparison between pre and post overtaking opportunities has been carried out for the majority of projects that have a specific objective relating to overtaking. The percentage of vehicles travelling through the survey site that carried out an overtaking manoeuvre pre and post opening has been compared to provide an indication of the effect that the project has had on overtaking. Commentary is provided confirming the project's effect on vehicle platoons.

The results of the overtaking surveys undertaken are considered to provide a reasonable indication of the overall effects of providing overtaking opportunities for other projects in the same geographic region. Where pre opening information is not available, the level of post opening overtaking has been presented along with commentary confirming the project's effect on vehicle platoons.

Data Sources

Pre Opening Overtaking Conditions

Confirmed through pre opening survey information collected to support the project's economic assessment.

Post Opening Overtaking Conditions

Confirmed through post opening survey information.

Travel Times

Comparison Between Pre and Post Opening Travel Times

A comparison between pre and post opening travel times has been carried out for the majority of projects that have a specific objective relating to travel times.

For those projects where overtaking surveys were undertaken, the mean vehicle speeds over the extents of the survey site has been used as a proxy for changes in travel times. The results of the overtaking surveys undertaken are considered to provide a reasonable indication of the overall change in travel times for other projects providing increased overtaking opportunities in the same geographic region. Where pre opening information is not available, commentary is provided on the likely effect of the project on travel times.

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.

Data Sources

Pre Opening Travel Times

Confirmed through pre opening survey information collected to support the project's economic assessment.

Post Opening Travel Times

Confirmed through post opening survey information.

Predicted Travel Times

Obtained from the pre-tender economic assessment undertaken during the project's preparation.

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.

Data Sources

Proposed Mitigation Measures

Presented in the Environmental Statement produced during the project's preparation.

Implemented Mitigation Measures

Confirmed through site visit.

Noise and Air Quality

A review of noise and air quality has been undertaken for projects that have a specific objective relating to the impact of the project on noise and air quality.

A comparison between pre and post opening traffic flows within the study area and/or a comparison between predicted and actual traffic flows has been undertaken to provide an indication of the effect of the project on noise and air quality, supported by commentary on available noise survey information.

Data Sources

Noise Levels

Confirmed through noise survey information collected as part of the project's preparation.

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 and 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.

In the case of bypass projects, it is necessary to collect details of any accidents on the bypassed sections of the old roads after projects open, as well as on the new projects themselves, in order to obtain a true comparison of accidents pre and post project opening.

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.

Data Sources

Personal Injury Accident Numbers

Obtained from the STATS19 data collection system.

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 the whether the predicted benefits of the project are likely to be realised.

A comparison where the actual observed flow is lower than the predicted flow 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 actual observed flow is higher than the predicted flow or actual travel times are less (i.e. speeds higher) than predicted, the economic benefits of the project may have been under predicted.

A project with predicted flows in the range +/-20% of the actual flows indicates that the economic benefits are within acceptable limits.

2.6 Integration

Commentary on Transport Integration, Transport & Land-use Integration and/or Policy Integration has been provided as appropriate for projects that have specific objectives relating to the Integration criterion.

Data Sources

Bus Services

Obtained from automatic traffic counters in the study area.

Land-Use

Outlined within the Structure Plan.

Local Government Policies

Outlined within the Local Plan.

2.7 Accessibility & Social Inclusion

Commentary on Community Accessibility and Comparative Accessibility has been provided for projects that have specific objectives relating to the Accessibility & Social Inclusion criterion.

Data Sources

Provision for Non-motorised Users

Confirmed through site visits.

Bus Services for Socially Excluded Groups

Confirmed through bus service information.

Cycling Provisions

Detailed within the Cycle Audit report produced during the project's preparation.

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.

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.

Data Sources

Predicted Project Costs

Obtained from the pre-tender economic assessment undertaken during the project's preparation.

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.

Data Sources

Predicted NPV and BCR

Obtained from the 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 07 and March 09 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.