Transport Scotland Trunk Road Project Evaluation - Report for Trunk Road Projects Opened between April 05 and March 07

2 Data Collection and Analysis

2.1 Economic Assessment Data

During the period that the projects in this report were originally assessed, there were a number of changes to the 'present value year' and the discount rate. In 2002, the 'present value year' changed from 1994 to 1998 whilst the discount rate remained at 6%. Then in January 2003 the discount rate changed from 6% to 3.5% (which requires Optimism Bias to be applied to both the project costs and the works' duration).

In July 2005 the 'present value year' changed again, this time to 2002 and the appraisal period was extended from 30 years to 60 years. The 3.5% discount rate is now used for the first 30 years of the 60 year appraisal period and reduces to 3.0% in years 31 to 60.

The following programs were used to assess the economic impact of projects that opened between April 05 and March 07:

  • the Strathclyde Integrated Transport Model (SITM), a four stage multi-modal model;
  • the TREVAL/PTEVAL modules (for highway and public transport, respectively) forming part of the CSTM (Central Scotland Traffic Model) package;
  • the NESA (Network Evaluation from Surveys and Assignments) computer program, developed and maintained by Transport Scotland (TS); and
  • the PEARS (Program for the Economic Assessment of Road Schemes) program (supported by NESA runs to assess the economic impact of accidents).

The version of SITM used had a 'present value year' of 1994 and a discount rate of 6%, whereas the version of TREVAL/PTEVAL had a 'present value year' of 1998 and a discount rate of 6%. For the NESA and PEARS assessments, the versions used had a 'present value year' of 1998 and a discount rate of 3.5%.

Cost comparisons have been carried out on the projects assessed in this report using:

  • a 'present value year' of 1998 and a discount rate of 6.0%; or
  • a 'present value year' of 1998 and a discount rate of 3.5%.

The economic assessment reports generally contain information on the traffic projections and cost estimates used in the original project assessments. Where necessary, this is supplemented by information from the project consultants responsible for the detailed project assessment.

2.2 Traffic Flow Data

Data Retrieval

The 'Before' traffic data used in this report is the information collected and used in the economic assessment during the detailed project design stage.

The amount of traffic data is dependent upon the area wide complexity of the project and ranges from outputs from a detailed traffic model to single one day link and junction surveys.

With the continued development of the Scottish Road Traffic Database (SRTDb), both the quality and quantity of observed traffic data have increased substantially in recent years. In 2007, there were approximately 1,650 automatic traffic counter sites across the trunk road network providing continuous traffic flow data. Of these, approximately 1,200 are capable of recording classified traffic count data and almost 1,300 are capable of recording speed data as well.

The level of accuracy and reliability of traffic flow information is improving as the extent of the trunk road network covered by SRTDb traffic counters increases.

Following the opening of each project, long-term Automatic Traffic Count (ATC) monitoring is undertaken on appropriate sections of the route.

Data Analysis

The traffic flow data analysis conducted for this report consists of factoring either the base year or the predicted opening year, design network flows to the actual opening year using growth factors obtained from the National Road Traffic Forecasts (NRTF) in use at the time of the detailed scheme assessment. For all the projects assessed in this report, NRTF(97) was applicable.

From the ATC information collected after project opening, the Annual Average Daily Traffic (AADT) flow is determined. This value is then compared with the low and high growth traffic predictions for the opening year.

The difference between the actual traffic flow and the low and high growth predictions is calculated and expressed as a percentage of the actual flow. A project with predicted flows in the range +/- 20% of the actual flows is accepted as being a reasonable range for future year forecast traffic flow comparisons, as established by the National Audit Office.

An assessment of the appropriateness of the road standard built is also undertaken. This compares the carriageway standard implied by the observed traffic flows with that actually constructed.

For all the projects assessed in this report, the carriageway assessment was carried out using DMRB, Volume 5, TA 46/97 'Economic Assessment and Recommended Flow Ranges for New Rural Road Links'. This technical memorandum uses observed opening year flows to determine the appropriateness of the carriageway standard constructed and was the memorandum in place at the time all the projects in this report were originally assessed.

2.3 Project Cost Data

It is clearly important to gauge the accuracy of project cost estimates used in detailed scheme assessment compared with the actual project out-turn costs. An under or over-estimate of project costs during the economic assessment will result in the economic benefits of the project being over or under predicted.

Data Retrieval

Information on the predicted project cost used in the pre-tender economic assessment is obtained and used as it is at this stage of the project preparation that the decision is taken on whether or not to proceed with the project and the output from the pre-tender economic assessment influences this decision.

Data Analysis

Retail Price Index (RPI) and discount corrections are performed to both the estimated and out-turn project costs in order to convert the figures to a common 'present value year' for prices and values. A comparison of predicted and actual costs is then made.

It should be noted that only out-turn costs incurred after the pre-tender economic assessment are included. 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. This is in accordance with HM Treasury Guidance, The Green Book - Appraisal and Evaluation in Central Government.

While the analysis of estimated and actual out-turn costs can provide guidance on the accuracy of the predictions, it should be noted that a number of issues will distort any comparisons and these include:

  • Timing of Expenditure - this will affect the conversion of costs to base year prices and values. If the actual project expenditure commences either sooner or later than predicted, the validity of the cost comparisons may be diminished.
  • Expenditure Profile - similarly, if the actual project expenditure profile varies significantly from the predicted profile then the price and value corrections will distort any comparisons. The actual expenditure on projects clearly takes place over a longer period than that expenditure profile estimates used in the cost benefit analysis, although the bulk of the costs should be concentrated over a few years as predicted.
  • On-Going Expenditure - expenditure on landscaping, land acquisition and claims can continue for a number of years after a project opens. Therefore, the actual costs presented to date may not be the final project costs.

2.4 Accident Data

Data Retrieval

"Before" accident data is defined as the 3 year accident statistics prior to the project opening.

"After" accident data is collected as part of the Stage 4 and 5 Road Safety Audits (RSAs) which are carried out 1 and 3 years respectively after project opening. These report on all accidents on the improved section of road and recommendations are made for ameliorative measures if appropriate. In the absence of RSA reports, the source "After" opening accident data is reviewed.

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 such circumstances, an additional accident study may be undertaken by the relevant local authority to supplement the Stage 4 and 5 RSAs for the project.

In the cases 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 before and after project opening.

Data Analysis

A comparison of injury accident numbers before and after project opening is undertaken. The number of injury accidents for the 3 years prior to a project opening is initially compared with the observed number of injury accidents for the project in its first year of operation. Two years later, when 3 years post opening accident data is available, a subsequent accident comparison with 3 years observed accident data is performed.

For projects that opened between April 05 and March 07, it has been possible to obtain accident data for the 3 years after opening for all of the reported projects. The 3 year after accident analysis is included in this report, where possible, together with a discussion of any of the issues raised in the Stage 5 RSA reports (if available).

A further examination of the causes of accidents may be undertaken if this broad accident comparison highlights accident types or numbers that require further investigation.

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. Often, these are treated in terms of Accident Investigation and Prevention work prior to planning the permanent solution. The 3 year before and after accidents therefore only demonstrate the minimum road safety improvement derived from the project.

2.5 Environmental Mitigation Measures

The project evaluation report includes a review of the environmental mitigation measures proposed within the Environmental Statements produced during the original project assessments to establish whether or not the measures proposed have been introduced and to provide comment on their success.

The Evaluation Report for Trunk Road Projects Opened between April 05 and March 07 includes a review of environmental mitigation measures for selected projects.