Highways Agency - Safe roads, Reliable journeys, Informed travellers

A419 COMMONHEAD JUNCTION IMPROVEMENT

7 TRAFFIC AND ECONOMIC ASSESSMENT

Introduction

7.1 At this stage in the scheme progress, the underpass and overpass options have been the subject of traffic modelling, forecasting and economic appraisal. This section of the report details the following aspects of the work:

• Traffic modelling
• Forecasting
• Effect of scheme options
• Economic performance

7.2 The throughabout option was found to be lacking in long term capacity and would operate satisfactorily only in the short term. This option therefore has not been considered in this section.

Traffic Modelling

7.3 The modelling for the A419 Commonhead junction improvement scheme has been undertaken following the completion of the A419 Blunsdon Study. A primary component of the assessment work for that project was the use of the 1999 Swindon Multi Modal Traffic Model (SMMTM).

7.4 Due to some small local deficiencies with the large area model (SMMTM) it was decided to develop a smaller cordoned model of the area around Commonhead junction (shown in Figure 7.1) and revalidate this much smaller area to current conditions.

Data Collection

7.5 In order to validate to present day conditions a comprehensive programme of traffic surveys was undertaken in May 2002 (Figure 7.2) which included the following:

• Automatic Traffic Counter (ATC) surveys of the A419 mainline
• Manual Classified Count (MCC) turning counts at A419 Commonhead Roundabout, and the A419 junctions with the M4 junction 15, A420 and A361 respectively;
• Self completion questionnaire on A361 Highworth Road
• Use of 1999 Roadside Interview (RSI) data for A420 Oxford Road

7.6 As with the original model three time periods were required for the smaller cordoned model:

• AM peak: 0800 - 0900
• PM peak: 1700 - 1800
• Typical interpeak: 1100 -1200

7.7 The four user classes from the SMMTM were also maintained:

• User class 1: Home Based Work trips - light vehicles
• User class 2: Other trips (exc. Employers Business) - light vehicles
• User class 3: Employers Business trips - light vehicles
• User class 4: Heavy Goods Vehicles

Traffic Assessment

7.8 A summary of the traffic assessment is described below.

7.9 The traffic assessment followed current guidelines as set out in the Design Manual for Roads and Bridges (DMRB) volumes 12 and 13, Guidance on the Methodology for Multi Modal Studies (GOMMMS) and the HA Bridging Document 'Applying the Multi Modal New Approach to Appraisal to Highway Schemes'.

7.10 The procedure following cordoning of the large model involved comparing the 1999 cordoned models with Traffic Appraisal in Urban Areas (TAUA) validation criteria, the addition of new Origin Destination data via the development, extraction and replacement of select link matrices with RSI site matrices; scaling of overall traffic levels from 1999 to 2002 and subsequently the use of matrix estimation within SATURN to refine the link flow match of modelled to observed and ensure the model replicated observed May 2002 traffic flows.

7.11 Each of the three time period models underwent this process followed by validation to test the comparison of assigned with observed flows using the widely accepted criteria in DMRB Vol 12 Traffic Appraisal in Urban Areas (TAUA) namely:

• Individual flows should be within 15% for flows in the range 700-2700vph;
• Individual flows should be within 100vph for volumes <700vph;
• Individual flows should be within 400vph for volumes >2700vph;
• The GEH statistic for individual flows should be less than 5 for >85% of cases.

7.12 Over the entire modelled area and across the 3 different time periods the models constructed perform well meeting the TAUA criteria (shown in Table 7.1) and thus provide a robust platform from which to conduct traffic forecasting and future option testing. The observed Annual Average Daily Traffic (AADT) traffic flows for the area around Commonhead junction are shown in Figure 7.3.

7.13 Following achievement of satisfactory validation, traffic forecasts for future years were produced. The forecasting of traffic levels requires the development of both forecast year networks and matrices.

7.14 The main network option for consideration involves the grade separation of the main A419 north to south movement. Two variants of this have been proposed, firstly to go under the existing road junction level and, secondly, to go over the existing road level. The connection from north to south of the A419 at the roundabout effectively negates the need for through traffic to circumnavigate the roundabout incurring delays at the signalised nodes, leading to the generation of substantial travel time benefits.

7.15 In addition the M4 onslips at junction 15 were improved to assume parallel lane entry so increasing their capacity. No other network changes were incorporated as part of the Do - Something scheme.

7.16 As a result of this assessment future year AADT traffic flows for 2005 and 2020 are shown in Figure 7.4 and Figure 7.5. These figures show traffic assignments for the do-minimum and the do-something following induced traffic assessments.

Effect of Scheme Options

7.17 The primary effect of the scheme option is to remove A419 through traffic from circulating links on the roundabout. This transfer of flow represents a reduction of between 70 and 80% on traffic entering the roundabout from the A419.

7.18 Total AADT traffic flows in 2005 (Figure 7.4 ) show little difference between the do-minimum and do-something.

7.19 In 2020 (Figure 7.5) traffic flows on the A419 north of Commonhead exceed approximately 70,700 AADT. South of the junction traffic flows are higher reaching approximately 78,400 AADT.

7.20 Do-minimum flows in the 2020 (high growth) southbound are lower than in 2005 indicating an increasing level of suppression south of the junction by 2020.

7.21 In both growth scenarios in 2020 the AADT on the grade separated link reaches approximately 58,000 AADT.

Economic Appraisal

7.22 The objective of an economic evaluation is to assess the value for money of scheme proposals. Value for money is determined by comparing those elements of scheme benefits that can be expressed in monetary values against the total cost of having the scheme in place. The following paragraphs summarise the work conducted.

7.23 The benefits of a scheme that are included in an economic assessment as specified in DMRB Volume 13 are:

• Time savings to travellers,
• Accident cost savings,
• Vehicle operating cost savings.

7.24 The economic appraisal for Commonhead junction improvement has been undertaken using the Department for Transport's packages of TUBA for time and vehicle operating cost savings and COBA11 to evaluate accident savings.

7.25 As a result separate models had to be constructed for both and evaluations undertaken for both the constrained and unconstrained assignments.

7.26 To undertake the COBA evaluation, traffic flow input was obtained from the SATURN model for the AM, IP and PM peak periods and factored to obtain 12 hour flows. The COBA network itself covered the same area as the cordoned SATURN models to ensure a compatibility of flow and geographical coverage when including the TUBA results.

7.27 The COBA model was developed as a 12 hour model and as such the detailed operational issues related to the peak period are only reflected through the use of COBA default flow group proportions and expansion factors.

7.28 The costs of the scheme for the purposes of the economic assessment are the overall costs of the scheme that are included in the total construction costs (including preparation and supervision of the scheme), compensation claims and land acquisition. The cost of the over option is estimated at £10.5 million, and under option at £16.5 million with no land costs being incurred. These costs are based on Quarter 1 of 2001. The period for economic assessment is 30 years from the scheme opening year.

7.29 The resultant economic summary of the over option is contained in Table 7.2. This is the final result of the evaluation and thus takes account of the impact of the induced traffic appraisal using variable trip matrix methodology.

NB. Costs are in thousands, evaluation period 30 years All values in 1998 prices discounted to 1998 at a 6% discount rate.

7.30 The economic assessment of the under option also derived substantial benefits but these are lower than those for the over option due to the higher cost of construction. The summary results for that evaluation is shown in Table 7.3.

NB. Costs are in thousands, evaluation period 30 years All values in 1998 prices discounted to 1998 at a 6% discount rate.

7.31 In addition to the value of time and vehicle operating costs, benefits are also augmented via accident benefits derived from COBA11. These will be primarily as a result of the removal of through traffic from the signalised roundabout as no reassignment/re-routing of traffic has been assessed due to the nature of the network. These are shown in Table 7.4.

7.32 Thus approximately £1 million in accident benefits are also accrued for both schemes.