1. Introduction and objectives

1. INTRODUCTION AND OBJECTIVES

1.1 The 1998 Roads Review announced a Blunsdon Road Based Study to investigate the problems of the A419 around Swindon. As a result two schemes were proposed to be added to the Targeted Programme of Improvements (TPI) programme, now known as the Programme of Major schemes:

Blunsdon Bypass was added to the roads programme in April 2002 and Commonhead junction improvement added recently in December 2002.

1.2 Further work has been carried out for Commonhead junction to decide if the new junction should be an underpass, overpass or a throughabout (a new signal controlled road built through the centre of the existing roundabout). This has been completed and has resulted in the scheme recently being added to the TPI programme, now known as the Programme of Major schemes.

1.3 This Scheme Assessment Report summarises the existing information for the site at Commonhead junction on the A419 near Swindon. It assesses the information including environmental, geotechnical and traffic, in the light of the three alternatives. The aim of the improvement would be to reduce congestion, improve safety and be constructed within the existing highway boundary.


1. Introduction and objectives

2. Existing conditions

2. EXISTING CONDITIONS

2.1 Commonhead junction is located on the A419 to the south east of Swindon town centre and is located approximately 1 km north of Junction 15 of the M4 (see Figure 1.1). It is an important junction for access into and out of Swindon. In its present form, Commonhead junction consists of a signal controlled at-grade roundabout. The current layout allows for future grade separation.

2.2 The road gradients at Commonhead junction, its approach roads and the A419 to the north of the roundabout are relatively level. Within the central reserve is existing mounding of varying height (up to a maximum of 3m), with mature planting on the crown and slopes. In the north west quadrant there is a mound (up to approximately 6.5m high) with mature planting to the road facing slopes, behind which is the housing estate at Liden. The new Great Western hospital is in the south west quadrant. The landscape to the east of the A419 is predominantly farmland, with the villages of Wanborough and Liddington around 1.5km distant. Commonhead junction lies on the boundary of the North Wessex Downs Area of Outstanding Natural Beauty (AONB).

2.3 The land generally slopes in a south-west to north-east direction. As such, the western side slopes of the northbound carriageway of the A419 are generally in medium to high cutting (up to approximately 5m in depth) and the eastern side slopes of the southbound carriageway of the A419 are generally on low embankment (approximately 1 to 2m in height).

2.4 The capacity of the existing roundabout is inadequate during periods of peak traffic flow, and even with traffic signals the junction still suffers congestion. There are regularly extensive queues on the A419 during peak periods. Traffic flows range from 55,000 to 66,500 vehicles per day of which 16% are heavy goods vehicles (HGVs) and journey delays can be up to 25 minutes. There have been 38 personal injury accidents (PIAs) recorded over the past 5 years.

2.5 The area remained largely unchanged between the late 1800s and 1977 with most of the land comprising pasture land, farm dwellings, country roads and woodland. However, rapid urbanisation to the north west of the Commonhead junction scheme has since ensued, as Swindon expanded.

2.6 The published geology for the site has been confirmed by ground investigation as Gault Clay overlying Lower Greensand. The Gault Clay lies between 4 and 6m thick across the majority of the site. The Lower Greensand comprises strongly cemented coarse grained sandstone material. The raised mound of material in the central reserve areas comprises re-worked Gault Clay, which, in parts, contains gravel and brick fragments.

2.7 Groundwater was struck in some of the boreholes drilled during a recent ground investigation at depths equivalent to 6.4m below existing ground level (begl) and 7.4m begl. Groundwater monitoring undertaken during and subsequent to the July 2002 investigation indicates that the groundwater levels have tended to rise since the ground investigation was completed. Groundwater levels would be expected to rise even further during the wetter winter months.

2.8 There are two significant surface water features within 2km of the site, namely Coate Water located 1.5km to the west of Commonhead junction and an attenuation pond 750m to the north of the junction. There are two watercourses (both unnamed on Ordnance Survey Maps) (OS) within the overall study area. Both these watercourses, which originate as springs from the high ground to the south east of the study area, flow in a generally northerly or north westerly direction, and ultimately flow into the attenuation pond. 2.9 The flood depth information report indicates that the site is not situated within the 0- 1m estimated 100-year flood depth. However, the northernmost part of the site is situated within the floodplain of the watercourses supplying the River Cole.

2.10 The Gault Clay is classified as a non aquifer of negligible permeability but the Lower Greensand and the alluvium to the north west are classified as minor aquifers.

2.11 The site is not situated within a Source Protection Zone for groundwater abstraction. Groundwater flow is considered to be towards the north i.e. towards the River Cole.


2. Existing conditions

3. Planning factors

3. PLANNING FACTORS

3.1 The traffic model developed for the Commonhead Improvement Study has taken account of planning and development considerations in respect of the following:

3.2 Although the Haydon III development is situated outside the traffic model study area to the north of Swindon, it represents a major residential development. Its effect, therefore, has been taken into account and necessary adjustments have been made for both development related vehicle trips and general vehicle trip growth.

3.3 Particular account of specific developments within the study area were dealt with individually, notably the new Great Western Hospital. Vehicle trip generation and distribution matrices were calculated for this development and its effect included in the junction assessment.

3.4 The final area of planning consents and potential future developments affecting the study area were dealt with via the use of TEMPRO estimates of forecast vehicle trip growth. TEMPRO contains trend based planning datasets, using published government population and household projections, and commercially available employment projections, all at county level. The underlying assumption is a continuation of past trends in birth rates, migration, household formation, and employment in each industrial sector.

3.5 In order to ensure that the projected household and employment datasets at county level fed into the software is reasonable and up to date, the Department for Transport consults Government Offices to produce further planning datasets which more closely reflect current planning policy.

Park and Ride

3.6 The first Deposit Draft of the Swindon Borough Local Plan 2011 contains a policy safeguarding a site for a Park and Ride facility adjacent to Commonhead junction, adjacent to the new Great Western Hospital.

3.7 None of the proposed schemes would prejudice its possible future development.


3. Planning factors

4. Description of scheme options

4. DESCRIPTION OF SCHEME OPTIONS

4.1 Three options to solve the problems at Commonhead junction have been considered as follows:

4.2 Option A (Overpass) would add a second level above the existing Commonhead junction for A419 through traffic. The removal of the through traffic would reducecongestion at the roundabout.

4.3 Option B (Underpass) would provide a similar reduction in congestion as Option A (Overpass) but would pass under the existing Commonhead junction.

4.4 Option C (throughabout) would provide a new traffic signal controlled layout at the same level as the existing Commonhead roundabout. This option however would not provide a long term solution to congestion and would provide congestion relief for around only 6 years.

Cost Estimates

4.5 The construction cost estimate for each option is as follows:

Option AOption BOption C
10.516.55.0

Note: These estimates are based on (£ million) 2001/Q1 values.


4. Description of scheme options

5. Engineering assessment

5. ENGINEERING ASSESSMENT

Engineering Standards

The preliminary design for this scheme has been completed in accordance with the following standards:

Option A (Overpass)

5.1 This option would entail re-aligning the existing A419 such that it crosses approximately 7.0m above the existing junction (see Figure 5.1). The proposed carriageway layout would be as the existing i.e. dual 2 lane carriageway. It would involve raising the existing embankments and the construction of two underbridges to take the A419 over the existing junction. There would also be the requirement to make provision for the slip road merge/diverge lanes.

5.2 This option together with earthworks of 1 in 2 side slopes would fit within the existing central reserve. The route could therefore be constructed with minimal effect on the existing A419 traffic, thereby reducing delays during the construction period.

5.3 It is proposed that the side road (Day House Lane, to the south east of Commonhead junction) egress from the A419 is closed for safety reasons because it would be located in the slip road merge area.

Option B (Underpass)

5.4 The horizontal alignment and carriageway layout of Option B (see Figure 5.2) would be the same as for Option A. However Option B would entail re-aligning the vertical alignment of the existing A419 such that it passes approximately 6.5m below the existing junction. This would involve the construction of the proposed alignment in cutting with two proposed overbridges above the re-aligned A419 to accommodate the roundabout. In addition there would be the requirement to make provision for the slip road merge/diverge lanes.

5.5 The presence of a high water table would result in structural design considerations, which would increase construction costs. A structure approximately 350m in length and anchored by tension piles would be required to ensure a satisfactory waterproof construction. An alternative method dealing with the high groundwater levels would be permanent de-watering by pumping, but the volume of water expected would not make this option feasible.

5.6 There is an added risk that future groundwater levels during the anticipated life of the structure may be even higher than those recorded to date, due to both seasonal and long term climatic effects. At the moment, water levels would be around a maximum of 2.0 to 2.5m above road level. As expected, if groundwater levels rise further by around 2m, then the length of waterproof structure required would increase from approximately 350m to around 500m. Construction would be further complicated by the need for temporary de-watering over a substantial length of the cutting.

5.7 The Environment Agency has commented on Option B to say that their Water Resources function would prefer to see a sealed cutting option as opposed to long term de-watering which would not be environmentally sustainable. In addition, they highlight the fact that soakaways would be inappropriate for the drainage in this area because of the presence of a high groundwater level and the likelihood of groundwater pollution. There would also be an increased risk of flooding in adjacent catchment areas as a result of the anticipated quantities of water that would require to be discharged from the cutting.

5.8 Ground conditions comprise predominantly Gault Clay, and preliminary design work suggests that the earthworks cutting slopes would need to be between 1 vertical in 3 horizontal and 1 vertical in 5 horizontal. With these slopes, the scheme would not fit within the existing central reserve.

5.9 Therefore, structural retaining walls would be required to contain the construction works within the existing central reserve and prevent encroachment into the existing A419 carriageways. Without retaining walls, the A419 would need re-aligning which would cause severe disruption to traffic during the construction of the works.

5.10 It is proposed that the side road (Day House Lane, to the south east of Commonhead junction) egress from the A419 is closed for safety reasons because it would be located in the slip road merge area.

Option C (Throughabout)

5.11 The Throughabout junction would involve re-construction of the approaches to the existing roundabout and the introduction of a new road link for the A419 through the centre island of the roundabout with associated traffic signals (see Figure 5.3). The proposed alternative would consist of a standard throughabout, with three approach lanes, together with the associated filter lanes.

Drainage

5.12 Whilst the outline drainage proposals have not yet been finalised for all three options, drainage direct to ground is not likely to be an option due to the predominance of clay just below road level. It is more likely that road drainage would pass into the nearest stream via use of balancing ponds.

Public Utilities

5.13 Information from service providers relating to significant plant that may be located on site are detailed in Table 5.1.

5.14 Swindon Borough Council's Environmental Health Office was contacted particularly with regard to pollution incident records and the presence of any overground or underground fuel storage facilities. Confirmation was subsequently received that no records of any pollution incidents or underground storage tanks are held by Swindon Borough Council.

Service ProviderDetails of Significant Plant and Location
British TelecomThe Swindon Exchange is shown crossing Commonhead junction in a north west - south east direction. A spur is also shown running in a northerly direction along the centreline of the central reserve to the north of the roundabout.

BT plant is also shown as follows:-
  • Crossing the A419 to the west of Great Moor Leaze Farm.
  • Running south along the western verge of the A419.
  • Northbound carriageway before crossing the A419 between Commonhead junction and the M4 motorway.
NTL, Comtel & Cable and WirelessNTL have indicated that they have plant crossing the A419 immediately to the north of Commonhead junction between two manholes. NTL also have plant in the verge of the northbound carriageway.

Comtel and Cable & Wireless also have plant crossing the A419 just to the north of the NTL plant.
Water (Clean and Foul)There are surface water sewers crossing the A419 at the south and north end of the lake which is situated to the north west of the roundabout.

There is a trunk foul sewer in the verge of the northbound carriageway of the A419.

There are water pipes in the southbound carriageway verge to the south of Commonhead junction.
ElectricityThere does not appear to be any high or low voltage cables located within, or crossing, the site area.

There are however cables supplying streetlights in all verges in the immediate vicinity of Commonhead junction.
TranscoTransco have confirmed that they do not have any plant that is affected by the proposed works.
Fisher GermanFisher German have confirmed that there are no Esso/Mainline pipelines in the area of the proposed works.

The following services would be potentially affected by the proposed schemes:

Assessment of Anticipated Departure from Standard

5.15 For Option A (Overpass), it has been determined that for the vertical curvature to comply totally with Departmental standards and to be able to maintain the scheme within existing land take, there would be a requirement for the provision of either retaining walls or the realignment of the existing A419 carriageways. To avoid this, a Departure from Standard has been applied for and approved.

5.16 The application of the Departure would not compromise driver safety because full visibility would only be unavailable over a short section. In order to mitigate this loss of visibility, warning signs advising motorists that there would be traffic merging onto the carriageway ahead would be proposed on the approaches to the slip road merging areas.

Summary

5.17 On engineering grounds, the impact of groundwater for construction of an underpass would be severe. It is not certain that waterproofing a structure in the underpass would be successful and groundwater pumping measures, as well as pumping to remove rainfall, would also be required. Pumping groundwater alone is not a sustainable operation.

5.18 An overpass solution does not have these high risks and can be constructed within the highway land currently available. The cost differential between the two is about £6M. At this stage the underpass solution would not seem a viable option.


5. Engineering assessment

6. Environmental Assessment

6. ENVIRONMENTAL ASSESSMENT

6.1 AIR QUALITY

Baseline Conditions

6.1.1 Information on existing air quality in the study area has been obtained from the following sources:

6.1.2 Consultation with Swindon Borough Council (SBC) has identified that no monitoring data exists within the study area. SBC have also confirmed that the study area does not lie within an Air Quality Management Area (AQMA). This means that properties adjacent to Commonhead junction do not experience pollution levels that exceed the Standards and Objectives set out in the Air Quality Standards (AQS) at the present time and are not expected to in the near future.

Environmental Effects

6.1.3 Detailed air quality predictions have been carried out for the baseline situation (2002 and 2005), Option A (over) 2005 and Option B (under) 2005. Air quality modelling has not been carried out for Option C (throughabout) as this would show no differences to the baseline situation.

6.1.4 Results from the assessment are displayed in Table 6.1

Table 6.1: Air quality scoring predictions
Option NO2 Score PM10 Score
Option A - Over -121.2 -13.4
Option B - Under -150.1 -15.1

6.1.5 The results in the above table show that both the 'over' and 'under' options result in improvements of NO2 and PM10 concentrations when compared with 'do minimum' . The 'under' option shows the most significant improvement in air quality concentrations, although differences between the two options are very small.

6.1.6 As previously mentioned, the air quality impacts of Option C (throughabout) would be neutral and therefore this option has not been examined in detail.

6.1.7 Air quality predictions show that objectives for NO2 and PM10 are currently not exceeded at residential receptors in the area. Improvements in air quality associated with the proposals would ensure that it would be unlikely that the air quality objectives would be exceeded.

6.1.8 The recently released tighter PM10 objectives are not currently included in the Air Quality Regulations for the purpose of Local Air Quality Management. Comparison with these 2010 objectives has therefore not been carried out. Mitigation of Impacts arising once the site is Operational (Road Traffic)

6.1.9 As the air quality impacts associated with both the 'over' and 'under' options are beneficial, it is not necessary to provide mitigation measures for operational impacts.

Construction Related Impacts

6.1.10 During the construction of either option, emissions to air would be generated, mainly consisting of dust and PM10. These would emanate from a variety of different sources including site excavations for foundations and groundwork, emissions from construction vehicles on site and dust from exposed soil. Such emissions present the potential to create nuisance in neighbouring residential areas.

6.1.11 The majority of dust (up to 80% of the total release) would be likely to be deposited within 200m of the source, with the remaining portion typically being deposited within400m of the source. Therefore, it may be expected that a discernible increase in deposited dust may be noted up to 400m from the source, but the greatest potential for nuisance would be within 200m of the source. However, through the application of mitigation and site management measures, the potential for 'nuisance' from this source would be minimised.

Mitigation of Construction Related Impacts

6.1.12 A mitigation strategy would be implemented and construction operations would be monitored to ensure that dust nuisance is minimised.

6.1.13 Liaison with SBC Environmental Health Department would be undertaken before and during construction to ensure that any air quality related issues that occur can be dealt with effectively and promptly.

Statement of Significance

6.1.14 Although differences between the 'over' and 'under' options are very small, the 'under' option would be the favoured option with respect to air quality.

6.1.15 Option C 'throughabout' would be the least favoured option in terms of air quality, as pollutant concentrations would remain unchanged, although these are unlikely to exceed air quality objectives.

6.2 CULTURAL HERITAGE

Baseline Conditions
Archaeology

6.2.1 A total of six sites and findspots of archaeological significance have been identified within an approximate 200m radius around Commonhead junction. A further eight archaeological sites and findspots have also been identified in the wider vicinity. These comprise four prehistoric sites and findspots, six Roman sites and findspots, one medieval site, two post-medieval sites and undated human burial. There are no listed buildings, conservation areas of historic landscape features within the existing highway boundary.

Environmental Effects

6.2.2 The impact on any archaeological resource caused by the redevelopment of the junction is similar for all three options, because they all impact within the existing highway boundary. It is considered that any archaeological resource within the circuit of the roundabout and the surrounding roads would have been destroyed by the construction of the A419. This view has been agreed with the Wiltshire County Archaeologist.

6.2.3 The construction of any ancillary structures such as a works compound outside of the junction could have a negative impact on any archaeological resource.

Mitigation

6.2.4 Any ancillary activity, particularly a temporary works compound, should be kept well away from the earthwork enclosure and all fields which contain ridge and furrow earthworks. If outside the existing highway boundary, the location of any compound and the extent of any archaeological mitigation measures should be agreed in advance with the Wiltshire County Archaeologist.

Statement of Significance

6.2.5 The impact on any archaeological resource caused by the redevelopment of the junction is similar for all three options. This is because the impact would be completely within the existing highway boundary. It is considered that any archaeological resource within the circuit of the roundabout and the surrounding roads would have been destroyed by the construction of the A419. This view has been agreed with the Wiltshire County Archaeologist.

6.3 ECOLOGY AND NATURE CONSERVATION

Baseline Conditions and Methodology

6.3.1 The study comprised two phases, a desk study and a general walkover the site.

6.3.2 The purpose of the desk study was to collect baseline data held by statutory and nonstatutory consultees and obtain views and concerns with regard to the proposals. Information was requested for the route corridor and a 2km study area around the corridor. The following organisations were consulted:

6.3.3 The walkover field survey was carried out on 16 July 2002. This survey mainly focused on the proposed junction improvement works along the A419. Phase 2 protected species surveys were also conducted for badgers, water voles and bats.

Environmental Constraints

6.3.4 There is a suitable habitat along the route for several protected species. Along the route corridor itself, the grassed road verges, embankments and hedgerows provide ideal habitat for reptiles and badgers.

6.3.5 The surrounding area of the existing A419 is characterised by pasture and arable fields, hedgerows and woodland areas. These habitats provide ideal areas for breeding birds and badgers.

6.3.6 The A419 route itself is not designated but eleven locally important and one nationally important conservation sites are located within the route corridor (Table 6.2 and Figure 1.1).

Table 6.2: Designated sites of nature conservation value within route corridor
Site Name Designation Notification
Coates Water Site of Special Scientific Interest (SSSI) This site has a diverse breeding bird population associated with two lakes and adjacent semi-natural vegetation. Breeding summer migrant birds and wintering waterfowl visit the site. The site also has an outstanding assemblage of dragonflies and damselflies.
Long Copse and Crook's Copse Site of Nature Conservation Interest (SNCI) This site is designated for its local importance for nature conservation.
Oaken Ground Copse SNCI This site is designated for its local importance for nature conservation.
Pinkcombe Wood SNCI This site is designated for its local importance for nature conservation.
Burderop Wood North SNCI This site is designated for its local importance for nature conservation.
River Ray and Burderop Plantations SNCI This site is designated for its local importance for nature conservation.
Day House Copse SNCI This site is designated for its local importance for nature conservation.
Lidcombe Down SNCI This site is designated for its local importance for nature conservation.
Medbourne Chalk SNCI This site is designated for its local importance for nature conservation.
Wanborough Meadows SNCI This site is designated for its local importance for nature conservation.
River Cole SNCI This site is designated for its local importance for nature conservation.
Lidcombe Down South SNCI This site is designated for its local importance for nature conservation.
Black Horse Pub Meadow SNCI This site is designated for its local importance for nature conservation.

6.3.7 The proposed junction improvements would not impinge on any statutory / nonstatutory designations. Table 6.2 summarises the responses received from the statutory and non-statutory consultees.

6.3.8 There are numerous mature trees on-site that are suitable for bat roosts. The study area also has numerous hedgerows, a large lake and small stream which would all provide good commuting and foraging habitat for bats. It should be noted that all species of bat and their roosts are protected under the Wildlife and Countryside Act 1981 and The Conservation (Natural Habitats &c.) Regulations 1994. Several mature trees along the route could potentially provide suitable roosting areas for bats.

6.3.9 During a night-time foraging survey undertaken on 16 July 2002, common pipistrelle (Pipistrellus pipistrellus) were recorded foraging along the hedgerows adjacent to the small stream and along the A419 dual carriageway. The pipistrelles were also recorded foraging around the lake and along the associated woodland edge. Daubenton's (Myotis daubentonii) were recorded foraging across the lake.

6.3.10 The grassed road verges along the route provide ideal basking areas for common lizards, slow worms and grass snakes. Slow-worm (Anguis frafilis), common or viviparous lizard (Lacerta vivipara), adder (Vipera berus) and grass snake (Natrix natrix) are protected by the Wildlife Countryside Act 1981 by part of Section 9 (1) and all of Section 9 (5). This means that they are protected against intentional killing and injuring (but not 'taking') and against sales and transporting for sale. Suitable habitat is restricted to the ditches, road embankments and hedgerows. No evidence of reptiles was recorded during the walkover survey.

6.3.11 Badgers (Meles meles) and their setts are protected by the Protection of Badgers Act (1992), under which it is an offence to harm either badgers or their setts. The hedgerows, woodland areas and grassed roadside banks provide ideal areas for badger setts and foraging grounds. Two dung pits and an old latrine were recorded during the walkover survey within an arable field close to the A419 dual carriageway. A well-used path was also recorded with signs of dogs and some signs of use by badgers within the area and to the south-east of the carriageway.

6.3.12 A small group of holes was recorded on Commonhead roundabout itself. These holes were used by badgers but have clearly been disused for several years. Rabbits now occupy these holes. A single hole was also recorded on the roundabout. This hole was blocked by debris and no evidence of badgers was recorded.

6.3.13 Water voles (Arvicola terrestris) have received legal protection by its inclusion on Schedule 5 of the Wildlife and Countryside Act 1981. This protects their place of shelter rather than the voles themselves, recognising that habitat loss and fragmentation has been a major contributory factor in the decline of the species. Suitable habitat exists on-site to support this species, i.e. wet ditches and small stream. Signs of water voles were searched for on the banks of watercourses and drainage ditches on-site. No evidence of water voles was found during the survey.

6.3.14 Numerous birds and old nests were recorded during the site walkover within the hedgerows and on the lake. The time of year at which the survey was carried out means that summer migrants could be recorded. The birds recorded included:

6.3.15 It should be noted that all species of wild bird and their nests are protected under the Wildlife and Countryside Act (1981).

6.3.16 The pond on-site is very shaded with no bank side and submerged vegetation. This waterbody is not considered suitable as a breeding pond for amphibians.

Environmental Effects

6.3.17 The drainage ditches could be affected along all three improvement options but the construction of culverts would maintain a link and, thereby, reduce any adverse effect. The existing road verges would be affected, which would reduce their value as habitat link features.

6.3.18 It is thought that road lighting can adversely affect invertebrates and disorientate birds and bats. However, the proposed lighting would only be around the new roundabout improvements. As the existing roundabout is lit, the adverse effects of lighting would be of a neutral significance.

Option A - Overpass

6.3.19 This option would result in the loss of grassed verges and landscape planting along the dual carriageway and within the roundabout.

Option B - Underpass
6.3.20 This option would result in the loss of grassed verges and landscape planting along the dual carriageway and within the roundabout.
Option C - Throughabout

6.3.21 This option would sever the planting in the centre of the roundabout. Again this option would result in the loss of grassed verges and landscape planting along the dual carriageway.

6.3.22 All options would sever wildlife corridors such as hedgerows, grassed verges and woodland edges.

Mitigation

6.3.23 This section sets out the measures that would be considered as part of the proposed works to prevent, reduce or, where possible, offset potential environmental impacts, which may occur during the construction and operation of the proposed scheme.

6.3.24 Given the previous records of badger road mortality in the area, it may be appropriate to consider installing badger tunnels (according to the design and specifications presented in DMRB Volume 10) where the well used runs were recorded. These tunnels should be supported by short lengths of badger-resistant highway fencing (approximately 25m from the tunnel headwalls in either direction) to 'funnel' badgers towards the entrances and baited at intervals following construction to encourage their future use by badgers.

6.3.25 Another badger survey should be conducted before construction takes place to ensure that badgers have not re-occupied the holes found on-site or constructed new ones within the vicinity.

6.3.26 All species of wild bird and their nests are protected under the Wildlife and Countryside Act (1981). For this reason, site clearance operations should be conducted outside of the breeding season which, in general, runs from March to July.

6.3.27 If any mature tree is to be felled, a survey by a licensed bat handler should be carried out. This would determine whether or not the tree is being used as a bat roost. If evidence of bats is found, English Nature should be consulted prior to any felling operations.

6.3.28 The landscape design should include replacement planting of any of the existing vegetation which would be lost, because bats are foraging and commuting along these features.

6.3.29 Planting should include locally native species. Fruit trees and shrubs would provide an additional food source and shelter for birds and small mammals.

6.3.30 Contractors compounds should be located in areas of low ecological value and well away from the more sensitive areas, i.e. the small stream and associated hedgerows.

Statement of Significance

6.3.31 The main direct impact from all three options would be the loss of grassed verges and landscape planting along the dual carriageways. Option C (throughabout) would allow the retention of severed existing vegetation on Commonhead roundabout.

6.3.32 Phase 2 protected species surveys have been carried out for bats, badgers and water voles. These surveys have highlighted that badgers and bats are using the study area for foraging and commuting.

6.3.33 The overall ecological effects of the proposals would be of neutral significance. Providing that the mitigation measures are implemented, all three proposed options areconsidered viable.

6.4 LANDSCAPE AND VISUAL CHARACTER

Baseline Conditions
Topography

6.4.1 Gently undulating lowland, typical of the Upper Thames Clay Vale, forms a broad floodplain landscape, rising gently to the south and west from 90 to 120m AOD. Several watercourses run through the landscape. To the south and east, this landscape is enclosed by the northern part of the North Wessex Downs, known as Lambourn Downs, which rise steeply to approximately 270m AOD. The northern slope of Lambourn Downs is indented with minor valleys and protruding hill formations. The minor valleys contain springs and watercourses.

6.4.2 Commonhead junction is situated at approximately 110m AOD within the lowland landscape. Both the roundabout and the A419 approach central reserves are mounded and planted. A noise bund of varying height up to approximately 6.5m is located adjacent to its north-west quadrant.

Settlement and Infrastructure

6.4.3 Commonhead roundabout forms a junction on the A419 approximately 1km north of the M4, linking with the A4259 to Swindon town centre to the west and two minor roads to the villages of Wanborough and Liddington to the east. Other minor roads link settlements in the lowlands.

6.4.4 Swindon, historically a railway town, has expanded with retail, light industrial and residential development, with the A419 generally forming its western boundary in this area. Liden, an area of residential development approximately 30 years old, lies adjacent to the noise bund in the north-west quadrant of the roundabout. To the north of the noise bund there is a large man-made balancing pond. In the south-west quadrant, the prominent new Great Western hospital building is located on rising ground above the vegetated road cutting. There are proposals to locate a Park and Ride facility between the hospital and the A419.

6.4.5 The villages of Wanborough and Liddington lie on the rising scarp slope of Lambourn Downs to the south and east. Both are attractive villages with good facilities.

6.4.6 Badbury lies on the scarp slope south of the M4, 2km from the site at 150-160m AOD and several farmsteads and hamlets populate the lowlands.

Landuse and Vegetation

6.4.7 The gently undulating lowland is mainly used as pastureland. Medium to large-sized fields are bounded with hedges and hedgerows, with mature trees. Where the field boundaries follow watercourses, the vegetation is particularly strong.

6.4.8 The scarp slope is also mainly used as pastureland. There is significantly more vegetation than on the lowland, with copses and woodland located particularly in the valleys and adjacent to the settlements.

6.4.9 The noise bund between Commonhead junction and Liden is well vegetated with mature trees and shrubs forming a dense deciduous woodland. Commonhead junction supports mature deciduous vegetation, mostly in good condition but with a small stand of dead trees on the northern side. The central reserves of the A419 on the approaches to the roundabout also support healthy stands of mature deciduous vegetation. There are also mature deciduous trees in the central reserve of the A4259 approaching the roundabout. The cutting adjacent to the new Great Western hospital site is vegetated. The eastern boundaries of the A419 are less well vegetated, with scattered trees to the north of the roundabout and a length of hedgerow adjacent to the layby to the south of the roundabout.

Footpaths and public access

6.4.10 The principal footpath route in this area is The Ridgeway. This is one of thirteen long distance routes in England, which have been designated as National Trails. It runs across Lambourn Downs from south-west to north-east, following the high land of the northern ridge and at its nearest is some 3km from the site. There are a number of other public rights of way within the study area, but no bridleways or other rights of way. No public rights of way are directly affected by the proposals.

Landscape Designations

6.4.11 The A419 lies at the edge of the North Wessex Downs Area of Outstanding Natural Beauty (AONB), with the side road to Upper Wanborough as its northern limit. AONBs are national designations, which recognise and protect attractive landscapes of national significance.

6.4.12 The landscape to the south and east of Swindon, as far north as Wanborough Marsh, is designated as an Area of Local Landscape Importance (ALLI). ALLIs are local designations, which recognise and protect attractive landscapes of local significance.

6.4.13 This area is also a designated Rural Buffer. The purpose of rural buffers is to maintain the separate identities of towns and villages, and to protect the open countryside.

Landscape Character and Quality

6.4.14 The landscape within which Commonhead junction is located is a broad scale rural landscape of undulating lowland pasture, contrasting with the urban mass of Swindon to the west. The south-eastern horizon is formed by the rising wooded ridge of Lambourn Downs, the most northerly area of the North Wessex Downs.

6.4.15 The landscape divides into the following Landscape Character areas:

Clay vale lowland:
Gently undulating lowland landscape typical of the Upper Thames Vale. Medium to large pasture fields with mature hedgerows, with several small watercourses. Farmsteads and hamlets largely comprise period buildings. A traditional and harmonious landscape. Good quality;
Scarp slope:
Steeper slopes rising to Lambourn Downs, indented with water-cut valleys containing streams and significant copses giving a more wooded appearance. Pasture fields of medium size combine with old villages to form a well balanced landscape. Good to high quality;
Urban residential:
Housing estate at Liden circa 1970 of cul-de-sacs and open front gardens, screened from the A roads by a well vegetated noise bund. Below average quality;
Hospital site:
Widely visible site with very large scale cream coloured main hospital building dominant in views from other areas. Poor to below average quality;
Noise bund:
Well vegetated noise bund of approximately 6.5m maximum height, with grassed lower slopes adjacent to housing; well used as informal Public Open Space with informal paths and virtually no littering. Good quality;
Rural villages:
Wanborough and Liddington, on the scarp slope, with period buildings, churches, pubs and other village facilities, with cultural implications of a strong rural community. Lower Wanborough, which includes more recent development, does not have views towards the site due to a shoulder of the ridge intervening. Good to high quality;
Ribbon development:
Along Ham Road, larger properties set within large plots form an individual character, with wide, well maintained grass verges and broad views west towards the A419 Swindon and the hospital site. Good quality;
Environmental Effects and Mitigation

6.4.16 Options A (overpass) and B (underpass) would require the removal of the mounding and vegetation within the central reserves on the A419 approaches and within the central roundabout. Option C (throughabout) would require the removal of much of the mounding and vegetation in the central roundabout. There is an area of vegetation, which may be retained in the middle of the roundabout in Option C. However, there are some dead trees amongst it, and the newly exposed edges may suffer windblow. Some replanting would be possible. It is likely that the majority of the mounding in the central reserves of the approaches would be retained. In this respect, Option C would have less immediate visual impact than Options A (overpass) and B (underpass). The retained vegetation would continue to screen the northbound carriageway in views from the east.

6.4.17 Option C has been largely discounted due to traffic problems and has not been assessed in detail. Options A (overpass) and B (underpass) have been assessed in terms of landscape and visual impact and Table 6.3 summarises this assessment, together with a summary of these impacts 15 years after opening with mitigation as described below.

Mitigation
Option A (Overpass): Opening year

6.4.18 Landscape mitigation is limited to within the existing highway estate (see Figure 6.1), apart from potential offsite planting schemes. Planting mixes would comprise native species indigenous to the area.

6.4.19 The existing boundaries of the A419 on the eastern side would be planted with hedgerows and trees, where no planting exists at present. The new embankments on the approaches would be planted with a dense shrub and intermittent tree mix of native species. The embankments within the roundabout would be gently mounded to give gentler slopes, using all the land in the central roundabout area and planted with a dense tree and shrub mix.

6.4.20 The proposed 4m noise fence on the bund at Liden may require the removal of some of the existing vegetation, for construction and maintenance. Both the removal of existing vegetation and the introduction of the noise fence to the view would have an adverse impact on views from properties at Liden. The noise fence would screen winter views of moving traffic. Additional planting between the noise fence and the properties at Liden, on the slopes of the noise bund, would assist in mitigating this impact.

6.4.21 Offsite planting could be offered to properties, which may suffer moderate to severe impacts. However, the above impacts are not severe; and offsite planting could be disadvantageous because it would also reduce the scale of view and obscure elements, which add landscape quality to the view.

Option A (Overpass): Environmental Effects after 15 years
Option B (Underpass): Opening year

6.4.22 Landscape mitigation is limited to within the existing highway estate (see Figure 6.2), apart from potential offsite planting schemes. Planting mixes would comprise native species indigenous to the area.

6.4.23 The existing boundaries of the A419 on the eastern side would be planted with hedgerows and trees, where no planting exists at present. The new cuttings on the approaches and within the roundabout would be planted with a dense shrub and intermittent tree mix of native species.

6.4.24 The remaining land in the central roundabout area would be mounded on either side of the cutting. The mounding would have 1:3 maximum road facing slopes, with 1:4 maximum outer slopes, to a maximum height of 3m above the roundabout carriageway level, well rounded across the top to give a smooth and gentle landform. These mounds would be planted with a dense tree and shrub mix.

Option B (Underpass): Environmental Effects after 15 years
Statement of significance

6.4.25 A summary of the key potential effects for the underpass and overpass proposed route options is provided in Table 6.3

6.4.26 This summary shows that Option A (overpass) would have slightly greater adverse visual impact than Option B (underpass). Neither option has been assessed as having a substantial adverse visual impact, despite abutting the AONB. The reasons for the low ratings can be summarised as follows:

Table 6.3: Summary of the Stage 2 visual assessment
RECEPTOR OPTION A (OVER) Visual Impact OPTION B (UNDER) Visual Impact
OPENING YEAR General: (for issues appropriate to each option)
  1. Liden
  2. Elevated Properties (AONB)
  3. Lowland Properties (AONB)
  4. Lowland Properties (not within AONB)
  5. Great Moor Leaze Farm
  6. The Meadow House
  • Moderate Adverse
  • Slight Adverse
  • Moderate Adverse
  • Slight Adverse
  • Moderate Adverse
  • Slight Adverse
  • Slight Adverse
  • No Change
  • Slight Adverse
  • Moderate Adverse
  • Slight Adverse
  • Slight Adverse
  • Slight Beneficial
  • Slight Adverse
  • No Change
  • No Change
  • No Change
  • No Change
  • No Change
  • No Change
YEAR 15 General: (for issues appropriate to each option)
  1. Liden
  2. Elevated Properties (AONB)
  3. Lowland Properties (AONB)
  4. Lowland Properties (not within AONB)
  5. Great Moor Leaze Farm
  6. The Meadow House
  • Slight Adverse
  • No Change
  • Slight Adverse
  • No Change
  • Slight Adverse
  • Slight Adverse
  • Slight Adverse
  • No Change
  • No Change
  • Slight Adverse
  • No Change
  • No Change
  • Slight Beneficial
  • Slight Beneficial
  • No Change
  • No Change
  • No Change
  • No Change
  • No Change
  • No Change

6.4.27 In landscape and visual terms, Option B is the more favourable. At opening, this assessment has found no visual impacts above Slight Adverse and, after 15 years, the impacts are assessed as Slight Beneficial to No Change. Whilst Option A is less favourable at opening, having a Moderate Adverse visual impact, these impacts are reduced to Slight Adverse after 15 years.

6.5 LAND USE AND COMMUNITY EFFECTS

Baseline Environmental Conditions

6.5.1 Land use was surveyed during June 2002 and information on existing land use in the study area has been obtained from the following sources:

6.5.2 All of the land directly affected by the scheme is 'non agricultural' and further detailed assessment of the quality of agricultural land is not necessary.

6.5.3 It is proposed that an access road off the southbound highway (to the south east of Commonhead junction) would be stopped off. This could have an effect on access to local farm businesses including Medbourne Farm.

6.5.4 Land on the roundabout itself is vegetated by mainly scrub/woodland with mown, amenity grassland in an approximately 5m zone around the perimeter of the roundabout.

6.5.5 There are a number of Public Rights of Way (PRoW), in the form of footpaths, within the 1km study area around Commonhead junction. There are no bridleways, or other forms of PRoW within the study area. There are no formal PRoW that are directly affected or severed by the proposals. However, there is evidence of worn paths or 'desire lines' in the grass leading up to, and crossing over the central reservation, immediately to the north of the roundabout. Therefore, the public use this part of the central reservation to cross over both carriageways (northbound and southbound) of the A419.

6.5.6 No other public or community land (e.g. Common Land or designated Public Open Spaces) would be affected by the proposals.

Environmental Effects

6.5.7 The land affected by all three options is 'non agricultural'. Therefore, there would be no adverse impact on agricultural land quality.

6.5.8 Access to local farm businesses, such as Medbourne Farm, may be adversely affected by an access road on the southbound carriageway (to the south east of Commonhead junction) being stopped up with underpass and overpass options. The access road would remain open with the throughabout. Therefore, there is no difference between the underpass and overpass options in terms of impacts on local agricultural businesses. The throughabout option would leave accesses to local farm businesses as existing.

6.5.9 There are no public rights of way (ProW) that are directly affected by any of the three options.

6.5.10 An informal path used by the public to cross over both carriageways (northbound and southbound) of the A419 could be adversely affected by all three options.

Mitigation

6.5.11 Due consideration would be given to maintaining adequate access to farms that may be affected by stopping off the slip-road along the southbound carriageway of the A419 to the south east of Commonhead junction. Alternative access would be provided, as appropriate.

6.5.12 Likewise, due consideration would be given to pedestrians that cross over the A419 immediately to the north of Commonhead junction. Where the movement of pedestrians is restricted by the options, alternative crossing points/arrangements would be incorporated within the design of the selected option.

Statement of Significance

6.5.13 The effect of restricting access to farm businesses as a result of stopping off the sliproad along the southbound carriageway (to the south east of Commonhead junction) could be readily mitigated by providing alternative access, or by ensuring that access is possible by an alternative route. Provided that access to farms is fully considered and access restrictions are mitigated, where necessary, the overall impact of all three options on local agricultural businesses would be neutral.

6.5.14 Provided that suitable access can be maintained for pedestrians who currently use an informal footpath to cross over both carriageways of A419 at a point immediately to the north of the Commonhead junction, then the overall effect of all three options on pedestrian movement would be neutral.

6.6 NOISE AND VIBRATION

Baseline Conditions
Environmental Noise Measurements

6.6.1 Noise measurements have been undertaken at two locations, representative of the nearest noise-sensitive dwellings to the east and west of the A419. The purpose of the measurements was to determine the LA10,18hour noise level for the period 06:00 to 24:00 of a typical weekday. The aggregate and calculated LA10,18hour noise levels for the two locations, based on the noise measurement results are shown in Table 6.4.

Table 6.4: Noise Measurement Position - free-field dB
Location Level Type Noise Level LA10,18hour (dB)
Moor Leaze House 24-hour Aggregate level 68
No. 41 Hallam Moor 3-hour corrected level 62
Existing Situation

6.6.2 Table 6.5 sets out the number of properties that are currently exposed to each range of noise levels.

Table 6.5: Existing Situation
Noise Level Range LA10,18hour (dB) Number of Properties
67.5 to 70.0 2
65.0 to 67.5 23
62.5 to 65.0 31
60.0 to 62.5 81
57.5 to 60.0 106
55.0 to 57.5 107
52.5 to 55.0 75
Less than 52.5 10

6.6.3 Table 6.6 shows the existing noise level calculated for a number of individual properties located within 300m of the proposed scheme.

Table 6.6: Noise Level at Various Properties - Existing Situation 2002.
Property Noise Level, LA10,18hour (dB)
Moor Leaze House 63
Great Moor Leaze Farm 60
33 Arnfield Moor 67
42 White Edge Moor 65
9 Hathersage Moor 61
63 Matley Moor 62
5 Abney Moor 56
20 Hallam Moor 57
55 Edale Moor 55
12 Tansley Moor 53
148 Ridge Nether Moor 55
1 Matley Moor 55
Hospital 56
Option A (Overpass)

6.6.4 In an opening year of 2005 and if no noise mitigation measures were included, the majority of the properties considered are predicted to experience an increase in the level of traffic noise, with the exception of five properties to the north east of Liden. Low noise road surfacing would be used. The increases in noise level range up to 3.0 dB(A). However, many of the increases are not considered to be significant, as they are below +1.0 dB(A). Table 6.7 sets out the corresponding number of properties that are likely to experience noise level changes within a noise level range.

Table 6.7: Change in Noise Level - Option A.
Range of Change in Noise Level, LA10,18hours (dB) Number of Properties
-1.4 to 0.0 5
0.0 to 1.0 74
1.0 to 2.0 216
2.0 to 3.0 139

6.6.5 Table 6.8 sets out the predicted change in noise levels immediately after the opening of the proposed scheme, at each of the individual properties considered.

Table 6.8: Noise Level and Change at Various Properties - Option A.
Property Noise Level, LA10,18hour (dB) Change (dB)
Existing 2005 Option A 2005
Moor Leaze House 63 64 +1
Great Moor Leaze Farm 60 61 +1
33 Arnfield Moor 67 7 <1
42 White Edge Moor 65 66 +1
9 Hathersage Moor 61 63 +2
63 Matley Moor 62 64 +2
5 Abney Moor 56 59 +3
20 Hallam Moor 57 60 +3
55 Edale Moor 55 56 +1
12 Tansley Moor 53 55 +2
148 Ridge Nether Moor 55 57 +2
1 Matley Moor 55 58 +3
Hospital 56 59 +2
Option B (Underpass)

6.6.6 In 2005, following the opening of Option B, the majority of the properties considered are predicted to experience a reduction in the level of traffic noise, with the exception of 97 properties to the north of Liden. Low noise road surfacing would be used. These increases are all less than 1.0 dB(A), and are therefore considered insignificant. Table 6.9 sets out the corresponding number of properties that are likely to experience noise level changes within a noise level range.

Table 6.9: Change in Noise Level - Option B.
Range of Change in Noise Level, LA10,18hour Number of Properties
-3.7 to - 1.4 43
-1.4 to 0.0 294
0.0 to 1.0 97

6.6.7 Table 6.10 sets out the predicted change in noise levels immediately after the opening of the proposed scheme, at each of the individual properties considered.

Table 6.10: Noise Level and Change at Various Properties - Option B.
Property Noise Level, LA10, 18hour (dB) Change dB
Existing 2005 Option A 2005
Moor Leaze House 63 62 -1
Great Moor Leaze Farm 60 59 -1
33 Arnfield Moor 67 67 <-1
42 White Edge Moor 65 64 -1
9 Hathersage Moor 61 60 -1
63 Matley Moor 62 60 -2
5 Abney Moor 56 55 -1
20 Hallam Moor 57 57 <-1
55 Edale Moor 55 56 +1
12 Tansley Moor 53 53 <+1
148 Ridge Nether Moor 55 55 <-1
1 Matley Moor 55 55 <-1
Hospital 56 56 <-1
Option C (Throughabout)

6.6.8 In 2005, following the opening of Option C, it is considered that none of the properties within 300m of the proposed scheme would experience a significant increase in noise levels. This is because the vertical alignment of the road would remain very similar to the current situation, resulting in the acoustic attenuation afforded by the existing vegetated barriers and soft ground of the intervening land remaining the same.

6.6.9 It is considered that noise levels at many of the properties close to the proposed scheme would be reduced as a result of low-noise road surfacing that will be used to construct the new sections of road.

Mitigation
Noise Insulation Regulations (NIR)

6.6.10 Traffic noise levels for Option A and B have been predicted for all properties within 300m of the proposed scheme.

6.6.11 Several properties were identified to the north east of Liden that are likely to experience façade noise levels that exceed the 68 dB LA10,18hour noise level specified in the NIR. However, none of these properties are predicted to experience noise level increases that exceed the 1.0 dB(A) increase criterion stated in the NIR.

6.6.12 Therefore, of the properties considered within this assessment and in accordance with the provisions of the NIR, it is considered that no properties are likely to be eligible for a grant in respect of noise insulation measures, whichever option is chosen. However as the scheme is developed the anticipated noise impact would be reviewed.

Ground-Borne Vibration

6.6.13 It is anticipated that none of the proposed options will give rise to significant levels of ground-borne vibration, since the most important factor in its generation is an uneven road surface. This is unlikely to be the case at Commonhead junction for a number of years.

Option A (Overpass) 4m noise reducing barrier

6.6.14 The proposed design incorporates a noise barrier in conjunction with stone mastic asphalt low noise road surface. Located to the north west of the scheme, the 4m high barrier would run along the top of the existing bund adjacent to the northbound slip road, and is designed to screen the extensive conurbation of Liden.

6.6.15 As a result, 431 properties would either benefit from a decrease in noise level or would have no change . 3 properties would still experience a noise increase. Table 6.11 sets out the change in the number of properties that are likely to experience noise level changes in each range.

Table 6.11 Change in - Option A with Barrier Mitigation Scheme.
Range of Change in Noise Level LA10, 18 hour (dB) Number of Properties
-6.0 to -8.0 26
-4.0 to -6.0 62
-2.0 to -4.0 176
0.0 to -2.0 167
0.0 to 2.0 3
Option B (Underpass)

6.6.16 Given that no properties are predicted to experience significant increases in noise levels after the opening of Option B with the use of low-noise road surface, it is considered that no further mitigation would be required.

Option C (Throughabout)

6.6.17 Given that noise levels are predicted to remain the same at all properties, or be reduced if the low-noise road surface is used, it is considered that no further mitigation would be required for Option C.

Statement of Significance

6.6.18 For Option A (overpass) which includes the 4m noise barrier and low noise road surface, three properties within 300m would be subject to an increase in noise level, with the remaining 431 properties within 300m subject to a decrease or no change in noise level. The maximum predicted decreases in noise level would be approximately -8.0 dB(A), and increases in noise level at the 3 properties predicted to experience them, would range up to 2.0 dB(A).

6.6.19 For Option B (underpass) including low noise road surface, 97 properties within 300m would be subject to an increase in noise level, with the remaining 337 properties within 300m subject to a decrease in noise level. The maximum expected decreases in noise level would be up to -4.0 dB(A), and increases in noise level would range up to 1.0 dB(A). It has therefore been identified that Option B would incur no significant increases in noise level.

6.6.20 For Option C (throughabout) including low noise road surface, no properties within 300m would be subject to an increase in noise level. All 434 properties within 300m would experience a decrease in noise level.

6.7 WATER QUALITY AND DRAINAGE

Baseline Environmental Conditions

6.7.1 Information on existing water quality and hydrology in the study area has been obtained from the following sources:

6.7.2 Information from these data sources has been supplemented where necessary, through direct consultation with the Environment Agency (EA). The EA has also been consulted to obtain their views on the proposed improvement options in relation to water quality and drainage issues.

Environmental Effects and their Mitigation

6.7.3 The key environmental issues that have been identified and addressed as part of this assessment are:

6.7.4 This assessment has been carried out in turn for each of the three proposed options:

  1. Option A: Overpass
  2. Option B: Underpass
  3. Option C Throughabout Pollution Control (Potential Environmental Effects)

6.7.5 The three proposed options all give rise to an increase in the area of hardstanding compared with the current situation, as follows:

  1. Option A: increase by 20,675 sq.m;
  2. Option B: increase by 20,675 sq.m; and
  3. Option C: increase by 7,220 sq.m.

6.7.6 Increasing the area of hardstanding would increase the total volume of surface water runoff from the A419 around Commonhead junction. However, this would not affect the concentrations of contaminants on the road surface as the traffic flows would not change significantly and it is from these that the mass loadings of contaminants on the road surface are determined.

6.7.7 Therefore, the three junction improvement options would give rise to an increase in the volume of surface water runoff, but the quality of this runoff is not expected to change significantly from the current situation (i.e. the concentration of key pollutants would be the same).

6.7.8 In view of the relatively coarse nature of DMRB Stage 2 calculations on pollution effects, any difference in pollution effect for each option compared to the existing situation is therefore considered to be negligible.

6.7.9 The purpose of the proposed works is to reduce congestion and improve road safety at the junction and its approach roads and therefore lead to a decrease in the number of accidents. For this reason, the probability of a serious accidental spillage for any of the proposed junction improvement options would be lower than that for the existing situation at Commonhead junction. It would therefore constitute an improvement to the existing situation.

Pollution Control (Mitigation)

6.7.10 In relation to pollution control, the drainage design for the preferred route option should include silt traps and oil interceptors to reduce the concentrations of suspended solids, hydrocarbons and heavy metals prior to discharge to the receiving water.

6.7.11 The existing drainage system for Commonhead junction could be used during construction to manage surface water runoff, with the drainage ditches alongside the carriageway being used to trap silt and other contaminants prior to discharge to the receiving water.

6.7.12 The overall impact of all three options in relation to pollution control is considered to be neutral, i.e. the adoption of suitable mitigation measures to control the quality of surface water runoff would not give rise to a deterioration in surface water or groundwater quality. Due to the lower volumes of runoff arising from Option C, (Throughabout) the extent of mitigation required for this option would be less than that for Option A (Overpass) and Option B (Underpass).

Flood Risk (Potential Environmental Effects)

6.7.13 No part of the proposed route options lies within the EA's indicative flood plain.

6.7.14 As for the pollution effects described above, each of the three options would lead to an increase in the area of hardstanding and would therefore lead to increased volumes of surface water runoff. A reach of Liden Brook between Moor Leaze Farm and the balancing pond is considered to be at risk of flooding. Therefore, the control of additional surface water runoff from the new areas of hardstanding is a key issue in order not to exacerbate existing flooding problems.

6.7.15 Some of the additional hardstanding area for Options A and B would lie either over or under existing areas of hardstanding. This would reduce the relative impact of these options in terms of the generation of additional surface water runoff volume, compared to Option C (throughabout), where all of the surface area is exposed to rainfall.

6.7.16 For Option B (Underpass), the proposed level of the cutting would be 2.0 to 2.5m below the water table level. Groundwater could seep into the underpass and would require removal by pumping which would incur additional costs. Winter water levels could rise which would increase the amount of pumping. Early consultation with the EA on this matter has concluded that the EA prefer not to be permanently dewatering the area, as this would be unsustainable.

Flood Risk (Mitigation)

6.7.17 Flood risk would require careful attention during the drainage design. Detailed flood risk calculations would be required during the Stage 3 Assessment to establish the most appropriate route of disposal of surface runoff and the extent of flow attenuation required prior to discharge. It might be possible to utilise the balancing pond to the north of the roundabout, should it possess sufficient capacity. If not, new attenuation facilities would be required.

6.7.18 As described above, Option B (Underpass) would require pumping to remove surface water runoff from the underpass, and possibly groundwater from the cutting. However, the pumping requirements for both of these aspects would be combined, thus reducing the overall additional cost.

6.7.19 The overall environmental impacts of all three options (following mitigation) in relation to flood risk are neutral.

6.7.20 Options A (Overpass) and B (Underpass) give rise to an increased surface area of hardstanding and therefore require a larger volume of surface water runoff to be attenuated prior to discharge to the receiving water. Option C (throughabout) requires significantly less attenuation and therefore would be slightly preferable in terms of cost.

Statement of Significance

6.7.21 With the proposed mitigation measures in place, the overall impact of each of the three options in relation to water quality and drainage would be neutral.

6.7.22 There is no basis, in terms of environmental impact, to rank the three options in order of preference, as the differences are not considered to be significant enough. In cost terms for water quality and drainage, the order of preference would be:

  1. Option C - Throughabout
  2. Option A - Overpass
  3. Option B - Underpass

6.7.23 The most significant water resource issue would be flooding and detailed flood risk calculations are recommended for the Stage 3 Environmental Assessment.

6.7.24 Depending upon the current state of the existing pollution control and drainage measures at Commonhead junction, there would be likely to be a slight improvement in pollution control and flood protection should a new scheme be constructed.

6.7.25 Groundwater levels are likely to fluctuate with seasonal and long term variations. Hence, groundwater coupled with surface water run-off would pose a serious problem for an 'underpass' solution, which would require substantial engineering works and groundwater pumping, all of which would make such a solution not environmentally sustainable.

Summary

6.7.26 For an 'underpass' solution, the effect of high ground water levels would pose a serious problem for construction. Permanent pumping could have a serious impact on the local hydrology and the cost of carrying out such an operation coupled with the need to provide waterproof structures would add significant additional cost.

6.7.27 The adverse environmental impact on groundwater would make an 'underpass' not a viable option. An 'overpass' solution has no such high environmental risk.

6.8 VEHICLE TRAVELLERS

Baseline Conditions

Landscape Character

6.8.1 Undulating clay vale lowland adjoins the southbound carriageway of the A419 in the study area; the rising scarp slope of the North Wessex Downs is approximately 1km to the south east of Commonhead junction. Much of this landscape is designated; the North Wessex Downs Area of Outstanding Natural Beauty adjoins the A419, and there is an Area of Local Landscape Importance and Rural Buffer designations.

Roadscape

6.8.2 The dual carriageway splits on the approaches to the junction, with mounding and mature vegetation on the widened central reserves. The roundabout itself is also mounded and planted; most of the vegetation is healthy with one small group of dead trees on the northern side.

6.8.3 The land to the east is slightly lower than the A419, and there is little intermittent boundary vegetation. To the west, the noise bund at Liden and the cut slope in the south west quadrant restrict views; both support mature vegetation.

Landscape Quality

6.8.4 The scarp slope is assessed as very attractive landscape with indented water-cut valleys, copses, medium sized pasture fields and rural villages. The villages of Liddington, Wanborough and Badbury, and the ribbon development at Ham Road, are themselves assessed as good landscape, with attractive and harmonious development well integrated with the adjacent landscape.

6.8.5 The clay vale lowland is assessed as good landscape, medium to large pasture fields with significant hedgerows particularly along watercourses, scattered farmsteads and hamlets.

6.8.6 The new Great Western hospital site is assessed as poor landscape, being degraded by the dominance of the new hospital building, which is so large that it dominates views in the area.

6.8.7 Liden is assessed as ordinary quality, enhanced by the good quality landscape of the adjacent noise bund, which screens Liden from the road and from the hospital building.

Extent of View

6.8.8 A419 northbound carriageway: No view: the mounding and vegetation between the carriageways and on the roundabout prevent views to the east. The rising cut slope by the new Great Western hospital and the Liden noise bund prevent views to the west. North of the junction, where the central reserve narrows, views are more open to the east.

6.8.9 A419 southbound carriageway: Open view to the east, no view to the west: the general lack of roadside vegetation allows mainly unrestricted views to the east and south across the undulating lowland, to the rising scarp slope of the North Wessex Downs. These attractive views extend across the M4 corridor, which is in cutting and not visible, to the rising hills to the south, south of the roundabout. Roadside vegetation adjacent to the layby restricts views to the east for a short length. Views to the west are restricted by the central reserve mounding and planting.

Driver Stress

6.8.10 Driver stress is influenced by a number of factors including road layout and geometry, surface riding characteristics, junction frequency, and speed and flow per lane. At present traffic volumes can be high and traffic speeds can be very low around the Commonhead junction at peak times which is likely to create driver stress.

6.8.11 The current junction arrangement is also likely to induce frustration for drivers remaining on the A419 because these drivers are required to interrupt their forward journey and negotiate the junction.

Environmental Effects (Options A - C)

Views from the Road - Option A

Opening Year

6.8.12 A419 northbound carriageway: Open view to the east, restricted view to the west: on the rising approaches to the junction, views to the east would be opened by removal of the central reservation mounding and planting and by the elevation of the carriageway.

6.8.13 The retained mounding and planting at Liden, and the vegetated cut slope by the new hospital, would continue to restrict views to the west.

6.8.14 For travellers in cars, views would be limited by the adjacent mounding where the carriageway crosses the roundabout. For travellers in raised cabs, longer views may be available, for example a glimpse along the A4259. In addition, there would be glimpses to the junction from the overpass and vice versa, introducing additional carriageway into the view.

6.8.15 A419 southbound carriageway: Open view to the east, restricted view to the west: the mainly unrestricted views to the east and south across the undulating lowland, to therising scarp slope of the North Wessex Downs, would be retained from a raised angle.

6.8.16 Views to the west would be opened across the northbound carriageway to the vegetation at Liden and the cut slope in front of the hospital.

6.8.17 For travellers in cars, views would be limited by the adjacent mounding where the carriageway crosses the roundabout. For travellers in raised cabs, longer views may be available, for example a glimpse along the A4259. In addition there would be glimpses to the junction from the main line and vice versa, introducing additional carriageway into the view.

Year 15

6.8.18 A419 northbound carriageway: Restricted view: on the rising approaches to the junction views to the east would be restricted by the growth of the proposed planting. The retained mounding and planting at Liden, and the vegetated cut slope by the new hospital, would continue to restrict views to the west.

6.8.19 A419 southbound carriageway: Restricted view: the mainly unrestricted views to the east and south across undulating lowland, to the rising scarp slope of the North Wessex Downs, would be filtered at the lower level by the growth of the proposed planting. There would continue to be views to the west, which were opened by the construction works, across the northbound carriageway, but again these would be limited by the growth of the proposed planting.

Views from the Road - Option B
Opening Year

6.8.20 A419 northbound carriageway: No view: The cut slopes adjacent to the carriageway would block views. On the approaches, as the carriageway enters and exits the cutting, limited views across the carriageway to the east would be opened by removal of the existing central mounding and planting. In addition, there would be glimpses to the junction from the underpass and vice versa, introducing additional carriageway into the view.

6.8.21 A419 southbound carriageway: No view: The cut slopes adjacent to the carriageway would block views. On the approaches as the carriageway enters and exits the cutting, there would be views to the east, and limited views across the carriageway to the west, opened by removal of the existing central mounding and planting. These views would be limited by the mounding at Liden and the hospital cut slope. There would be glimpses to the junction from the main line and vice versa, introducing additional carriageway into the view.

Year 15

6.8.22 A419 both carriageways: No view: The cutting would block both views. The development of the proposed planting would add interest on the cut slopes but would also restrict any wider views on the approaches to the cutting.

Views from the Road - Option C
Opening year

6.8.23 A419 northbound carriageway: Restricted view to the east and west: Most of the existing mounding and planting in the central reserve would be retained, restricting views to the east. The retained mounding and planting at Liden, and the vegetated cut slope by the new hospital, would continue to restrict views to the west. There would be additional carriageway in the view at the junction, complicating the foreground. Some vegetation may be retained on the roundabout, visually breaking up the roadscape.

6.8.24 A419 southbound carriageway: Open view to the east, restricted view to the west:

Most of the existing mounding and planting in the central reserve would be retained, restricting views to the west. Views to the east across the adjacent landscape would be open. There would be additional carriageway in the view, complicating the foreground. Some vegetation may be retained on the roundabout, visually breaking up the roadscape.

Year 15

6.8.25 A419 northbound carriageway: Restricted view: The retained mounding and planting would continue to restrict views to the west and east.

6.8.26 A419 southbound carriageway: Intermittent view: Views to the east and south across the undulating lowland, to the rising scarp slope of the North Wessex Downs, would be filtered at the lower level by proposed mitigation planting on the boundary. Views to the west would continue to be restricted by existing mounding and planting.

Driver Stress

6.8.27 Option A (overpass) and Option B (underpass) are the favoured options in terms of driver frustration. These two options would be more likely to allow drivers to drive at a speed consistent with his or her own wishes in relation to the general standard of the road. As the 'over' and 'under' options would ease congestion more than the 'throughabout', these are considered to be equally the most favourable options.

6.8.28 The junction improvements would also provide the opportunity for good design and layout of signs, which can go a long way towards eliminating the cause of stress from new road schemes.

6.8.29 An assessment of driver stress for each option, utilising the tables in DMRB has identified the following anticipated stress levels for each option:

6.8.30 Lower vehicle speeds result in the 'high' rating for the 'throughabout' option. In terms of driver stress there are no preferences between the 'over' and 'under' options.

Statement of Significance

Views from the Road

6.8.31 Option A: Restricted View: Wider views than at present in the opening year would be replaced in the long term by views which would be more restricted than at present, due to the growth of proposed landscape mitigation.

6.8.32 Option B: No View: Views would be blocked where the main line enters cutting. Views into the cutting would be available for travellers on the slip roads. In the long term, the proposed landscape mitigation would add interest, but would restrict any remaining views where the carriageway emerges from cutting.

6.8.33 Option C: Intermittent/Restricted View: In the short term some views may be open across the landscape to the east. In the long term these views would be filtered by the growth of the proposed landscape mitigation planting.

Driver Stress

6.8.34 In terms of driver stress, both the 'over' and 'under' options would be likely to reduce driver stress from high to moderate, whereas the 'throughabout' option would be likely to have little effect. The beneficial effects from the 'over' and 'under' options result from the reduction in congestion at the junction and the resultant higher vehicle speeds. There is no preference between the 'over' and 'under' options in terms of driver stress.


6. Environmental Assessment

7. Traffic and economic assessment

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:

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:

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

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

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:

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.

Table 7.1. Validation Performance: Cordoned Model
% of flows meeting criteriaValidation Criteria
AM Peak Inter Peak PM PeakFlows > 700 100 100 100
Flows < 700 91.8 84.7 87GEH > 5 91.9 83.5 91.5

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:

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.

Table 7.2 Economic Summary of Over Option
 LowHigh
Present Value of Benefits4543876798
Indirect Tax Revenues-1119-1427
Present Value of Costs-7827-7827
Net Present Value3649267544
Benefit : Cost Ratio5.6629.629

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.

Table 7.3 Economic Summary of Under Option
 LowHigh
Present Value of Benefits4543876798
Indirect Tax Revenues-1119-1427
Present Value of Costs-12722-12722
Net Present Value3159762649
Benefit : Cost Ratio3.4845.924

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.

Table 7.4 Accident Benefits Over/Under Option
 UnitsLow GrowthHigh GrowthWeighted
Accidents
Links£'000,s142166154
Junctions£'000,s815918867
Reduction in Casualties
FatalNumber-- 
SeriousNumber3.33.83.55
SlightNumber56.565.060.75

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


7. Traffic and economic assessment

8. Programme

8. PROGRAMME

8.1 Targeted Programme of Improvement (TPI) announcement - December 2002

This timetable could be improved if a public inquiry could be avoided. In which case works could start in 2005.


8. Programme

9. Conclusions and recommendations

9. CONCLUSIONS AND RECOMMENDATIONS

9.1 Three options to solve the congestion and accident problems at Commonhead junction have been considered as follows:

9.2 The at-grade throughabout was rejected because it would not provide long term capacity, only around 6 years.

9.3 The 'overpass' and 'underpass' options would provide both adequate capacity and reduce the number of accidents.

9.4 Effects on groundwater, noise and visual effects are the main environmental impacts of the 'underpass' and 'overpass' options. The noise levels with the 'overpass' option, including mitigation would decrease or stay the same apart from three properties. Of the 434 properties assessed only 3 would experience a noise level increase. With the 'underpass' option there would be minimal noise increases at 97 properties but generally noise levels would decrease overall.

9.5 In terms of visual effects, the 'overpass' option would have moderate adverse visual impact for views across the AONB and winter views from Liden. However, gentle mounding and planting on the roundabout and planting on the embankments would soften the impact to slight adverse visual impact after 15 years. The 'underpass' option would have slight adverse visual impact for views across the AONB through loss of vegetation. Similarly, the provision of gentle mounding and planting on the roundabout would soften the impact to no change in visual impact after 15 years.

9.6 In terms of impact on groundwater, the 'underpass' would pose a high risk. Despite the need for substantial waterproof structures, groundwater may also need to be pumped to keep the new road free of water. The resultant de-watering on the local hydrology would have an adverse effect.

9.7 The 'overpass' option is more cost effective and has better economics than the 'underpass' option. The cost of the 'underpass' option would be around £6.0M more than the 'overpass' option with the need to permanently de-water and/or waterproof the cutting base, due to the presence of high groundwater. This means the 'underpass' would be poorer value for money than the overpass.

9.8 It is therefore considered that the 'underpass' is not a viable option and it is recommended that the 'overpass' option should be taken to public consultation as the proposed layout.


9. Conclusions and recommendations