South marston technical note final with appendices

Technical Note Date June 2012

Subject South Marston Supplementary Planning Document - Part Three Transport Assessment

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1 Introduction 1.1 This technical note sets out the main findings of the work commissioned by Swindon Borough Council and

South Marston Parish Council in support of the development of a Supplementary Planning Document (SPD) for the village of South Marston. The purpose of the SPD is to provide a planning framework setting out the expectations and requirements for the sustainable development of the village of South Marston. The study has been conducted in three parts:-

• Part One – Examination of existing transport challenges • Part Two – Examination of the transport impacts of the Eastern Development Area

• Part Three – Examination of the transport impacts of the proposed Eastern Villages

Part One

1.2 Part One of this study assessed options to mitigate the impacts of existing issues on the road network in and around South Marston village. Issues were identified from site observations, discussions with South Marston Parish Council and the Traffic Sub-group, and an assessment of the ‘Village Strategy’.

1.3 The identification of traffic issues, and potential options to address these, were based on existing volumes of traffic through the village. It did not include the potential transport impact of proposed growth at the Eastern Development Area (EDA) at this stage.

Part Two

1.4 Part Two of the study assessed the transport impact of the EDA on South Marston and was based on data available in Autumn 2010. The study area was as per Part One of the study and included Thornhill Road to the Keypoint roundabout, Old Vicarage Lane to the A420 and Highworth Road.

1.5 The assessment was undertaken at a more strategic level given uncertainties regarding the masterplan for the EDA.

1.6 The Part Two report sought to address the optioneering solutions identified in Part One to understand whether the conclusions drawn were still valid with development traffic (i.e, ‘future-proof the identified solutions). The land use and highway planning context for the assessment of options was based on:-

• The Planning for Real exercise conducted by New Masterplanning in the Summer of 2010, which sought to establish some principles for the development of the village; and

• Long standing options identified in the 2006 South Marston Village Report.

1.7 Both reports are available on Swindon Borough Council’s website.

8 June 2012 South Marston Supplementary Planning Document - Part Three Transport Assessment

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Part Three

1.8 This technical note reports on an updated transport modelling assessment of the impact of the proposed Eastern Villages development on South Marston. The main changes between this assessment and the previous 2011 Part Two assessment are as follows:-

• The strategic model which underpins our modelling exercise has been updated with revised housing and employment growth numbers to reflect the latest version of the Core Strategy rather than Regional Spatial Strategy housing growth trajectories

• It includes changes to planning trajectories in terms of estimated housing and employment completions to 2026

• It reflects the latest Eastern Villages layout rather than the more linear EDA layout along the A420

• It includes revised background traffic growth assumptions, and

• Focuses on highway options that were not resolved in Part One and Two, or where further testing was deemed necessary using the latest growth and layout assumptions.

1.9 The assessment has been conducted for the morning and evening peak periods in line with best practice guidance set out in the “Guidance on Transport Assessments”. A 2026 forecast year has been chosen as this conforms to the ‘plan period’ of the emerging Core Strategy. It should be noted that the morning and evening peak periods represent conditions on the network when traffic volumes are expected to be at their highest. At other times of the day and on weekends, traffic volumes on the network will be considerably lower.


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2 Options Considered in Parts One and Two 2.1 To provide context, this section identifies the options that have been considered from Part One through to

Part Three.

2.2 Following discussions with South Marston Parish Council and a review of the 2007 South Marston Village Strategy, Part 1 identified the transport challenges listed in Table 2.1 below.

Table 2.1 - Key Existing Transport Challenges and Recommendations from Part One Description Existing Issue Preferred Options from Part

One of the Study Options discounted

Pedestrian and vehicular conflict around Pound Corner

Insufficient carriageway and footway available

Realignment of the carriageway at Pound Corner to provide footway on the northern side of Old Vicarage Lane

-signalisation (environmental issues) - junction widening (land take) -Realignment (limited pedestrian benefit) -Footway on south side (alternative pedestrian routes to Thornhill Road available to the rear of the school)

Gaps in footway provision

Thornhill Road close to the industrial estate. Vicinity of South Marston Hotel.

Continuous footway from Pound Corner to Keypoint roundabout and from Pound Corner to South Marston Hotel

None

Accessibility improvements

Lack of Dropped Kerbs / Tactile paving

To provide dropped kerbs / tactile paving in locations where these are not provided

None

High Traffic Speeds/ Through Traffic

Little evidence to support view in terms of speeds/accidents or traffic volumes.

20mph zone for village centre & 30 mph between village centre and A420 for consistency. Some additional traffic calming measures

20mph smaller zone (would only include the existing village within the existing built up area)

2.3 Whilst the recommendations in Part 1 address existing concerns, the study also identified issues to be

considered within the Part Two assessment of the impacts of the Eastern Development Area. These issues are listed in Table 2.2 below


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Table 2.2 - Additional Transport Challenges associ ated with the EDA / Eastern Villages development Issue Option tested in Part Two

Can the masterplanning principles set out by SBC be achieved in transport terms?

Function of Pound Corner? Function of Old Vicarage Lane / A420 Junction?

Are the ‘southern’ and ‘eastern’ link roads necessary?

Testing of options with and without the southern link road Testing of options with and without the eastern link road

Implications of Rowborough What are the implications of Rowborough- Through traffic on the performance of: - Pound Corner - Keypoint roundabout- A420 / Old Vicarage Lane Junction

Will the proposed options identified as Part One be sufficient to mitigate the impact of the EDA?

Test options identified in Table 2.1

Are the proposed and existing traffic calming suitable with increased volumes of traffic? Are further measures required?

Existing traffic calming measures

2.4 Part Two of the study concluded the following:

• Without mitigation, there will be a significant increase in journey times through the village particularly in the morning peak and in the southbound direction. The approaches to the Gablecross junction and the Old Vicarage Lane / A420 junction will experience the most significant delays;

• Evening peak hour congestion will be less pronounced than in the morning peak;

• The majority of Rowborough traffic will be accessing destinations to the south and via the A420 and will not be travelling through South Marston village centre;

• Existing traffic calming measures which reduce the width of the available carriageway (chicanes) will generate queues and delays for vehicle streams which do not have priority;

• Footway provision on Old Vicarage Lane, south of Nightingale Lane will need to be extended to link the new development to the rest of the village and also to the remainder of the Eastern Villages Development;


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• Carriageway widths on Old Vicarage Lane, south of Nightingale Lane will need to be widened.

2.5 In relation to Pound Corner, Part Two identified that in its current form (priority junction) there would be

insufficient capacity to cater for the anticipated increase in traffic volumes unless additional links to Thornhill Road and / or capacity improvements were considered at the junction. The study identified that relocating the junction to the west of the potential school extension could be considered. However, as some of the playing fields would be lost to accommodate the diversion of the northern end of Old Vicarage Lane, this was not considered further.

2.6 The Part Two study also concluded that the provision of an eastern bypass for South Marston to accommodate Rowborough traffic would only provide a marginal improvement to journey times through the village. Coupled with low forecast traffic volumes using the road, and the cost and environmental impact of providing such a route, the study recommended that there was no justification for its inclusion in any future masterplanning exercise for the village.

2.7 Similarly, the provision of a link road to the south of an expanded village and connecting Thornhill Road with Old Vicarage Lane was also previously suggested in order to reduce traffic volumes in the village. Modelling indicated that if this link road was extended to Rowborough then the resultant delays and queues experienced at the Keypoint and Gablecross roundabouts would increase significantly.

2.8 In addition, the queues and delays experienced on Old Vicarage Lane would also increase because of the increase in opposing flows where Old Vicarage Lane met the new road. Part two concluded that from a road hierarchy point of view, the A420 was considered to be the more suitable route for Rowborough movements rather than the provision of a new link road skirting the southern edge of the expanded village.

2.9 A further disbenefit was that a link road to the south of the new development would create a physical barrier for pedestrians and cyclists and would result in single-sided development along an artificially lit highway edge. Hence it was concluded that this would adversely impact on the rural character to the south of the village and could potentially make the village more vulnerable to further encroachment to the south.

2.10 In place-making terms, the Part Two study also concluded that a circuitous residential road connecting Old Vicarage Lane with Thornhill Road and running within the development would be preferable as this would directly serve the expanded community and would be designed to minimise through-traffic. The new roads would be designed according to best practice to create the appropriate road hierarchy. The alignment of the road would need to be considered in further masterplanning exercises and through the preparation of the South Marston SPD.

2.11 The final recommendation from Part Two was that in order to accommodate traffic flows from the Eastern Villages development, the existing junction at Old Vicarage Lane/A420 would need to be upgraded. Part Two explored online and offline improvements. Online improvements would require the provision of a signalised T-junction with flared approaches on all arms. Offline improvements would require the provision of a new junction and underpass underneath the railway line and to the east of the current location.

2.12 The above recommendations were contingent on highway improvements being in place elsewhere on the network and in particular on the A420 and at White Hart Junction.


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3 Assumptions for Part Three Assessment Using the work from Part One and Part Two as a starting point, the Part Three modelling work has been underpinned by a number of land use and network assumptions both internal and external to South Marston Village.

Land Use Assumptions

3.1 Committed / Consented developments and housing and employment allocations identified in the Core Strategy to 2026 are included in the model. For example, this includes the strategic sites at Wichelstowe, Commonhead and Tadpole Farm. It also includes urban extensions to Swindon in Wiltshire and an assumed level of background traffic growth which accounts for growth from neighbouring areas such as Oxfordshire.

3.2 The largest and most influential development on South Marston village will be the strategic site known as the Eastern Villages development. The Eastern Villages ‘core’ development quantum is shown in Table 3.1 below

Table 3.1 Proposed Eastern Villages Development Co mposition

Land Use Type Quantum

Residential 7500 dwellings

B1 Office 2000 jobs

B8 Warehousing 1500 jobs

Primary School 1680 places

Secondary School 1200 places

Retail (District Centre only) 500 jobs

Source: Swindon Borough Council

3.3 Of the 7,500 dwellings which would be constructed on the Eastern Villages sites, the following breakdown of residential units has been assumed. These numbers are indicative and used for transport modelling purposes only. They may be subject to change.


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Table 3.2 Proposed Eastern Villages – Islands of D evelopment

Number of Dwellings

South Marston 500

Rowborough 1200

Priors Farley 300

Longleaze 1600

Great Stall 1325

Lotmead 575

Earlscourt 1200

Lower Earlscourt 800

The Marsh 0

Totals 7500

Source: Swindon Borough Council

3.4 An allowance has also been made for the conversion of the Crown Timber and Thornhill Industrial sites to

residential from their current use.

Baseline Network Assumptions

3.5 In order to establish a baseline/ reference case by which future changes to the highway network in South Marston can be assessed against (and for impacts to be isolated), a number of network assumptions were included in the modelling.

3.6 The most pertinent assumptions external to South Marston Village (i.e. the A420 and beyond) are shown in Table 3.3 below and have been identified as being potential options to accommodate growth to 2026 and also facilitate access to the Eastern Villages. This includes public transport and highway interventions

Table 3.3 Baseline Network Assumptions external to South Marston Village

Ref No.

Network Assumptions

1 Existing access points to A420 from the north - signalised junction at Gablecross, A420 Old Vicarage Lane junction modified to provide a new signalised junction and new eastern access from ‘Rowborough’

2 3 access points onto A420 for EV traffic from the south

3 Reconfigured Police Station access

4 Link between Rowborough and Old Vicarage Lane as indicated by the DLA masterplan

5 Green Bridge across the A419 for public transport, walking and cycling only

6 A419/420 White Hart junction 'full improvement - diamond interchange'

7 Use of existing link across A419 via Merlin Way Roundabout (but no direct access from new development)

8 No link at Dorcan

9 Truck stop at B2/B8 allocation to the south of the development


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3.7 Network assumptions internal to South Marston and identified in the baseline assessment included the following:-

Table 3.4 Baseline Network Assumptions internal to South Marston Village

Ref No.

Network Change

1 Link between Rowborough and Old Vicarage Lane

2 20mph zone for South Marston

3 Circuitous residential roads through an expanded South Marston Village

4 New junction – Old Vicarage Lane / A420

Option Tests

3.8 The options to be tested against the baseline assessments were discussed in consultation with the South Marston Village Transport Group and were broken down into three themes as follows:-

• Highworth Road options

• Connections to Rowborough; and

• Options for Pound Corner including traffic circulation patterns around the village centre

Highworth Road

3.9 Options for the treatment of Highworth included:-

• Closure to through traffic (access only)

• Conversion to northbound operation only

• Conversion to southbound operation only

3.10 Following discussions with Swindon Borough Council and South Marston Transport Group, all these options

were discounted because of the:

• Impact on wider highway network

• Residents would be inconvenienced by the increased time and costs of making journeys to and from Highworth,

• Part two modelling results suggested that the majority of movements from Rowborough were south to the A420; and

• Likely enforcement issues


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3.11 Consequently none of these options were taken further and were not included in the modelling assessments.

Connections to Rowborough

3.12 Two options were considered in the modelling exercise:-

• All movements junction allowing turning movements between Old Vicarage Lane and the link road to Rowborough; and

• A limited movements ‘left in/left out’ junction between Old Vicarage Lane and Rowborough with no right turns from either Old Vicarage Lane (South) to Rowborough or right turns from Rowborough to Old Vicarage Lane

3.13 The purpose of these option tests was to understand whether limiting movements at this junction had a significant impact on traffic volumes at Pound Corner and on Old Vicarage Lane.

Pound Corner Options

3.14 Two options were considered in the modelling exercise:-

• The incorporation of the preferred option for Pound Corner (i.e. realigned carriageway with a footway on the northern side of Pound Corner as identified in Part One) coupled with a new junction on Thornhill Road between Rawlings Close and Manor Park; and

• A clockwise one way system starting at Pound Corner and comprising of Old Vicarage Lane, new residential roads, a new junction on Thornhill Road between Rawlings Close and Manor Park; and Thornhill Road

3.15 The purpose of these tests was to understand how the Pound Corner junction performed and how traffic volumes would displace within the existing village.

3.16 Plans showing the options for Pound Corner and infrastructure required to accommodate Rowborough traffic are shown in Appendix A and Appendix B. They include the following options:-

• Inset A – Pound Corner two way system (Appendix A – Drawing Number T108838-PR-09) • Inset A – Pound Corner one way system (Appendix B – Drawing Number T108838-PR-10) • Inset B – Old Vicarage Lane / Rowborough Link Junction (Appendix A and B) • Inset C - Old Vicarage Lane / A420 Junction (Appendix A and B) • Inset D – Thornhill Road new junction in vicinity of Rawlings Close (Appendix A and B)


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4 Assessment Outcomes

Introduction

4.1 Operational impacts were measured from a combination of the following:-

• Changes in traffic volumes between the options

• Queuing at junctions

Impact of the Rowborough Link

4.2 The PARAMICS model results indicate that with the introduction of an all movements roundabout the proportion of Rowborough traffic travelling north towards Pound Corner and south towards the A420 on Old Vicarage Lane would be as shown in Table 4.1

Table 4.1 Direction of Traffic from Rowborough Lin k to Old Vicarage Lane

Direction AM PEAK PM PEAK

Vehicles Proportion of Traffic


Towards A420 254 82% 150 93%

Towards Pound Corner 55 18% 12 7%

TOTAL 309 162

4.3 The corresponding results for traffic travelling to Rowborough is shown in Table 4.2

Table 4.2 Direction of Traffic to Rowborough Link from Old Vicarage Lane

Direction AM PEAK PM PEAK



From A420 direction 94 87% 145 90%

From Pound Corner direction 14 13% 17 10%

TOTAL 108 162

4.4 The results confirm the Part Two conclusions that the majority of Rowborough traffic will travel south on Old Vicarage Lane. Traffic volumes at Pound Corner were found to be very similar regardless of the form of junction introduced to connect Rowborough with Old Vicarage Lane.

4.5 At the Old Vicarage Lane / A420 junction, queues are forecast to be of the order of 200 to 250 metres which would extend back towards the new Rowborough link road junction. The increase in flows at the Old Vicarage Lane / A420 junction would necessitate the provision of a new junction as described in section 2.10 of this technical note.


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4.6 In summary, the modelling results indicate that introducing a new all movements junction on Old Vicarage Lane to connect with Rowborough would not have an adverse impact on other junctions within South Marston.

Pound Corner Options

4.7 Tables 4.3 and 4.4 show the changes in traffic volumes at Pound Corner between existing data collected in 2009 and 2026 modelled flows with a two way system and a one way system.

Table 4.3 Pound Corner Traffic Volumes with and wi thout a one way system – AM Peak hour

Origin Destination Existing Flows (2009)

Two Way system and footway improvements at Pound Corner (2026)

With one way system and alternative footway improvements that don’t require relocation of hedges and walls

(2026)

Highworth Road Old Vicarage Lane 41 49 271

Highworth Road Thornhill Road 96 221 - Old Vicarage Lane Thornhill Road 69 36 - Old Vicarage Lane Highworth Road 34 129 - Thornhill Road Highworth Road 55 101 228 Thornhill Road Old Vicarage Lane 45 30 134

Table 4.4 Pound Corner Traffic Volumes with and wi thout a one way system – PM Peak hour

Origin Destination Existing Flows (2009)

Two Way system and footway improvements at Pound Corner

(2026)

With one way system and alternative footway improvements that don’t require relocation of hedges and walls

(2026)

Highworth Road Old Vicarage Lane 19 50 160

Highworth Road Thornhill Road 38 111 - Old Vicarage Lane Thornhill Road 43 20 - Old Vicarage Lane Highworth Road 22 68 - Thornhill Road Highworth Road 85 122 190 Thornhill Road Old Vicarage Lane 35 21 67

4.8 The results indicate that the provision of a new junction on Thornhill Road (Inset D) will reduce the number of movements between Thornhill Road and Old Vicarage Lane and between Old Vicarage Lane and Thornhill


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Road at Pound Corner even if a two way system is retained. This will reduce the level of opposing movements at Pound Corner and modelling indicates that no queuing is expected in either the morning or evening peak periods.

4.9 Whilst no queuing is forecast in either peak with two way operation retained, overall traffic volumes at Pound Corner will increase on present day levels albeit from a very low base. As such, the provision of adequate footway provision (such as the preferred northern footway option identified in Part One of this study) will become more pressing.

4.10 The provision of a one way system affords the opportunity to provide enhanced footway provision at Pound Corner and on Old Vicarage Lane without the need to relocate the existing hedges and walls at the junction. However, with a one way system there will be a significant increase in the volumes of traffic at the northern end of Old Vicarage Lane adjacent to the primary school making road crossings more hazardous.

4.11 This is because all Highworth Road to Thornhill Road traffic will be redirected onto Old Vicarage Lane. In addition traffic that would have turned right from Thornhill Road to the new link road adjacent to Rawlings Close (Inset D) will be required to turn right at Pound Corner instead. There are also a number of journey time disbenefits of redirecting traffic to a longer more circuitous route.

4.12 Notwithstanding the above, the modelling indicates that there are no significant capacity issues associated with a one way system although the heavier right turning movement from Thornhill Road to Old Vicarage Lane may be required to give way to left turning traffic from Highworth Road to Old Vicarage Lane.

Other impacts

4.13 The PARAMICS model also identifies the impacts of queuing back from the Gablecross roundabout towards the Keypoint roundabout and Thornhill Road. The model indicates that with development traffic, queuing will occur on the Thornhill Road arm of the Gablecross roundabout which will extend back to the Keypoint Roundabout and beyond. Modelling suggests that queues of between 300m and 400m will develop in the AM Peak and between 250m and 350m in the PM Peak.


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5 Conclusions and Recommendations

5.1 This technical note has provided a view on the peak hour modelling undertaken in support of the

development of an SPD for South Marston. It should be noted that the morning and evening peak periods represent conditions on the network when traffic volumes are expected to be at their highest. At other times of the day and on weekends, traffic volumes on the network will be considerably lower.

5.2 The Part Three assessment has been conducted using a new set of assumptions to that previously used and has focused on the remaining outstanding highway issues to be resolved. These are the:-

• Form and function of Pound Corner; and

• Connections to Rowborough

5.3 The analysis has not only focused on the impact at Pound Corner but also where Old Vicarage Lane meets

the A420; where Thornhill Road meets the A420 at Gablecross and also on the new junction proposed on Thornhill Road

5.4 All options show a significant increase in the volume of traffic using existing roads. However, the increases are from a very low base.

5.5 With the exception of the A420 junctions, all the junctions in South Marston are expected to operate with minimal queuing. This assumes that a package of measures is in place for the A420 and A419 which deters vehicles from rat running through the village. The modelling conducted seems to suggest that rat running can be avoided provided the journey times on the A419 and A420 make these routes more attractive to use.

5.6 The study has concluded that an all movements connection to Rowborough in the form of a roundabout is unlikely to result in a significant increase in traffic volumes through South Marston village

5.7 In terms of Pound Corner, two options were modelled. The two way system with a new junction on Thornhill Road adjacent to Rawlings Close would offer the potential to reduce flows between Old Vicarage Lane and Thornhill Road. Consequently whilst traffic volumes would increase overall at Pound Corner the new junction would help to offset some of this increase. This junction would have to be considered in the context of the overall masterplan for the village. The Part One recommendation of providing a footway on the northern side of Old Vicarage Lane could also be accommodated providing a safer route for pedestrians to school. However, hedges and walls at the junction may need to be relocated or cut back which may be unacceptable to residents of abutting properties.

5.8 The one way system would increase volumes of traffic significantly at the northern end of Old Vicarage Lane which would make crossing the road more hazardous. However, this would be offset by the provision of an enhanced programme of footway works which would provide better segregation between pedestrians and vehicles. This could be provided without the need to relocate the hedges and walls at Pound Corner. However, the one way system would have the disbenefit of increasing journey times for Highworth Road to Thornhill Road bound movements. Evidence also suggests that vehicle speeds on one way streets are normally higher because of the lack of opposing traffic.

5.9 A further option which has not been tested but which is likely to be able to operate satisfactorily would be to retain two way operation on Thornhill Road between Pound Corner and the new junction but revert Old Vicarage Lane to a one way southbound operation only at the northern end. Under this arrangement only northbound traffic on Old Vicarage Lane would be inconvenienced by a longer diversion route. This arrangement would permit footway improvements to be made on Old Vicarage Lane without the need to


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relocate the walls and hedges at Pound Corner. It is recommended that this option be considered alongside the aforementioned options.


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APPENDIX A

SOUTH MARSTON VILLAGE OPTIONS – MODIFICATIONS TO POUND CORNER AND NEW JUNCTION BY RAWLINGS CLOSE

AND THORNHILL ROAD


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APPENDIX B

SOUTH MARSTON VILLAGE OPTIONS – MODIFICATIONS TO POUND CORNER AND ONE WAY SYSTEM


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Real Estate

South marston technical note final with appendices