Options Appraisal (Part 2) Engineering Options · 1/3/2014 · Options Appraisal (Part 2) –...

Options Appraisal (Part 2) Engineering Options

Document: CTRAEB/730/DOC HIG001

South Bristol Link

Bristol City Council & North Somerset Council

13 June 2013

Halcrow Group Limited

Burderop Park, Swindon, Wiltshire SN4 0QD

tel 01793 812479 fax 01793 812089

halcrow.com

Halcrow Group Limited is a CH2M HILL company

Halcrow Group Limited has prepared this report in accordance with

the instructions of client North Somerset Council for the client’s sole and specific use.

Any other persons who use any information contained herein do so at their own risk.

© Halcrow Group Limited 2013

Options Appraisal (Part 2) Engineering Options

South Bristol Link


13 June 2013

Options Appraisal (Part 2) – Engineering Options

Document history


South Bristol Link


This document has been issued and amended as follows:

Version Date Description Created by Verified by Approved by

0.0 10/01/13 Draft Gary Hodge P Paterson P Paterson

0.1 08/03/13 Draft (Updated) Gary Hodge P Paterson P Paterson

1.0 13/06/13 For planning application Gary Hodge P Paterson P Paterson


Contents

1 Introduction 1

1.1 Scope of this report 1

1.2 Report format 1

2 Background 1

2.1 Context 1

2.2 The scheme 1

3 Assessment of alignment options 2

3.1 Introduction 2

3.2 A370 to A38 2

3.2.1 Introduction 2

3.2.2 Pre-consultation route 3

3.2.3 Option 1 - alignment east of Colliter’s Brook 4

3.2.4 Option 2 - alignment through Ashton Vale trading estate 4

3.2.5 Option 3 - alignment west of Hanging Hill Wood 4

3.2.6 Option 4 - alignment west of Colliter’s Brook 4

3.2.7 Options 5A and 5B - alignments west of pre-consultation route north of railway tying into option 3 or option 4 5

3.2.8 Comparison of options 5

3.2.9 Railway crossing 6

3.2.10 Conclusion 7

3.3 A38 to Highridge Road junction 7


3.4 King Georges Road 8


3.5 Reserved Corridor 9


3.6 Hareclive Road junction to Cater Road Roundabout 10

4 Assessment of junction options 12

4.1 A370 roundabout 12


4.1.2 Conclusion 14

4.2 Brookgate junction 14


4.2.2 Brookgate junction location 14

4.2.3 Conclusion - location 14

4.2.4 Brookgate junction performance 15

4.2.5 Conclusion – performance 16

4.3 A38 junction 16



4.3.2 Conclusion 18

4.4 Highridge Road junction 19


4.4.2 Conclusion 20

4.5 Queens Road junction 20


4.5.2 Conclusion 21

4.6 Hareclive Road junction 21


4.6.2 Conclusion 25

5 Assessment of provision for cyclists & pedestrians 26

5.1 Feedback from the Consultation Exercise 26

5.2 Development of the Scheme 26

6 Preferred planning application scheme 27

6.1.1 Preferred scheme 27

Appendix

Appendix A Technical notes - Alignments

A.1 Technical note no. 1 – Assessment of Option 2

Appendix B Technical notes – Junctions

B.1 Technical note no. 1 and Addendum - Hareclive Road Junction: Option Design / Assessment

B.2 Technical note no. 2 - Queens Road Junction: Option Design / Assessment

B.3 Technical note no. 3 - Highridge Road Junction: Option Design / Assessment

B.4 Technical note no. 4 and Addendum - A38 Bridgwater Road Junction: Option Design / Assessment

B.5 Technical notes no. 5 – Brookgate junction

B.6 Technical note no. 6 - A370 roundabout


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1 Introduction

1.1 Scope of this report

This report covers the alternative routes and junction options considered for the

South Bristol Link following the pre-planning application public consultation in May

and June 2012. It explains the advantages and disadvantages of each in terms of

engineering and gives the reasons for the preferred route to be taken forward to the

planning application. This assessment has been undertaken in the context of ensuring

that the route alignment meets the aims and objectives of the scheme and considers

its overall environmental and social impact.

Scheme development prior to the pre-planning application public consultation in

2012 is covered in the ‘Options Appraisal (Part 1): Route Alignment Options’ report.

1.2 Report format

The remainder of this Report comprises a further four chapters as follows:

Chapter 2 provides background information and explains the origin of the route

corridor under consideration.

Chapter 3 summarises the assessment of the alignment options considered,

culminating in the selection of the option to be taken forward to the planning

application.

Chapter 4 explains the junction assessment and the reasoning for the particular

selection at each location.

Chapter 5 presents the development of the scheme layout as it relates to pedestrians

and cyclists.

Chapter 6 summarises the reasons for the preferred scheme to be included with the

planning application.

2 Background

2.1 Context

The existing political, physical, landscape, environmental and urban constraints have

limited the extent and scope to look at significant deviations in alignment. Options

considered are generally within a 500m corridor through the rural section and along

the line of the existing road/reserved corridor in the urban area.

2.2 The scheme

The proposed South Bristol Link (SBL) will provide a transport link between the A370

Long Ashton bypass within North Somerset and Hengrove Park within the Hartcliffe

area of south Bristol. This will incorporate 4.5km of new and upgraded highway

between the A370, the A38 and the Cater Road roundabout, including a continuous

shared cycleway and footway along the route corridor.

The route includes a number of junctions with other roads at the A370, Brookgate, the

A38, Highridge Road, Queens Road and Hareclive Road. It also crosses the Network

Rail Bristol to Taunton line.


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The SBL is one of a package of transport schemes that together will create a rapid

transit network across the sub-region, linking key employment, housing and leisure

areas. The objectives for the scheme are to facilitate regeneration and growth in South

Bristol (whilst reducing congestion on surrounding roads) and to improve

accessibility from residential and employment areas in south Bristol to the city centre

and to the strategic transport network, including Bristol Airport.

The entire SBL route is to be classed as an Urban All-Purpose Road (UAP) in

accordance with TA 79/99.

3 Assessment of alignment options

3.1 Introduction

Design workshops were held on 2 July 2012, 31 August 2012 and 10 October 2012 to

discuss the various alignment options identified and to agree the alignment to be

included in the planning application as the preferred route. These alignment options

are explained in this chapter. The route has been divided into sections for ease of

reference. The sections are listed below:

• A370 to A38

• A38 to Highridge Road junction

• King Georges Road

• Reserved Corridor

• Hareclive Road junction to Cater Road roundabout

3.2 A370 to A38

3.2.1 Introduction

This section of the scheme is approximately 2km in length and runs south west from

the A370 close to Long Ashton Park & Ride to the A38 Bridgwater Road at Castle

Farm. The route corridor runs through a rural landscape close to urban development.

There is a proposed junction at Brookgate to provide access to Ashton Vale Trading

Estate and a crossing of the Bristol to Taunton railway line. The route then continues

through the Colliter’s Brook valley adjacent to Hanging Hill Wood before joining the

A38 at Castle Farm.

Through the consultation exercise, concern was expressed about the alignment in this

section, particularly south of the railway in the vicinity of Yew Tree farm. The

concern focussed on the impact on the farm and the environmental impact of the

alignment put forward.

In considering these concerns, 6 route options were examined in addition to the pre-

consultation route. The options are listed below:

Option 1 Alignment east of Colliter’s Brook

Option 2 Alignment through Ashton Vale trading estate

Option 3 Alignment west of Hanging Hill Wood

Option 4 Alignment west of Colliter’s Brook

Option 5A Alignment west of pre-consultation route north of railway tying into

option 3


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Option 5B Alignment west of pre-consultation route north of railway tying into

option 4

These routes are shown on figure 3.2.1.

Figure 3.2.1 A370 to A38 route options

All options have been designed to TD 9/93 and provide a two lane single carriageway

of 3.375m width with 3m bus lanes on both sides between Brookgate junction and the

A38. Curve widening has been applied in accordance with TD 9/93 on sections where

the radius is low.

3.2.2 Pre-consultation route

The pre-consultation route for this section in June 2012 sets the base line for the

options considered in this report. The alignment is shown on figure 3.2.1 and runs

from a junction on the A370 which is 300m west of the Long Ashton Park & Ride to a

junction with the A38 which is just to the east of Castle Farm.

This option runs south easterly through open fields from the A370 and crosses under

the railway line close to Colliter’s Brook. The route crosses the brook and runs on the

east side of the Colliter’s Brook valley then southwards east of Castle Farm where it

joins the A38.


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Except for option 2 all other routes considered are variations to the west of this route

and only option 5 differs in its alignment north of the railway line.

The options are described in the sections below.

3.2.3 Option 1 - alignment east of Colliter’s Brook

The main difference between this route and the pre-consultation route is that the

section south of the railway line through the Colliter’s Brook valley is moved up to

30m west and runs closer to the brook in an attempt to reduce severance of farm land

and avoid landfill sites to reduce stabilisation requirements.

The route is shown on figure 3.2.1 and a comparison of the advantages and

disadvantages is presented at the end of this section.

3.2.4 Option 2 - alignment through Ashton Vale trading estate

The route of option 2 is shown on figure 3.2.1. This option was put forward to utilise

existing roads and make use of an existing crossing point of the railway. The route

bisects Ashton Vale Trading Estate and the alignment design did not enable the use

of the existing railway crossing without demolition of existing properties. This option

was discounted and not considered further due to the impact on the trading estate

and the potential demolition of properties when other viable options were available.

See Appendix A.1 for ‘Technical note no. 1 - Assessment of Option 2’.

3.2.5 Option 3 - alignment west of Hanging Hill Wood

This route is the most westerly option considered and was introduced to reduce the

impact on Hanging Hill Wood. The route differs from the pre-consultation route

south of the railway. The route heads south west from the railway, avoiding crossing

Colliter’s Brook sidelong up the valley slope and passes over the Viridor landfill site

with a change in level of 60m. The route passes Hanging Hill Wood at the top of the

valley then continues south to join the A38 west of Castle Farm. All other options join

the A38 east of the farm.



3.2.6 Option 4 - alignment west of Colliter’s Brook

This route differs from the pre-consultation route south of the railway. The route

keeps to the west of Colliter’s Brook and takes advantage of the existing track that

accesses the Viridor landfill site. The route runs through the edge of Hanging Hill

Wood and then crosses Colliter’s Brook as it continues south to join the A38 east of

Castle Farm. This option reduces the severance of farm land.




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3.2.7 Options 5A and 5B - alignments west of pre-consultation route north of railway tying into option 3 or option 4

These routes cross over the railway 200m west of the pre-consultation route where

the railway embankment height is lower. The route runs from the same point on the

A370 but heads further west going over the railway. Then the route crosses the

Viridor land fill site to join either Option 3 or 4 as Options 5A and 5B respectively.

Option 5A stays towards the top of the Colliter’s Brook valley running to the west of

Hanging Hill Wood to join Option 3. Option 5B heads down the valley side through

the corner of Hanging Hill Wood to join Option 4.

The routes are shown on figure 3.2.1 and a comparison of the advantages and

disadvantages is presented below.

3.2.8 Comparison of options

Table 3.2.1 below sets out a comparison framework for the options, based on 7 key

environmental and social impacts, and presents the advantages and disadvantage for

each alignment. Where any impact is not regarded as an advantage or disadvantage

for a particular option it is simply recorded as neutral. It should be noted that the use

of the terms ‘lower’ and ‘higher’ are comparative terms between options and are not

an absolute statement on the extent of the impact.

Option 2 was excluded from this assessment process given the adverse impact the

option would have on the trading estate and the undesirable alignment resulting

from the crossing of the railway.

Option Advantages Neutral Disadvantages

Consultation

route

Lower impact on Colliter's Brook

Lower impact on Hanging Hill Wood

Impact on landfill Higher impact on Yew Tree Farm slope

Higher impact on ecological sites

Higher visual impact

Relatively straight (higher speeds)

1 Lower impact on landfill

Lower impact on Hanging Hill Wood

Lower visual impact

Impact on Yew Tree Farm slope

Impact to Colliter’s Brook

Impact on ecological sites


3 Lower impact on Colliter's Brook

Lower impact on Yew Tree farm slope

Lower impact on ecological sites

Sinuous alignment (reduced speed)

Impact on Hanging Hill Wood Higher impact on landfill


4 Lower impact on landfill


Lower visual impact




Higher impact on Hanging Hill Wood

5A Lower impact on Colliter's Brook



Lower impact on ecological sites

Impact on Hanging Hill Wood Higher impact on landfill


Additional connecting roads

5B Lower impact on landfill


Lower visual impact



Higher impact on Hanging Hill Wood


Additional connecting roads

Table 3.2.1 Initial Environmental/Social assessment of route from A370 to A38


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The results indicated that Options 1 and 4 had fewer disadvantages whilst the

consultation route and Options 5A and B had the most. It was agreed that Options 1

and 4 would be developed to make a further assessment of their suitability.

In particular, further topographical surveying was undertaken to allow a better

understanding of the impacts of both Options 1 and 4 in the vicinity of Colliter’s

Brook. There is an existing access track that runs to the west of the brook to serve the

landfill site immediately south of the railway and the focus of the assessment was to

determine whether the line of this track could be utilised while minimising the

impact on Hanging Hill Wood. This proved to be the case and hence Option 4 was

favoured over Option 1, the latter having a more significant environmental impact.

From the point where the SBL crosses Colliter’s Brook at Hanging Hill Wood the

route climbs up to the A38. A number of options for the vertical alignment were

considered in this area with a view to:

• facilitating the crossing of the SBL by cyclists and pedestrians,

• minimising the impact on the adjacent environment of Hanging Hill Wood

and Colliter’s Brook, and

• minimising the impact on the existing landfills.

The preferred option achieves these objectives by keeping the alignment close to

existing ground level, allowing passage of pedestrians and cyclists beneath the SBL at

Colliter’s Brook and minimising the footprint of the scheme and hence the

environmental impact on Colliter’s Brook.

3.2.9 Railway crossing

This section of the report discusses the reasons why the crossing of the Bristol to

Taunton railway has moved west of its original location in the pre-consultation

design.

On 7 November 2012, a value engineering design workshop for the railway crossing

was held to discuss the mitigation of the then current alignment’s impact on Bristol

and Wessex Water’s mains equipment. Three options were proposed for discussion;

options A to C, with a view to reducing the costs to divert the utilities. These options

are summarised below along with option D which moves the crossing further to the

west.

Option A - This was based on the bridge not moving

The cost to divert the water mains equipment would be in the region of £2.5m which

was deemed as significant and this option was discounted due to the additional cost.

Option B - Move the bridge to the east by narrowing the structure and reducing

carriageway width.

Although potentially the bridge could be moved to the east to avoid the diversion of

the water mains equipment this option was discounted due to flooding issues and

potential impact on the Brookgate industrial units.

Option C - Move the bridge to the west and onto Viridor land.


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This option requires the relocation of 2 ponds and encroaches further on to Viridor

land. Allowing for a 3m clearance from the western water main and adjustments to

connect into Brookgate this was a viable option with minimal impact on the water

mains equipment and was worth pursuing. This also allows for the retention of the

existing arch structure to provide a footway/cycleway route.

Option D - Relocate bridge to a new position (further west).

This was discounted due to English Heritage objections (visual impact from Ashton

Court) and the cost of constructing the road on the landfill.

From the points raised, it was evident that Option C advantages far outweigh any

advantages of the other options. There are also fewer disadvantages and hence

Option C was the favoured location to take forward.

3.2.10 Conclusion

The alignment development for this section of the SBL between the A370 and the A38

established a preferred route that significantly reduced farm severance and had less

environmental impact on account of following the alignment of an existing access

track.

3.3 A38 to Highridge Road junction

3.3.1 Introduction

This section of the scheme is approximately 1.1km in length and runs south east from

the A38 Bridgwater Road at Castle Farm to the Highridge Road junction at the south

east corner of Highridge common. The first 750m runs through farm fields whilst the

last 350m runs through and alongside Highridge Common. The final 250m is along

Highridge Green road.

The first 450m approximately follows the line of the pre-consultation route, although

the junction with the A38 is slightly further east to accommodate the lime kilns (see

Section 4.3 below). The central 400m has been realigned by introducing back to back

curves to accommodate comments made by landowners and residents in the

consultation process to reduce the impact on property and the common. The more

sinuous alignment also has the effect of reducing speed as you enter the common.

The last section has been moved from the pre-planning application alignment, which

was further into the common, to follow the line of the existing Highridge Green road

as much as possible, reducing the impact on the common.

Following confirmation of the proposed exchange land the distance between the

scheme and Highridge Cottage was increased from approximately 10m to 20m, by

moving the alignment northwards. This has provided a wider corridor and has

improved the connectivity between the existing common and the exchange land.

The preferred route is shown on figure 3.3.1 below.


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Figure 3.3.1 A38 to Highridge Road junction

3.4 King Georges Road

3.4.1 Introduction

This section of the scheme is approximately 500m in length and runs along the

existing King Georges Road. There is very little scope to change the alignment from

that of the existing road due to the proximity of the residential properties and the

constraints imposed by the junctions with Highridge Road at the northern end and

Queens Road at the southern end.

The consultation exercise identified a number of concerns about the layout of the SBL

in this section, principally focussed on severance, the potential loss of parking and the

impact on the residential setting of the road.

A number of cross-sections were therefore considered in order to achieve an optimal

scheme layout, providing effective passage for vehicles using the SBL while retaining

the urban/residential environment of the road. The initial thoughts were to have the

combined cycletrack/footpath and the footway immediately adjacent to the road but,

given the number of existing utilities that impact on the potential for tree planting,

the amount of landscaping is optimised by retaining the cycletrack and footways on

the existing alignments adjacent to the residential premises. The ability to maximise

the landscaping along this section of the route was a key consideration from the

feedback on the scheme.

Consideration was also given to the ease with which pedestrians could cross the road.

Alternatives including a full central reserve, a partial central reserve or no central

reserve were considered and the preferred option of a median strip with a number of

kerbed islands was considered to provide the optimum layout.


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The inclusion of the median strip for much of the length of King Georges Road

provides an informal waiting space for pedestrians choosing to cross the road whilst

allowing residents to access their driveways as they currently do. It also provides

space for overtaking in the event of vehicles breaking down. The 1.5m wide flush

central reserve will be finished in a contrasting coloured material. This colour

contrast reduces the negative visual impact of a wider black macadam surface

although it can be overrun by vehicles.

Formal pedestrian crossing points are provided at regular intervals along the road via

kerbed islands 2.0m wide. The islands have the appropriate dropped kerbs and tactile

paving with surface materials to match the central strip.

The preferred route is shown on figure 3.4.1 below and is considered to address the

concerns raised during the consultation exercise by making provision for those who

wish to cross the road and access driveways, while retaining an appropriate width for

vehicles using SBL.

Figure 3.4.1 King Georges Road

3.5 Reserved Corridor

3.5.1 Introduction

This section of the scheme is approximately 600m in length and runs through a

corridor that has been reserved for the route. The route will be at grade to limit the

impact on adjacent property.

In the northern half of this sub-section the alignment of the SBL is highly constrained

by adjacent premises, but as the route moves towards Hareclive Road the corridor

widens.

The consultation exercise identified severance as a key concern and this was the focus

of the subsequent design development.

Alternatives were considered in terms of where to locate the SBL within the corridor.

A more curved alignment than was presented at the public consultation in 2012 was

favoured in order to help reduce vehicle speeds and improve safety. This also

allowed the avoidance of significant underground water storage tanks. As with King


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Georges Road, consideration was also given to the cross-section of the road through

this sub-section. As more width is available it was concluded that the introduction of

a wide central reserve in the southern half would be preferable, splitting the

carriageway and hence reducing the perception of the width of carriageway. A

shared footway/cycleway runs parallel and adjacent to the north side of the

carriageway. The footway along the south side of the corridor deviates away from the

carriageway where the corridor is wider.

The planted central median helps to break up the dominance of the road through this

existing publically accessible green corridor. The median will contain park

style/ornamental shrub planting and semi mature trees. It also provides a central

median for pedestrian crossing facilities linking the residential areas on either side of

the reserved corridor. Within the central median two crossings will be non-signalised

and one will be signal controlled.

Where the footway is not adjacent to the westbound carriageway, kerbs will be

splayed and the grass verge will be reinforced to allow broken down vehicles to pull

off the road.

The preferred route is shown on figure 3.5.1 below and is considered to significantly

mitigate the concerns about severance and the creation of a ‘road’ corridor.

Figure 3.5.1 Reserved Corridor

3.6 Hareclive Road junction to Cater Road Roundabout

This section of the scheme is approximately 300m in length and runs through a

corridor adjacent to residential property on the south side and retail/commercial

property to the north. The route will be at grade to limit the impact on the adjacent

property.

The public consultation identified concerns relating to accessibility for pedestrians,

particularly at the junction with Hareclive Road, and access to the commercial

premises to the north.

The evaluation of this section of the SBL therefore focussed on the footprint of the

junction with Hareclive Road and this has been significantly reduced from the layout


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presented at the pre-application public consultation in May/June 2012. The proposed

roundabout has been replaced with a traffic signal controlled junction, greatly

improving the connectivity for pedestrians and hence addressing the key concern for

this part of the route.

As with the route alignment at Highridge Common, the alignment of the SBL has

been kept, as far as possible, on the alignment of the existing road to maximise the

opportunities for ‘greening’ the remaining corridor.

From the new signalised junction at Hareclive Road, the SBL will extend eastwards

on a realigned section of Whitchurch Lane. This will connect the SBL route to the

existing Cater Road roundabout. Access to existing commercial properties will be

provided by the use of a median strip to allow turning without causing a blockage,

hence addressing the concerns raised during the consultation.

Existing footways are present on either side of Whitchurch Lane in proximity to the

Hareclive Road junction. These footways will be re-provided as part of the

realignment proposals for Whitchurch Lane. These will feed into pedestrian crossing

points on all arms of the new Hareclive Road traffic signals.

Two new signal controlled crossings will aid the crossing of Whitchurch Lane for

access to the superstore.

The SBL will terminate at the Cater Road roundabout, with vehicles able to connect to

the existing highway.

The preferred route is shown on figure 3.6.1 below.

Figure 3.6.1 Hareclive Road junction to Cater Road Roundabout


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4 Assessment of junction options

4.1 A370 roundabout

4.1.1 Introduction

The pre-consultation design included a three arm roundabout for the junction with

the A370 located 300m west of Long Ashton Park & Ride.

At the Initial Design Workshop on 2 July 2012, it was agreed that the location of the

roundabout junction on the A370 would be reviewed following feedback from the

pre-planning application consultation. In particular, comments questioned whether

or not the junction could be moved closer to the Long Ashton Park & Ride facility.

This would also eliminate the need for a farm accommodation bridge for cattle to

cross the line of the route. The layout of this option is shown on figure 4.1.1 below.

The assessment of this option is given in Appendix B.6 ‘Technical note no. 6 - A370

roundabout’.

However, during the review process it was identified that this alternative crosses

land reserved for future extension to Long Ashton Park & Ride and it was agreed that

this option should be discounted.

Figure 4.1.1 A370 roundabout relocated

At the Design Workshop on 31 August 2012, it was agreed that the layout of the

roundabout junction on the A370 would be revised to take account of comments from

the Stage 1 Road Safety Audit and from North Somerset Council’s initial design

review. The layout of this option is shown on figure 4.1.2 below. The assessment of

this option is given in Appendix B.6 ‘Technical note no. 6 - A370 roundabout’.


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Figure 4.1.2 A370 roundabout- revised layout

At the third Design Workshop on 10 October 2012, there were some concerns raised

by North Somerset Council (NSC) about the revised location of the A370 junction

which was moved approximately 60m north-west of the pre-consultation design

location to meet stopping sight distance (SSD) requirements. It was thought that the

revised junction location could raise objections from English Heritage and the

residents of Long Ashton. Accordingly, a further revised layout was developed and is

shown on figure 4.1.3 below. The assessment of this option is given in Appendix B.6

‘Technical note no. 6 - A370 roundabout’.

Figure 4.1.3 A370 roundabout– preferred layout


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4.1.2 Conclusion

The assessment of these options is given in Appendix B.6, ‘Technical note no. 6 - A370

roundabout’ and concludes that the layout shown on figure 4.1.3 should be taken

forward as the preferred layout. This is because it is the best option in terms of

meeting the objectives of the scheme, being safer and performing better in operational

terms than the pre-consultation design.

4.2 Brookgate junction

4.2.1 Introduction

In addition to the junctions with the A370 and A38 there is an intermediate junction

to provide a connection at Brookgate to improve access to Ashton Vale Trading

Estate. This section describes the assessment of the location of the junction and the

layout in terms of optimum operational performance.

4.2.2 Brookgate junction location

At a meeting held with the landowners of Ashton Vale Trading Estate in August

2012, it was agreed with North Somerset Council to assess an alternative arrangement

for the Brookgate junction in order to avoid the area used for storage by some

businesses on the trading estate. The alternative layout is shown on figure 4.2.1

below.

Figure 4.2.1 Brookgate junction alternative

4.2.3 Conclusion - location

The assessment of this option is given in Appendix B.5 ‘Technical note no. 5A -

Brookgate junction: Location’ and concludes that the pre-consultation design should

be taken forward as the preferred location. By comparing the two junction’s


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advantages and disadvantages, the pre-consultation route has less impact on the

surrounding environment and is a better, shorter, alignment.

4.2.4 Brookgate junction performance

Two options have been considered for the preferred layout of Brookgate junction. The

only difference is the termination treatment proposed for the bus lane on the

northbound SBL approach, in order to assist buses in executing the right turn to the

bus link connection. Option 1 includes a bus gate whereas Option 2 does not. The

layout drawings for each option are shown in figures 4.2.2 and 4.2.3 below:

Figure 4.2.2 Brookgate junction Option 1


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Figure 4.2.3 Brookgate junction Option 2

4.2.5 Conclusion – performance

The assessment of these options is given in Appendix B.5 ‘Technical note no. 5B –

Brookgate junction: Option Design / Assessment’ and concludes that the Option 2

layout for the Brookgate junction should be taken forward.

4.3 A38 junction

4.3.1 Introduction

The junction with the A38 Bridgwater Road adjacent to Castle Farm is a key junction

on the proposed South Bristol Link route. The traffic forecasts predict high volumes

on both the A38 and South Bristol Link approaches and consequently a high level of

turning conflict. Three options have been considered as follows:

• Figure 4.3.1 Option 1 - Signalised Roundabout;

• Figure 4.3.2 Option 2 - Signalised Crossroads; and

• Figure 4.3.3 Option 1A - Signalised Roundabout (moved to avoid lime kilns).

Options 1 and 2 are considered in Appendix B.4 ‘Technical note no. 4 - A38

Bridgwater Road Junction: Options 1 and 2 Design / Assessment’. Option 1A is

considered in Appendix B.4 ‘Technical note no. 4 addendum - A38 Bridgwater Road

Junction: Option 1A Design / Assessment’.


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Figure 4.3.1 Option 1 - Signalised Roundabout

Figure 4.3.2 Option 2 - Signalised Crossroads


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Figure 4.3.3 Option 1A - Signalised Roundabout (moved to avoid lime kilns)

4.3.2 Conclusion

The assessment of these options is given in Appendix B.4 ‘Technical note no. 4 and

Addendum - A38 Bridgwater Road Junction: Option Design / Assessment’ and

concludes that the signalised roundabout layout (Option 1 or 1A) should be taken

forward as the preferred layout. This is because it offers far more in operational terms

than the signalised crossroads. The level of spare capacity will also ensure that a

degree of traffic growth can be accommodated in the weekday peak hours beyond

the opening year.

In terms of location the signalised roundabout layout Option 1A should be taken

forward as the preferred layout. This layout accommodates all local constraints and

allows the retention of the lime kilns and moves the roundabout further from the

Grade II listed Castle Farm. A local realignment of the existing A38 is necessary to

prevent the loss of the lime kilns but this is acceptable. In operating terms, the

junction provides the same level of reserve capacity as Option 1.


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4.4 Highridge Road junction

4.4.1 Introduction

Two options have been considered for the junction at Highridge Road, a main option

providing what is considered to be an optimum layout and a secondary option

catering for pedestrian crossings on all arms but with the smallest possible ‘footprint’

and hence impact on Highridge Common. The layout drawings for each option are

shown in figures 4.4.1 and 4.4.2 below:

Figure 4.4.1: Highridge Road junction Option 1

Figure 4.4.2: Highridge Road junction Option 2


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4.4.2 Conclusion

The assessment of these options is given in Appendix B.3 ‘Technical note no. 3 -

Highridge Road Junction: Option Design / Assessment’ and concludes that the

Option 1 layout for the Highridge Road junction should be taken forward. This gives

a much better level of reserve capacity (PRC) than Option 2. Furthermore, the level of

service afforded to pedestrians and cyclists is considerably enhanced.

4.5 Queens Road junction

4.5.1 Introduction

Two options have been considered for the junction at Queens Road, a main option

considered to provide the optimum layout and a secondary option with controlled

pedestrian crossings on all four arms of the junction. The layout drawings for each

option are shown in figures 4.5.1 and 4.5.2 below:

Figure 4.5.1: Queens Road junction Option 1


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Figure 4.5.2: Queens Road junction Option 2

4.5.2 Conclusion

The assessment of these options is given in Appendix B.2 ‘Technical note no. 2 –

Queens Road Junction: Option Design / Assessment’ and concludes that the Option 1

layout for the Queens Road junction should be taken forward. Whilst both options

give a similar operational performance it is considered that there would be safety

implications in including an additional crossing on the King Georges Road arm,

given the restricted visibility.

4.6 Hareclive Road junction

4.6.1 Introduction

Six options have been considered for the junction at Hareclive Road with the

intention of addressing concerns expressed through the public consultation relating

to accessibility and connectivity for pedestrians. The layout drawings for each option

are shown in figures 4.6.1 to 4.6.6 below:

Options 1 to 4 are considered in Appendix B.1 ‘Technical note no. 1 - Hareclive Road

Junction: Options 1 to 4 Design / Assessment’. Options 3A and 3B are considered in

Appendix B.1 ‘Technical note no. 1 addendum - Hareclive Road Junction: Option 3A

and 3B Design / Assessment’.

Option 1 was developed to provide a ‘minimal footprint’ for the SBL/Hareclive Road

junction; whilst also severing the existing Whitchurch Lane linkage along the Lidl

store frontage to the north of this new junction.

Option 2 is effectively the same as Option 1 except that the Whitchurch Lane linkage

north of the crossroads is maintained.


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Option 3 maintains the Whitchurch Lane linkage to the north but, in this case, 7 of the

12 vehicle movements at the crossroads junction are prohibited. This allows a

significant reduction in the ‘footprint’ of the crossroads junction; and offers a very

significant enhancement to the level of service afforded to pedestrians and cyclists

crossing at the junction.

Option 4 again retains the Whitchurch Lane linkage to the north. Unlike Option 3 the

crossroads layout in this case retains 10 of the possible 12 vehicle movements; with

only the right turn from the SBL(E) approach and the left turn from the Hareclive

Road(N) approach prohibited.

Option 3A essentially retains the layout at the SBL/Hareclive Road junction

developed in Option 3. However, there are a number of key changes affecting the

surrounding highway layout; needed in part to accommodate traffic flows prevented

from executing turning manoeuvres at the new crossroads junction.

The Option 3B variant differs only in that the left turn from Hareclive Road(S) is

additionally prohibited at the new SBL junction. This imposes a further restriction on

vehicle movements but does result in a further reduction to the junction ’footprint’.

Figure 4.6.1: Hareclive Road junction Option 1


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Figure 4.6.5: Hareclive Road junction Option 3A


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Figure 4.6.6: Hareclive Road junction Option 3B

4.6.2 Conclusion

The assessment of these options is given in Appendix B.1 ‘Technical note no. 1 and

Addendum - Hareclive Road Junction: Option Design / Assessment’. The first

technical note concludes that the Option 4 layout for the Hareclive Road area

provides the best compromise between making adequate provision for local vehicle

access and providing a suitable level of service in respect of the crossing facilities for

pedestrians and cyclists. ‘Technical note no. 1 Addendum - Hareclive Road Junction:

Option 3A and 3B Design / Assessment’ was subsequently prepared to address

comments by BCC officers to reduce the footprint of the junction. This concluded that

the Option 3A layout, which retains the left turn from Hareclive Road (S), is

preferable and that this arrangement is taken forward as the ‘preferred’ design for

this section of the South Bristol Link. Further discussions with BCC and undertaking

a Road Safety Audit have resulted in the introduction of bus lay-bys on Hareclive

Road north of the junction.


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5 Assessment of provision for cyclists & pedestrians

5.1 Feedback from the Consultation Exercise

The extensive consultation exercise in May and June 2012 identified a number of

concerns relating to the proposed provision for pedestrians and cyclists along the

length of the route.

These can be summarised as follows:

• Connectivity to existing facilities

• Ease of passage along the route, with the combined cycletrack/footway

crossing from one side of the SBL to the other at two locations

• Transverse severance created by the introduction of the SBL

As part of the scheme development, these concerns were considered and workshops

were held with the Non-Motorised User groups. The amendments to the layout

resulting from this process are described below.

5.2 Development of the Scheme

A number of changes to the layout presented for consultation have been made to

address the concerns raised and these are described below.

• At the northern end of the scheme, adjacent to the A370, the shared use

cycletrack/footway has been extended to connect to Festival Way.

• The shared use facility has also been provided adjacent to the bus link that

connects to the AVTM scheme, further improving connectivity.

• Diversions to existing Public Rights of Way (PROW) have been included

where appropriate and where possible these have been provided beneath the

proposed SBL to reduce potential points of conflict. Examples are at

Longmoor and Colliter’s Brook underbridges.

• Footway provision at Brookgate has been revised to improve connectivity.

• Along the entire length of the scheme the shared facility has been located on

the one side (to the north/east) greatly simplifying the longitudinal passage for

pedestrians and cyclists, particularly through the junctions.

• All junction designs have been reviewed to improve the passage for

pedestrians, avoiding all-red phases wherever possible. Junction footprints

have been minimised to improve the connectivity for pedestrians.

• In King Georges Road, a median strip has been included to provide an

informal crossing facility along the length of this section. Five formal

pedestrian islands have also been included.

• In the reserved corridor, transverse connectivity has been maintained by

providing signalised and non-signalised crossing points. A wide median has

been included to facilitate the crossing of the road in this area.


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• The proposed roundabout at Hareclive Road has been replaced by a signalised

junction with a small footprint, greatly improving connectivity between the

residential areas and the superstore.

Along the alignment the project considered options whereby existing Public Rights of

Way could be upgraded for cyclist use. However, given constraints such as

environmental impacts and the need to reduce the impacts upon landowners the

chosen alignment, parallel to the highway, is considered the most appropriate

alignment.

6 Preferred planning application scheme

6.1.1 Preferred scheme

Following on from the pre-planning application public consultation in May and June

2012, a detailed review of the scheme was undertaken. This review considered

ongoing engineering and environmental considerations, feedback received from the

consultation and ongoing discussions with the various scheme stakeholders.

The scheme layout has therefore been developed with regard to optimising:

• The alignment, with particular regard to the section between the A370 and the

A38.

• The junction layouts, with particular consideration to achieving a balance

between traffic capacity, facilities for pedestrians and cyclists and junction

footprint.

• The route for pedestrians and cyclists along the route by keeping the shared

facility to one side.

• The context of the scheme by integrating the engineering design with the hard

and soft landscaping design.

This process has culminated with the scheme layout included with the planning

application and illustrated on the scheme layout drawings.


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Appendix A

Technical Notes - Alignments


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Appendix A Technical notes - Alignments

A.1 Technical note no. 1 – Assessment of Option 2

Date: 20 September 2012

Introduction

At the Design Workshop on 31st August 2012, it was agreed that the proposed route

options for the South Bristol Link would be further explored to find the best option.

During this process, Option 2, which travels south of the railway and cuts through

Brookgate in Ashton Vale Trading Estate, has been discounted because the route is

not achievable for a number of reasons.

This technical note justifies the decision to discount Option 2.

The following are reasons in favour of Option 2

Follows existing road north of railway

Option 2 makes use of the existing route - Brookgate - through Ashton Vale Trading

Estate reducing material and construction costs. This would also promote and help

businesses within the trading estate due to exposure to motorists and commuters on

the South Bristol Link.

Avoids Colliter’s Brook

This option also avoids Colliter’s Brook which will reduce the environmental impacts

of the scheme. The other options – Options 1 and 4 both lie within close proximity to

the brook and will have significant environmental and possibly financial implications

to the scheme.

Reduces impacts on landfill

Option 2 also avoids the Yanley Farm landfill sites which is an advantage over

Options 3 and 4 which pass through the landfill sites. This would not only reduce the

financial costs due to the need to remediate or dispose of contaminated landfill

materials but also the buildability of the road would be improved and the

environmental implications would be reduced. Future maintenance or monitoring

costs of the road through contaminated areas will also be avoided making this a

favourable route.

The following are reasons against Option 2

Bisects the trading estate

Option 2 divides the trading estate making it an undesirable option.

Longer route

This route through the trading estate is a longer route compared to Options 1 and 4.

This would be a disadvantage for this option from a financial point of view even if

the existing road (Brookgate) is used. The route through Brookegate will need to be

widened and upgraded to make it suitable for the level of expected traffic on the SBL.


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Poor alignment at railway crossing

Road constructions at railway crossings are made much easier if the road is

perpendicular to the road. This helps with the buildability of the scheme and

improves visibility and safety for motorists. The proposed railway crossing lies on a

bend due to the tight constraints making the alignment unsafe and difficult to

construct. The bend is also a much tighter radius (160m for a Design Speed of 70kph)

which is a Departure from standards further adding to road safety concerns.

Likely to give rise to objection from Yew Tree farm

The route will also cut through land owned by Yew Tree farm which will likely give

rise to an objection should this option be progressed. This would affect the farms

herding activities, making it an undesirable route.

High cost of works on hillside

The associated construction costs of this route will be significant due to the level of

cut required through the hillside to meet geometric design standards. Compared to

Options 1, 2 and 3, the level of cut and fill is much greater for Option 2. The

maximum grades are also steep (beyond the max of 6% for roads and 5% for cycle

routes) making it a Departure from standards. This is not desired by the Client.

Land acquisition for cyclists

In additional to the above points, the layout/route for cyclists north of the rail

crossing has to be on the carriageway due to the limited available space through

Brookgate. Cycle routes on the carriageway are not a desired option due to road

safety concerns.

Conclusion

From the points raised, it is evident that Option 2 has some advantages but the

disadvantages far outweigh the advantages. The route option is not feasible for

further investigation due to the design constraints, cost implications and associated

design issues.

Appendix B

Technical Notes - Junctions

Appendix B Technical notes – Junctions Excluding annexes with drawings and traffic analysis data, the former being included in the main report text as figures.

B.1 Technical note no. 1 and Addendum - Hareclive Road Junction: Option Design / Assessment

Date: 17 December 2012

Section 1 - Introduction

Background

Halcrow has been commissioned by North Somerset Council (NSC) to progress the

design of the South Bristol Link road from its proposed junction with the A370 Long

Ashton Bypass to Cater Road Roundabout. As a significant length of the proposed

route lies within Bristol City Council’s (BCC) administrative boundary the

preliminary scheme design has involved close liaison with BCC officers. Taking on

board comments arising from earlier public consultation on an alignment and

junction layouts developed by Mott MacDonald; the required outcome of the current

work is to produce a robust design for the scheme to preliminary design level for

inclusion in a planning application.

The route alignment interfaces with existing roads and junctions in five key locations

as follows:

• A370 Long Ashton Bypass;

• A38 Bridgwater Road;

• Highridge Road / Highridge Green;

• Queens Road; and

• Hareclive Road

In addition to this there is proposed junction between the A370 and A38 to provide a

linkage to Brookgate; in order to achieve improved access to Ashton Vale Trading

Estate. This Technical Notes describes the various options considered for the

Hareclive Road junction; and the results of operational assessments undertaken using

LINSIG. The flows used in each case have been provided by Atkins for the 2016

‘Opening Year’ scenario. For comparison purposes; only the weekday AM and PM

peak hours have been considered.

Section 2 - Options Considered

General

Four options have been considered. The layout drawings for each are included in

Annex A; and are as follows:

• Drawing No CTR-AEB-730 SK401: Option 1;

• Drawing No CTR-AEB-730 SK402: Option 2;

• Drawing No CTR-AEB-730 SK403: Option 3; and

• Drawing No CTR-AEB-730 SK404: Option 4.

The following sub-sections describe the features of each of these options; and any

potential issues in respect of design standards or vehicle/pedestrian accessibility

where they exist. Operation in traffic terms is covered specifically in Section 3.

Option 1

Option 1 shown in Drawing No CTR-AEB-730 SK401 was developed following

comments from the BCC planners about seeking a ‘minimal footprint’ for the

SBL/Hareclive Road junction; whilst also aiming to sever the existing Whitchurch

Lane linkage along the Lidl store frontage to the north of this new junction. Severance

of the ‘Whitchurch Lane linkage’ would have the effect of increasing the traffic

pressure on the crossroads; as forecast flows between Whitchurch Lane and Cater

Road Roundabout are predicted to remain significant with the SBL in place.

Furthermore, to maximise the open space created it would be desirable to re-locate

the access into Lidl to the western end of their frontage as shown on the drawing.

This would obviously require some alterations to traffic circulation and parking

within the Lidl site; which may not be acceptable to the retailer.

The layout aims to accommodate all turning movements and, because the right turn

from the SBL(E) will be significant with severance of the Whitchurch Lane linkage, a

specific filter phase would be required to deal with this. In addition a right turn lane

of reasonable length would be needed to ensure that waiting right turners making an

‘opposed’ gap-seeking turn in the main road stage do not impede left and straight-

head vehicles.

One major disadvantage of this layout from the pedestrian viewpoint is that the

controlled crossings can only be accommodated with a full pedestrian stage, with a

limited green man time for each crossing in what could be a long cycle time with four

stages likely to appear cyclically. Another potential issue is exit blocking into the

junction as a result of right turners attempting to access the Lidl access; thus

impeding other traffic leaving the junction via Hareclive Road(N). However, the

latter is only likely to happen during Stage 3 when the two Hareclive Road

approaches run concurrently. During other stages vehicles on the Hareclive Road (N)

approach will be stopped and provided the yellow box zone is not obstructed, right

turners will be able to turn in relatively unimpeded.

Option 2

Option 2 shown in Drawing No CTR-AEB-730 SK402 is effectively the same as Option

1 except that the Whitchurch Lane linkage north of the crossroads is maintained. This

will have the effect of removing a significant amount of traffic which would

otherwise be required to route via the crossroads (Option 1). The traffic figures

provided by Atkins for 2016 show that, in the AM peak hour, the westbound

movement from Cater Road roundabout to Whitchurch Lane is 400pcu/hr; with the

same volume making the eastbound movement from Whitchurch Lane towards the

roundabout. In the PM peak hour the forecast volumes are 450pcu/hr and 350pcu/hr

respectively.

Maintaining the linkage to the north would permit the right turn from the SBL(E) and

the left turn from Hareclive Road(N) to be prohibited. It would remove the need for

the right turn filter stage for the SBL(E) approach; so reducing the normal sequence of

stages to three.

However, a major disadvantage with this layout from the pedestrian viewpoint

remains the fact that the controlled crossings can only be accommodated with a full

pedestrian stage, with a limited green man time for each crossing in what could still

be a long cycle time.

Option 3

Option 3 shown in Drawing No CTR-AEB-730 SK403 is a ‘worked up’ version of a

layout provided by the Traffic Signals team at BCC. This maintains the Whitchurch

Lane linkage to the north but, in this case, 7 of the 12 vehicle movements at the

crossroads junction are prohibited. This allows a significant reduction in the

‘footprint’ of the crossroads junction; and offers a very significant enhancement to the

level of service afforded to pedestrians and cyclists crossing at the junction.

The main disadvantage of this layout is the restriction it places on vehicle

movements. This will increase the number of vehicles using the section of

Whitchurch Lane to the north, as well as increasing traffic pressure on the two major-

minor ‘Y’ junctions with the SBL and Hareclive Road respectively. This impact is

considered further in Section 3 when discussing likely operational and diversionary

impacts.

One potential safety risk with this layout is non-compliance with the turn bans;

particularly where contravening the restriction is easy and the diversion route

inconvenient. The left turn movement from the SBL(E) to Hareclive Road is a case in

point. Any such non-compliance would put pedestrians at risk, as the crossings run

‘in parallel’ with the main east-west and north-south traffic movements. To

discourage and make it harder for vehicles to execute the right turns from the

Hareclive Road approaches a central island is shown on the two SBL arms. As the

two SBL arms and the two Hareclive Road arms run concurrently in each case; the

presence of ‘opposing’ vehicles may act as a sufficient deterrent to drivers attempting

a right turn in violation of the restrictions. However, there may still be a temptation

for drivers to contravene the banned turns in low flow conditions if the alternative

route is viewed as inconvenient or lengthy.

Option 4

Option 4 shown in Drawing No CTR-AEB-730 SK404 again retains the Whitchurch

Lane linkage to the north. Unlike Option 3 the crossroads layout in this case retains 10

of the possible 12 vehicle movements; with only the right turn from the SBL(E)

approach and the left turn from the Hareclive Road(N) approach prohibited. These

movements are conveniently and more directly accommodated by the Whitchurch

lane linkage; which also serves to remove circa 800pcu/hr from the crossroads in the

weekday peak hours (2016).

The cross-roads layout provides controlled crossing routes on all four arms of the

junction. Unlike Options 1 and 2; these crossings can be arranged to operate on a

‘walk-with-traffic’ basis by the insertion of traffic islands into the layout as shown on

the drawing. This will significantly enhance the level of service offered to non-

motorised users (NMU); although not to the level afforded by Option 3.

The Option 4 layout attempts to be a ‘win all’. It does not rely on the large number of

banned vehicle movements which may prove to be unpopular with local drivers.

Furthermore, it is considered to provide a more than adequate level of service for

NMU; certainly far in excess of that possible with a full pedestrian stage in Options 1

and 2. The necessary addition of traffic islands to incorporate crossings on a walk-

with-traffic’ basis does make this crossroads ‘footprint’ larger; but not excessively so

when compared with Options 1 and 2.

Section 3 - Traffic Operation

General

The likely traffic operation of the signalled cross-roads options has been assessed

using LINSIG. Summaries of the results obtained are included in Annex B. Potential

operation of the two major-minor ‘Y’ junctions to the north of the crossroads with

Options 2, 3 and 4 is not specifically assessed. However, a qualitative assessment is

made given knowledge of the likely level of the ‘give way’ volumes and typical

maximum capacities attainable for these (PICADY).

Option 1

The summary LINSIG results in Annex B show that this option would fail to achieve

a satisfactory operational performance. In the AM peak hour the predicted Practical

Reserve Capacity (PRC) is -26% using a maximum cycle time of 120 seconds. The PRC

in the PM peak is -23.1% using the same high cycle time. Notwithstanding the

queuing and delay impacts for drivers the level of service afforded to pedestrians and

cyclists will be very poor; with only a small green man ‘window’ available every two

minutes. For movements requiring the use of two crossings, such as Hareclive Road

(S) west side to Lidl, the potential wait delay will be very significant.

The LINSIG results are perhaps not surprising; as the severance of the Whitchurch

Lane linkage to the north results in a large increase in the volume of traffic required

to route via the crossroads. Given its very poor performance in operational and

‘pedestrian convenience’ terms this option is not considered to be practicable.

Option 2

The summary LINSIG results in Annex B show that this option would achieve a

satisfactory operational performance. In the AM peak hour the predicted Practical

Reserve Capacity (PRC) is +12.6% using a cycle time of 70 seconds. The PRC in the

PM peak is +15.5% using the same low cycle time. The uplift in performance

compared to Option 1 is due to the displacement of circa 800 vehicles to the

Whitchurch Lane ‘link’; and the subsequent ability to remove the right turn filter

stage for the SBL(E) approach from the cyclic sequence.

Notwithstanding the ability of this layout to cater for the expected vehicle demands;

there remains a concern about its performance with respect to ‘wait’ times and delay

for pedestrians and cyclists. As with Option 1 the crossing movements are only

catered for in the full pedestrian stage. Whilst the low cycle time possible will assist

in reducing the wait time for this stage to appear, the delay for pedestrians using

routes across the junction requiring more than one crossing point is potentially high.

For routes involving the use of one crossing the potential wait time would be

between 9 - 63 seconds with a 70 second cycle time in operation. However, for routes

requiring two crossing points this wait time range would increase to 72 - 126 seconds

for the same cycle time; as pedestrians would be unable to cross both in the same

pedestrian stage due to the short ‘window’ time given to the green man.

It is considered that, despite a workable layout in traffic terms, this option fails to

provide a sufficient level of pedestrian/cyclist convenience and accessibility. This is

demonstrated by the wait time delays which could be experienced with even a low

(70 second) cycle time in operation. In view of this it is considered that this option

should not be considered further. To give an acceptable level of NMU accessibility

such crossing facilities need to be incorporated on a ’walk with traffic’ basis; thus

affording the opportunity to give longer green man times in the operational cycle

time.

Option 3

Not unexpectedly the summary LINSIG results in Annex B show that this option

would achieve a high level of operational performance at the crossroads. In both peak

hours the predicted Practical Reserve Capacity (PRC) is +32.6% with a cycle time as

low as 40 seconds. This is due to the ‘conflicts’ removed with the banned turns, which

allows the junction to operate with just two signal stages. These banned turns also

displace some traffic from the junction entirely. The Atkins forecasts show that; in

addition to the movements between the SBL(E) and Whitchurch Lane mentioned

previously, the other significant turning movements which would be affected by the

banned turns proposed are as follows:

• Hareclive Road(S) to SBL(E) - circa 100-150pcu/hr in both peak hours; and

• SBL(E) to Hareclive Road(S) - circa 150pcu/hr in both peak hours.

In the LINSIG assessments these vehicles are assumed to use the ‘local’ alternative

route via the Whitchurch Lane link to the north; and so still route via the junction via

the north-south Hareclive Road axis. Despite this the junction is shown to operate

with considerable reserve capacity and; given the low cycle time and ‘walk with

traffic’ operation of the pedestrian crossings, the level of service for NMU will be very

good.

The concern with this option is the traffic displacement effects; particularly the likely

effect on the two major-minor ‘Y’ junctions at either end of the Whitchurch Lane

linkage. As previously stated the expected westbound right turn flow into this link

from the SBL without the added effect of additional displaced traffic is circa 400-

450pcu/hr. Adding in the extra traffic displaced by the banned left turn from the

SBL(E) to Hareclive Road(S) would increase this demand to 550-600pcu/hr. The

maximum capacity for a ‘give way’ right turn from the major road at a priority

junction is 700-800pcu (PICADY); this ‘maximum’ based on negligible opposing flow.

This is hardly likely to be the case here and, with expected demand about 77% of the

theoretical maximum capacity, it is highly probable that queuing and delay would

occur. Potential likely outcomes are:

• Right turn queuing would extend beyond the right turn lane; impeding

westbound ‘through’ traffic on the SBL;

• Some drivers may try to avoid this delay by continuing to the crossroads and

making a left turn into Hareclive Road, in violation of the banned turn; and

• Drivers would be encouraged to use Whitland Road or other residential roads

further south in the Hartcliffe estate to avoid these potential delays. This is

obviously less than desirable as the SBL should ideally seek to remove traffic

currently using residential roads; not add to the problem.

The potential ‘displacement’ problems in the eastbound direction would most likely

mirror this; as drivers prevented from turning right from Hareclive Road(S) to the

SBL(E) would add to the left turn minor road ‘give way’ demand at the same ‘Y’

junction with the SBL to the east of the crossroads. In the AM peak hour this demand

could rise to 550pcu/hr; and in the PM peak circa 450pcu/hr. The maximum ‘give

way’ capacity for a left turn from a minor road is circa 700pcu; so it is highly likely

that this level of demand will create congestion problems. As a result, it is highly

likely that drivers will be encouraged to use Whitland Road or other residential roads

in this direction as well.

Option 3; whilst very good considered in ‘isolation’, has the potential to create some

significant operational problems at other local junctions and to promote the

undesirable use of adjacent residential roads by displaced traffic. For this reason

alone the further consideration of this option is not recommended; even though the

crossroads itself is shown to have considerable reserve capacity and a high level of

service for pedestrians. It is not considered that the risk of these displacement effects

can be justified by the very high PRC which the junction does nevertheless achieve.

Option 4

The summary LINSIG results in Annex B also show that this option would achieve a



PM peak is +15.7% using the same low cycle time.

This layout avoids the displacement effects discussed in respect of Option 3; and still

produces a positive reserve capacity using a modest cycle time. The PRC would be

higher with a longer cycle time; say 120 seconds, but the shorter cycle time will

reduce potential wait time for pedestrians. As stated earlier all the pedestrian

crossings have been arranged to operate on a ’walk-with-traffic’ basis; with facilities

available on all four arms. It is accepted that the crossing time available over the

SBL(E) arm may be limited since it only runs in Stage 3 - Hareclive Road(N); which

could be short. However, the arrangement of signal stages gives good pedestrian

accessibility and a minimal wait time in both directions for the SW-NE movement via

the crossings provided on the western and northern arms of the junction. It is

accepted that the service of service for pedestrians and cyclists is not as good as

Option 3; but this layout does not rely on the numerous banned turns. This layout is

considered to provide the best compromise between making adequate provision for

local vehicle access and providing a suitable level of service in respect of the crossing

facilities for pedestrians and cyclists.

Section 4 - Conclusions

This Technical Note has considered four different layout options for the

SBL/Hareclive Road junction and the associated Whitchurch Lane linkage to the

north. Operational assessments have been undertaken using LINSIG and the results

presented in Annex B. Drawings showing the four options are included in Annex A.

The conclusions relating to the four options are as follows:

Option 1

• This layout fails badly in operational terms. The results demonstrate that

maintaining the Whitchurch Lane linkage to the north will be critical to the

performance of the crossroads junction. Removing it could add around another

800 vehicles to the junction demand in the peak hours (2016); and

• The required use of a full pedestrian stage fails to give a satisfactory level of

service for pedestrians and cyclists attempting to cross the SBL in this location.

Option 2

• LINSIG results show that this arrangement would operate satisfactory;

potentially with a cycle time as low as 70 seconds in the two peak hours (2016);

but

• Notwithstanding the above, required use of a full pedestrian stage will still fail

to give a satisfactory level of service for pedestrians and cyclists attempting to

cross the SBL in this location.

Option 3

• LINSIG results show that this arrangement would operate very well due to the

vehicle-vehicle and vehicle-pedestrian conflicts removed by banning turns. A

high level of service for pedestrians would be achieved; but

• Notwithstanding the above, traffic displacement created by banning 7 of the 12

vehicle movements possible will create a lot of additional pressure on the two

major-minor ‘Y’ junctions to the north. A comparison of the expected ‘give

way’ flows at the eastern SBL/Whitchurch Lane junction with maximum

capacities achievable under negligible opposing flow conditions (PICADY)

suggests that this junction would fail in operating terms. The likely outcome is

diversion to Whitland Road or other residential roads though Hartcliffe to the

south.

Option 4

• LINSIG results show that this arrangement would operate satisfactory;

potentially with a cycle time as low as 80 seconds in the two peak hours (2016);

• The layout provides controlled crossing facilities on all arms of the junction on

a ’walk-with-traffic’ basis. This combined with the low cycle time possible will

give a satisfactory level of pedestrian accessibility across the SBL; albeit not as

good as Option 3; and

• Option 4 caters for 10 of the 12 vehicle movements at the crossroads. The two

prohibited movements are conveniently and more directly catered for by the

Whitchurch Lane linkage to the north. As such, the potential displacement

effects described for Option 3 would be avoided.

On the basis of the above conclusions it is considered that the Option 4 layout for the

Hareclive Road area provides the best compromise between making adequate

provision for local vehicle access and providing a suitable level of service in respect

of the crossing facilities for pedestrians and cyclists. It is thus recommended that this

arrangement is taken forward as the ‘preferred’ design for this section of the SBL.

Technical note no. 1 Addendum - Hareclive Road Junction: Options 3A and 3B Design / Assessment

Date: 18 February 2013


Background


design of the South Bristol Link Road from its proposed junction with the A370 Long








This Technical Note is an Addendum to Technical Note 1 (TN1) issued on 17

December 2012 which examined various options for the Hareclive Road junction.

TN1 considered four options, with Option 4 recommended. However, following

consideration by BCC and a follow-up meeting; officers expressed a preference for

Option 3, which prohibits 7 of the possible vehicle movements at the SBL/Hareclive

Road junction. TN1 identified a need for further changes to adjacent junctions should

Option 3 be taken forward as the preferred layout. The two variants described below

incorporate these changes.


General

Two variants to Option 3 have been considered. The layout drawings for each are

included in Annex A; and are as follows:

• Drawing No CTR-AEB-730 SK403A: Option 3A; and

• Drawing No CTR-AEB-730 SK403B: Option 3B.

The following sub-sections describe the changes made to Option 3 in each case; and

any potential issues in respect of design standards or vehicle/pedestrian accessibility

where they exist. Operation in traffic terms is covered specifically in Section 3.

Option 3A

Option 3A shown in Drawing No CTR-AEB-730 SK403A essentially retains the layout

at the SBL/Hareclive Road junction developed in Option 3. However, there are a

number of key changes affecting the surrounding highway layout; needed in part to

accommodate traffic flows prevented from executing turning manoeuvres at the new

crossroads junction. These are as follows:

• The SBL/Whitchurch Lane junction to the east of Hareclive Road is signalled.

This is necessary to accommodate the higher traffic flow turning right into

Whitchurch Lane and the corresponding left turn out. The left turn into and the

right turn from Whitchurch Lane are both prohibited; which would allow the

junction to operate with a simple two stage Method of Control. Controlled

crossings are provided over the SBL(W) and Whitchurch Lane arms;

• The former alignment of Hareclive Road north of the new SBL junction is

retained; and so the existing priority junction with Whitchurch Lane. Unlike

the previous Option 3 layout; the traffic movements on Whitchurch Lane are

now ‘controlling’, with drivers on the Hareclive Road approach required to

‘give way’; and

• Existing bus stops on Hareclive Road affected by the new SBL crossroads

junction are shown re-located to the north; in the short length of Hareclive

Road between this new junction and Whitchurch Lane.

Option 3B

The Option 3B variant shown in Drawing No CTR-AEB-730 SK403B differs only in

that the left turn from Hareclive Road(S) is additionally prohibited at the new SBL

junction. This imposes a further restriction on vehicle movements but does result in a

further reduction to the junction ’footprint’.


General

The likely traffic operation of both variants has been assessed using LINSIG.

Summaries of the results obtained are included in Annex B. This includes the likely

operation of the SBL/Whitchurch Lane ‘Y’ junction to the east of the SBL/Hareclive

Road junction as well. In the results these are denoted as Controller 2 (C2) and

Controller 1 (C1) respectively.

Option 3A

The summary LINSIG results in Annex B show that that both junctions would

achieve a satisfactory operational performance. In the AM peak hour the predicted

Practical Reserve Capacities (PRC) for controllers C1 and C2 are +45.1% and +52.4%

using a cycle time of 60 seconds. The PRC in the PM peak are +43.2% and +45.1%

using the same low cycle time.

With the low cycle time possible at both junctions the level of service for pedestrians

will be very good.

Option 3B

Not unexpectedly, the summary LINSIG results for Option 3B, also in Annex B, show

a similar level of performance to variant 3A. In the AM peak hour the predicted

Practical Reserve Capacities (PRC) for controllers C1 and C2 are again +45.1% and

+52.4% using a cycle time of 60 seconds. The PRC in the PM peak are +46.6% and

+52.4% using the same low cycle time.

As with Option 3A the level of service for pedestrians will be very good.


Following BCC comment on TN1 this Addendum Technical Note (TN6) has

considered two variants to the Option 3 layout preferred by officers. Operational

assessments have been undertaken using LINSIG and the results presented in Annex

B. Drawings showing the two variant options (3A and 3B) are included in Annex A.

The conclusions arising are as follows:

Option 3A

• The SBL/Hareclive Road and SBL/Whitchurch Lane junctions both perform

satisfactory using the 2016 AM/PM predicted flows provided by Atkins. The

low cycle time achievable in each case would enable a high level of service to

be provided for pedestrians and cyclists; and

• Existing bus stops lost on Hareclive Road due to the new junction can be

satisfactory re-located in the short length of Hareclive Road between this

junction and Whitchurch Lane.

Option 3B

• As with Option 3A the SBL/Hareclive Road and SBL/Whitchurch Lane

junctions both perform satisfactory using the 2016 AM/PM predicted flows

provided by Atkins.; and

• Additionally prohibiting the left turn from Hareclive Road (S) to the SBL does

not ‘add’ to the level of pedestrian accessibility achievable; it is already very

good with Option 3A. The only benefit of banning this turn is the reduced

junction ‘footprint’. However, this has to be balanced against the potential

increased use of Gatehouse Road in the westbound direction by local drivers

attempting to access the westbound SBL via the Queens Road junction.

On the basis of the above conclusions it is considered that the Option 3A layout

which retains the left turn from Hareclive Road (S) is preferable. Further restricting

driver accessibility at the new cross-roads to achieve a further small reduction in the

junction ‘footprint’ is not considered a sensible ‘balance’; given the need to reduce

potential traffic routing along Gatehouse Road.

B.2 Technical note no. 2 - Queens Road Junction: Option Design / Assessment

Date: 20 December 2012


Background











as follows:





• Hareclive Road


linkage to Brookgate; in order to achieve improved access Ashton Vale Trading

Estate. This Technical Notes describes the various options considered for the Queens

Road junction; and the results of operational assessments undertaken using LINSIG.

The flows used in each case have been provided by Atkins for the 2016 ‘Opening

Year’ scenario. For comparison purposes; only the weekday AM and PM peak hours

have been considered.


General

Two options have been considered; a main option and a secondary option examined

to assess an aspiration of the planners at BCC (Option 2) for controlled pedestrian

crossings on all four arms of the junction. The layout drawings for each are included

in Annex A; and are as follows:



The following sub-sections describe the features of each of these options; and any

potential issues in respect of design standards or vehicle/pedestrian accessibility

where they exist. Operation in traffic terms in covered specifically in Section 3.

Option 1

Option 1 shown in Drawing No CTR-AEB-730 SK405 was developed from an initial

layout provided by the signals team at Bristol City Council (BCC). The only

significant difference from this BCC concept layout is the additional prohibition of

the left turn from the SBL(E) approach into Queens Road(S). This was considered

necessary to satisfactorily accommodate the vehicular access into the club car park on

the Queens Road(S) arm, whilst still allowing the pedestrian crossing on this

approach to run concurrently with westbound traffic on the SBL in Stage 1 of the

signal control sequence. In the former BCC layout this left turn was accommodated

by a filter lane joining Queens Road to the south of this crossing. This conflicted with

the private access; whilst the BCC layout did not consider the BRT platform stop

needed on the westbound SBL approach. A further constraint is the presence of a

large underground Wessex Water tank in the area just north of the private access to

the club car park. This cannot be subjected to traffic loading and effectively makes a

separate left filter lane unviable.

In addition to the left turn restriction from the SBL(E) mentioned above; there are a

number of other banned turns proposed. These are as follows:

• SBL(E): Right turn prohibited, as well as the left turn;

• Queens Road (S): Right turn prohibited; and

• Queens Road (N): Left turn prohibited.

These turn restrictions all affect movements to and from the new proposed section of

carriageway through the ‘green corridor’ and, as such, do not impose any constraint

on the movements which drivers can make at this junction at the moment. These

restrictions are necessary to reduce vehicle-vehicle and vehicle-pedestrian conflicts, in

order to achieve a sufficient level of layout capacity at a junction which is tightly

constrained by surrounding buildings. These restrictions have the added advantage

of providing a considerably enhanced level of service for pedestrians using the

crossing over the SBL(E) arm; as it is able to run concurrently with a ‘green man’

aspect during the whole of the side road Stage 3.

With respect to design standards set out in TD 50/04 ‘The Geometric Layout of Signal

Controlled Junctions and Signalised Roundabouts’ there are ‘Departures’ in respect of

inter-visibility between stop-lines. Paragraph 2.16 of TD50/04 states that “At new

signalised junctions major obstructions to inter-visibility within the junction inter-visibility

zone, such as that caused by buildings should be avoided. Under these conditions each

obstruction to visibility shall be considered as Departure from Standard and measures shall be

taken to mitigate the effects on inter-visibility”. The inter-visibility envelope is shown on

Drawing CTR-AEB-730 SK405. This shows that required inter-visibility set out by

TD50/04 is sub-standard in the SW and SE corners of the junction; particularly in the

SW corner where the Queens Head PH makes achieving this impossible. The stop-

line on Queens Road(S) needs to be set-back to accommodate the swept path of larger

vehicles turning right from the SBL(W) approach. Moving it forward is also

complicated by the access to the club. To reduce risk the three pedestrian crossings

have been located such that drivers on any approach will have a clear view of any

pedestrians or cyclists waiting to cross on either side. Similarly, pedestrians or

cyclists waiting to cross will have a good view of approaching vehicles on any arm.

The key residual risk is the lack of driver-to-driver inter-visibility across the SW

corner. These departures will need to be reviewed and signed off by the local

highway authority, in this case BCC.

‘Frontage’ issues that could impact on the general operation of this junction have

been identified as follows:

• Loading/unloading associated with the Queens Head PH: Access for deliveries

would appear to be via a small yard with an entrance just west of the junction

on King Georges Road. In practice large service vehicle may simply try to pull

up onto the footway in this location, rather than enter the yard and reverse out.

This could impede westbound traffic when this parking occurs; particularly

when vehicles turning right from the King Georges Road are present; and

• Loss of on-street parking for Queens Head patrons: Loss of adjacent

‘convenient’ parking on-street for customers using the Queens Head was

raised at consultation. At present this is available on King Georges Road; but

would have to be prevented on all four approaches to the new signalled

junction. There is no opportunity to provide alternative ‘close-by’ parking for

PH users within the existing highway boundary.

Option 2

Option 2 shown in Drawing No CTR-AEB-730 SK406 is essentially similar to Option

1, but provides a controlled crossing on the King Georges Road approach.

Consideration of this additional crossing was requested by BCC planners. Whilst

improving pedestrian accessibility it does, however, affect the Method of Control. It is

also considered to have potential safety implications.

The inclusion of this crossing will require a full pedestrian stage; as it cannot ‘run’ in

conjunction with any of the traffic phases. If this is needed anyway; the crossing on

the Queens Road(S) approach can be serviced in the same stage. As such, there would

be no need to prohibit the left turn from the SBL(E). The retention of this movement is

thus shown on this layout.

Given the poor and sub-standard inter-visibility achieved across the SW corner the

western crossing would be ‘blind’ to drivers turning left from Queens Road(S), albeit

at slow speed. Pedestrians waiting on the south side of this corner would also have a

very late view of these left turning vehicles. This increases the risk of a vehicle-

pedestrian collision.


General

The likely traffic operation of the two signalled cross-roads options has been assessed

using LINSIG. Summaries of the results obtained are included in Annex B. Specific

comment is provided in the following sub-sections.

Option 1


satisfactory operational performance although a high cycle time would be needed. In

the AM peak hour the predicted Practical Reserve Capacity (PRC) is +9.7% using a

maximum cycle time of 120 seconds. The PRC in the PM peak is +20.1% using the

same high cycle time.

The LINSIG modelling shows that the likely operation of this junction in the weekday

peak hours will be highly influenced by the volumes of traffic making the ‘opposed’

right turning movements from the King Georges Road and Queens Road(N)

approaches. The latter is particularly critical as there is no space for right turners to

wait clear of straight-ahead traffic on this approach. The Atkins forecasts for 2016

suggest that there will be no drivers attempting the right turn from the Queens

Road(N) approach in either peak hour. However, to assess what this effect might be a

‘sensitivity test’ was carried out assuming a right turn demand of 20 vehicles in each

peak hour. The results indicate that even a small number of right turners could have a

material effect on operating conditions at this junction. Indeed, the resultant PRC

value falls to -3% in both the AM and PM peak hours. In view of this consideration

could be given to additionally prohibiting the right turn from the Queens Road(N)

approach in order to mitigate this operational risk; especially as the forecast level of

demand for this movement is predicted to be very small.

Option 2

The summary LINSIG results in Annex B show that this option would also achieve a

satisfactory operational performance although a high cycle time would again needed.

In the AM peak hour the predicted Practical Reserve Capacity (PRC) is +7.5% using a

maximum cycle time of 120 seconds. The PRC in the PM peak is +14.8% using the

same high cycle time. The need for a full pedestrian stage means that the overall

vehicular capacity achievable is not as high as that predicted for Option 1.

Furthermore, this layout would also be subject to the same potential reduction in the

PRC with a small demand for the right turn from the Queens Road(N) approach.


This Technical Note has considered two different layout options for the SBL/Queens

Road junction. Operational assessments have been undertaken using LINSIG and the

results presented in Annex B. Drawings showing the two options are included in

Annex A. The conclusions relating to the two options are as follows:

Option 1

• There are ‘Departures from Standard’ in respect of the inter-visibility achieved

between stop-lines (TD50/04) in the SW and SE corners of the junction. The

position of the pedestrian crossings ensure that vehicle-pedestrian inter-

visibility is satisfactory, but driver-driver inter-visibility across the SW corner

is severely restricted by the Queens Head PH; and

• The junction is likely to operate satisfactorily in the weekday peak hours

(2016); although it will need a high cycle time of around 120 seconds in these

periods. Performance will be significantly affected if even a small demand for

the right turn from the Queens Road(N) approach occurs in practice; although

not predicted by the Atkins forecasts.

Option 2

• Has the same ‘Departures from Standard’ identified with Option 1. The

inclusion of the crossing on the west side of the junction increases safety risk

with the Departure relating to the SW corner. In short, the pedestrian view of

vehicles turning left from Queens Road(S) would be extremely restricted, and

vice versa;

• The junction would operate with a similar level of capacity to Option 1; but

again is expected to need a long cycle time in the weekday peak hours (2016). It

would also be subject to the same potential reduction in performance with

even a small number of right turners on the Queens Road(N) approach; and

• It would allow the left turn from the SBL(E) approach to be accommodated.


Queens Road junction should be taken forward. Whilst both options give a similar

operational performance it is considered that there would be safety implications in

including an additional crossing on the King Georges Road arm. This is because

pedestrian-driver inter-visibility across the SW corner is very sub-standard, and

constitutes a ‘Departure from Standard’ with both options. In such instances the aim

should be to mitigate any potential risks this creates, not exacerbate them.

B.3 Technical note no. 3 - Highridge Road Junction: Option Design / Assessment

Date: 4 January 2013


Background











as follows:





• Hareclive Road




Highridge Road junction; and the results of operational assessments undertaken

using LINSIG. The flows used in each case have been provided by Atkins for the 2016




General

Two options have been considered; a main option and a secondary option examined

to assess the operation of a junction layout catering for pedestrian crossings on all

arms but with the smallest possible ‘footprint’ and hence impact on Highridge

Common. The layout drawings for each are included in Annex A; and are as follows:

1. Drawing No CTR-AEB-730 SK407: Option 1; and

2. Drawing No CTR-AEB-730 SK408: Option 2.

The following sub-sections describe the features of the two options; and any potential

issues in respect of design standards or vehicle/pedestrian accessibility where they

exist. Operation in traffic terms in covered specifically in Section 3.

Option 1

Option 1 shown in Drawing No CTR-AEB-730 SK407 was developed from an initial

layout provided by the signals team at Bristol City Council (BCC). The only

significant difference from this BCC concept layout is the addition of the BRT

platforms; and road marking arrangements shown close to the stop-line on the

SBL(NW) arm to provide a sheltered waiting area for vehicles turning right into the

garage. The eastbound BRT platform has been re-located to the exit side of the

junction. In the original Mott MacDonald layout this bus-stop was situated in a lay-by

on the SBL(NW) approach close to the stop-line; with a pre-signal proposed to assist

egress from this lay-by. Adopting the revised ‘exit’ position for the eastbound

platform makes this pre-signal unnecessary. It also ensures that any bus priority

request made by a suitably equipped bus travelling eastbound can be used effectively

to ensure the bus passes through the stop-line with minimal delay.

To improve operation and tighten up the junction layout the following prohibited

turns are proposed:

• King Georges Road: Right turn prohibited. There is limited opportunity for

providing sheltered ‘waiting’ space for any right turners on this entry. Banning

this turn thus removes the risk of ahead and left turning vehicles being blocked

and impeded during all or part of the green signal period for this approach;

and

• Highridge Road (E): Left turn prohibited. This is necessary to safely

accommodate the eastbound BRT stop; whilst it also allows the junction

‘footprint’ in the SE corner of the layout to be significantly reduced and the

crossing point/stop-line on the King Georges Road brought forward. It also

allows the latter crossing to be ‘called’ on a ‘walk-with-traffic’ basis in Stage 3

of the signal cycle.

These two turn restrictions do affect existing movements possible at this junction.

Traffic forecasts suggest that vehicles making these movements are small; although it

is accepted that most will be associated with access to properties along King Georges

Road. As such, removing these turns will require residents to use Queens Road for

access to/from the north, or alternatively the SBL(NW) link and Highridge Green.

The junction layout, proposed Method of Control and associated traffic islands allow

the ‘staggered’ Toucan crossing on the Highridge Road(E) arm and the pedestrian

crossing on the other three arms to operate on a walk-with-traffic basis. This will

allow longer ‘green man’ times to be provided and help to reduce pedestrian/cyclist

waiting delays; particularly if the operational cycle time needed for capacity purposes

is long. As previously stated, likely traffic operation is considered in Section 3.

With respect to design standards set out in TD 50/04 ‘The Geometric Layout of Signal

Controlled Junctions and Signalised Roundabouts’ there are ‘Departures’ in respect of

inter-visibility between stop-lines. Paragraph 2.16 of TD50/04 states that “At new

signalised junctions major obstructions to inter-visibility within the junction inter-visibility

zone, such as that caused by buildings should be avoided. Under these conditions each

obstruction to visibility shall be considered as Departure from Standard and measures shall be

taken to mitigate the effects on inter-visibility”. The desirable inter-visibility envelope

defined in TD50/04 is sub-standard in the SW and NE corners of the junction;

particularly in the SW corner where the boundary wall to No 83 blocks inter-visibility

between the stop-line on the King Georges Road approach and the crossing on the

Highridge Road(W) exit. Risk might be reduced by removing the controlled crossing

points on the Highridge Road(W) arm. However, it is considered that pedestrians

would still try to cross in this location and, as such, it is considered that having a

controlled facility here would provide a net safety benefit albeit the sub-standard

inter-visibility across the SW corner.

In the NW corner drivers on the SBL(NW) approach will have a clear view of the

crossing on the Highridge Road(E) exit; although the ‘visibility envelope’ is partially

across third party land. As such, it constitutes a departure although the associated

safety risk is considered very low. This and the aforementioned inter-visibility

departure associated with the SW corner will need to be reviewed and signed off by

the local highway authority, in this case BCC.

The following ‘frontage’ impacts are worthy of note; and are discussed below:

• Access to/from the local garage on the SBL(NW) approach; which is very close

to the stop-line, has been a key consideration. To ensure that drivers waiting to

turn right onto the forecourt do not obstruct following vehicles a waiting ‘bay’

has been provided; although this will have the effect of shorting the length of

the right turn lane available for traffic on the SBL(NW) approach. There is a

risk that vehicles turning right at the main junction may simply choose to

queue back across this ‘bay’ rather than impede ahead and left turning traffic.

This risk will clearly depend on the expected volume of ‘opposed’ right turning

traffic at different times of the day. The latest Atkins forecasts (2016) suggest

that this volume in the AM and PM peak hours will be 100pcu and 150pcu

respectively. Assuming a ‘maximum’ 120 second cycle time the typical queue

associated with the right turn would be 3-5 vehicles. During the green period

for the SBL (Stage 1) these right turners will move forward into the junction

but; in the PM peak hour, it is accepted that the potential queue for this

movement will lead to drivers waiting within and obstructing the right turn

‘bay’ for the garage forecourt. However, at this time of day the likelihood of

customers arriving at the garage is also considered very low;

• The drawing shows that the driveway access from No 83 is within the junction.

This is unavoidable. Consideration will need to be given as to whether to

provide a separate signal and ‘call’ facility to allow vehicles leaving this

driveway to exit easily. The problem with so doing is that any signal for this

driveway mounted, say, on the adjacent central island, may easily be seen and

misconstrued by a waiting driver at the King Georges Road stop-line unless the

green aspect is suitably masked. Conversely, the driver waiting to exit the

driveway could easily see the secondary signal for the King Georges Road

approach; although this head could be moved and/or suitably shielded. As

currently shown, it is proposed that a driver attempting to exit this driveway

would do so in one of the available inter-stage periods; and

• The position of the driveway access to No 80 is such that right turn

access/egress is prevented by the long traffic island on the Highridge Road(E)

arm. Whilst this island might be shortened to make the right turn in possible,

the right turn out would be difficult to accommodate with the ‘staggered’

crossing arrangement proposed.

Option 2

Option 2 shown in Drawing No CTR-AEB-730 SK408 examines the ‘minimum’

junction footprint possible by removing traffic islands proposed in the Option 1

arrangement. A Toucan crossing is maintained on the Highridge Road(E) arm and

pedestrian crossing facilities provided on the other three arms as before. However, all

are now ‘single’ crossings. On the SBL(NW) arm the crossing length is circa 18m; and

so longer than the 15m ‘maximum’ recommended for a single crossing. This is partly

due to the exit width; which is designed to allow buses a straight approach into the

westbound platform stop to achieve satisfactory ‘docking’. However, the exit width

here must also allow for the swept path of a large vehicle turning left from the

Highridge Road(W) approach; with more than a right angle turn needed for this

movement.

The adoption of this layout will require a full pedestrian stage to service the

crossings; as most cannot ‘run’ in conjunction with any of the traffic phases. This will

increase wait times and delay for both pedestrians and cyclists; particularly for those

required to use more than one crossing point in negotiating the junction. In addition,

positive signal control will be needed for the driveway access from No 83 with a full

pedestrian stage in the signal cycle. Otherwise a driver attempting to exit may do so

when the pedestrian stage is active; leading to a risk of collision with pedestrians or

cyclists crossing with a right of way.

The aforementioned inter-visibility departures in the SW and NE corners of the

junction would remain; as would the potential risk of the right turn ‘bay’ into the

garage forecourt being blocked by ‘opposed’ waiting right turners making the

movement into Highridge Road(W). The right turn from King Georges Road and the

left turn from Highridge Road(E) are both prohibited as before, and for reasons

discussed earlier in describing the Option 1 layout. It is noted that the crossing on the

King Georges Road arm could additionally run on a ‘walk-with-traffic’ basis in Stage

3 of the signal cycle by virtue of the left turn ban.


General




Option 1




PM peak is +13.9% using a slightly lower cycle time of 80 seconds. These low cycle

times coupled with the ‘walk-with-traffic’ arrangement for the pedestrian crossings

will ensure that wait times for both pedestrians and cyclists are not excessive.

However, the cycle time used in practice will depend on whether this junction is to

operate normally under SCOOT control and, if so, whether it will be in the same

SCOOT region as the SBL/Queens Road junction. As previously demonstrated in

Technical Note 2 the SBL/Queens Road junction is likely to need a high cycle time of

120 seconds in the weekday peak hours to operate satisfactorily. If this junction is

included in the same SCOOT region then the same cycle time will be ‘forced’.

Option 2

The summary LINSIG results in Annex B show that this junction would operate

marginally over-capacity in the AM peak hour; with a predicted PRC of -1.5% at the

maximum desirable cycle time of 120 seconds. In the PM peak hour a PRC of +7.2% is

achievable; but only using a high cycle time of 120 seconds.

The reason for the decrease in performance is the necessary addition of a further

stage in the Method of Control to accommodate the full pedestrian stage (Stage 4).

During this stage all vehicle phases are held at red. This pedestrian stage will only

give a short ‘green man’ window of 7 seconds for three of the four crossings so, with

an expected cycle time of circa 120 seconds the wait time delays will be long. For

example, a pedestrian arriving to use the crossing over the SBL(NW) arm could

experience a wait delay between 10 and 113 seconds. If the desired route were to

involve the use of a second crossing a further wait delay of circa 120 seconds would

ensue. Notwithstanding the poor traffic performance it is considered that this layout

would fail to provide the required level of service for pedestrians.


This Technical Note has considered two different layout options for the

SBL/Highridge Road junction. Operational assessments have been undertaken using

LINSIG and the results presented in Annex B. Drawings showing the two options are

included in Annex A. The conclusions relating to the two options are as follows:

Option 1

• There are ‘Departures from Standard’ in respect of the inter-visibility achieved

between stop-lines (TD50/04) in the SW and NE corners of the junction. The

vehicle-pedestrian inter-visibility across the SW corner is most restricted due to

the boundary wall to No 83. However, on balance, it is considered that

providing a controlled crossing facility on this arm has safety benefits

outweighing the restricted inter-visibility risk;

• The proposed traffic islands and prohibited movements allow all crossings to

operate on a ‘walk-with-traffic’ basis. This will help to minimise potential wait

time delays for pedestrian and cyclists crossing the SBL in this location; so

reducing severance and improving NMU accessibility;

• Consideration may need to be given to providing a signalled egress for drivers

leaving No 83; as this driveway access is unavoidably within the junction

‘footprint’. If included at detailed design stage care will be needed in the

positioning of the signal head to avoid the green aspect being seen in error by

drivers waiting at the stop-line on King Georges Road;

• The ‘sheltered’ right turn access ‘bay’ into the garage forecourt may be blocked

on occasion by right turning vehicles on the SBL(NW) approach choosing to

queue across this, rather than block/impede following straight-ahead traffic.

Forecasts suggest the greatest risk will occur in weekday PM peak hour; but

the number of drivers seeking access to the garage is likely to be low or

negligible at this time of day; and

• The junction is likely to operate satisfactorily in the weekday peak hours

(2016); with a reasonable degree of reserve capacity (PRC) achievable in the

peak periods using a cycle time of 80-90 seconds.

Option 2

• Has the same ‘Departures from Standard’ identified with Option 1;

• A smaller junction ‘footprint’ is achievable with removal of the traffic islands

but crossing delays for pedestrians and cyclists will be much higher with this

layout. This is because most of the crossings can be served only in the full

pedestrian stage. This will only provide a short ‘green man’ time; so

pedestrians needing to use more than one crossing in negotiating the junction

will experience considerable ‘wait’ time;

• The driveway egress from No 83 will need to be positively signalled to prevent

safety risks associated with a vehicle making this egress manoeuvre during the

full pedestrian stage; and

• The junction is likely to operate marginally over-capacity in the weekday AM

peak hour (2016); but just within capacity in the PM peak hour. However, a

high cycle time of circa 120 seconds would be needed, which would not be

desirable for pedestrians and cyclists using crossings with only a small ‘green

man’ window afforded during the full pedestrian stage.


Highridge Road junction should be taken forward. This gives a much better level of

reserve capacity (PRC) than Option 2. Furthermore, the level of service afforded to

pedestrians and cyclists is considerably enhanced. The reason for the latter is the fact

that ‘walk-with-traffic’ provision is enabled by the various islands incorporated in the

layout; which also allows a reduced number of stages to be used in the normal cyclic

Method of Control. Crossing provision in Option 2 is reliant on the use of a full

pedestrian stage. This not only impacts on the achievable vehicular capacity of the

junction but will also result in high wait delays for pedestrians, particularly as a cycle

time approaching 120 seconds will be needed.

B.4 Technical note no. 4 and Addendum - A38 Bridgwater Road Junction: Option Design / Assessment



Background


design of the South Bristol Link Road (SBL) from its proposed junction with the A370

Long Ashton Bypass to Cater Road Roundabout. As a significant length of the

proposed route lies within Bristol City Council’s (BCC) administrative boundary the







as follows:





• Hareclive Road



Estate. This Technical Note describes the various options considered for the A38

Bridgwater Road junction and the results of operational assessments undertaken

using LINSIG. The flows used in each case have been provided by Atkins for the 2016




General

The junction with the A38 Bridgwater Road is probably the key intermediate

intersection on the proposed SBL Route. The traffic forecasts predict high volumes on

both the A38 and SBL approaches and consequently a high level of turning conflict.

On the southbound SBL approach there is a very high right turn volume predicted,

notably in the PM peak hour, which has had a large influence on the layout design

and options considered. Two options have been considered as follows:

• Drawing Nos. CTR-AEB-730 SK409/410: Option 1 - Signalled Roundabout; and

• Drawing No CTR-AEB-730 SK411: Option 2 - Signalled Crossroads.

The above drawings are included in Annex A. The following sub-sections describe

the features of the two options; and any potential issues in respect of design

standards, physical constraints or vehicle/pedestrian accessibility where they exist.

Operation in traffic terms in covered specifically in Section 3.

Option 1

General Description

Drawing No CTR-AEB-730 SK409 shows a signalled roundabout option with an

Inscribed Circle Diameter (ICD) of 80m. A diameter of this size is considered

necessary to ensure that there is sufficient internal circulatory space to accommodate

a degree of queuing at internal stop-lines without operation being unduly

compromised by the need to keep these queues very short to prevent exit blocking.

On the southbound SBL approach the bus lane is extended to the roundabout entry

and separate signalling used to control bus egress onto the circulating carriageway.

This removes the need for a remote pre-signal installation on this approach.

Toucan crossings are proposed on the A38(NE) arm to provide controlled crossing

provision for the shared use footway/cycleway running along the east side of the SBL.

Additional controlled crossings are proposed on the northern SBL arm to provide a

pedestrian linkage to/from the northbound BRT platform stop. Drawing CTR-AEB-

730 SK410 is simply a variant of the signalled roundabout layout with the BRT

platform stops on the northern SBL arm removed. This also removes the need for the

controlled pedestrian crossings on this arm. On the southern SBL arm there is a need

to maintain the footway route along the south side of the A38. Non-signalised

crossing points over the entry/exit carriageways are shown to maintain this linkage;

although these could be controlled if desired.

The proposed road marking layout allows the right turn from the southbound SBL

approach to be made using either the centre or outside lanes at entry. On the other

approaches right turners would be directed into the outside entry lane only.

Proposed Mode and Method of Control

The proposed normal mode of control would be Micro-processor Optimised Vehicle

Actuation (MOVA); which offers flexible adaptive control. This has been successfully

employed at other roundabout junctions in North Somerset; such as M5 Junction 19

and the A370 Hutton Moor and A370 Winterstoke Road roundabouts in Weston-

super-Mare. It is also the Highways Agency’s mode of choice at all new signalled

junctions, including signalised roundabouts.

The Method of Control adopted could be ‘single stream’ MOVA; in effect treating the

roundabout as a single junction with rigid linking, or multi-stream ‘Linked’ MOVA.

For modelling purposes ‘single stream’ MOVA operation has been modelled, with

stages used to both control the various entries, the bus gate and also provide

‘clearance’ events for removing ‘excess’ internal queues when required. The two right

turns of significance are those from the SBL(N) and the A38(SW); so the stages

considered necessary with ‘single’ stream MOVA control for the roundabout are as

follows:

• Stage 1: A38(SW) and A38(NE) approaches;

• Stage 2: A38(SW) RT ‘internal’ clearance and SBL(S). Called by queue detection

in front of the stop-line on the circulating section adjacent to the SBL(N) arm;

• Stage 3: SBL)N): BRT Bus Phase;

• Stage 4: SBL(N) and SBL(S) approaches; and

• Stage 5: SBL(N) RT ‘internal’ clearance and A38(SW). Called by queue

detection in front of the stop-line on the circulating section adjacent to the

A38(NE) arm;

Constraints

There are a number of physical constraints which have affected the roundabout

position. A key one is the position of Winford Bridge, where Colliter’s Brook passes

under the A38. The layout design shown in both Drawings CTR-AEB-730 SK409 and

410 accommodates the existing bridge within the central island, so making it

unnecessary to extend the length of the structure. To the south of the bridge it would

be necessary to culvert an existing open section of Colliter’s Brook to accommodate

the SW section of the circulatory carriageway whilst, to the north of the bridge, it may

be necessary to re-route the existing culvert.

Other key constraints are imposed by Castle Farm and its immediate curtilage to the

west of the proposed roundabout. Following consultation with the owners the layout

design has sought to:

• Maintain the existing vehicular access onto the A38;

• Avoid any impact on the walled garden/courtyard in front of the main

farmhouse;

• Maintain the bulk of the existing belt of trees screening the eastern side of the

farm. With the SBL in place this would also assist in reducing the noise and

visual impact of the new link; and

• Maintain the access track from the farm to the NE; this linking to a new track

and ‘cross-over’ point in order to maintain agricultural access to the fields on

the east side of the SBL.

Other identified constraints include some old lime kilns located just south of the A38

and north of Colliter’s Brook. These are not readily accessible at the moment and are

heavily overgrown; but there is nevertheless an aspiration to protect/maintain these

given their historical interest. Unfortunately, a feasible roundabout of 80m ICD

within the existing A38 corridor; and avoiding all the above mentioned constraints,

makes it impractical to avoid these. As such, these would be lost if the signalised

roundabout layout shown in Drawing No CTR-AEB-730 SK409 was constructed.

Some of the existing walls associated with the kilns can be seen underlying the

A38(NE) junction with the proposed roundabout.

Option 2

Option 2 shown in Drawing No CTR-AEB-730 SK411 shows a signalised crossroads

arrangement. This is similar in principle to the previous Mott MacDonald (MM)

proposal; although the layout has been reworked to provide:

• Two lanes for both the straight-ahead and right turn movements on the SBL(N)

approach; and an improved exit width/geometry for the A38(SW) arm to

accommodate the double right turn. The latter also allows the straight-ahead

movement from the A38(NE) to be made using two lanes;

• Separate ‘full phase’ right turn provision on all the other arms; which is

dictated on safety grounds by the size of the layout and the conflicting flow in

two lanes opposing each right turn movement;

• A ‘staggered’ Toucan crossing facility on the A38(NE) arm. This was

previously indicated as being a single crossing; and

• The addition of controlled pedestrian crossing facilities on the SBL(S) arm to

maintain the existing footway linkage along the south side of the A38.

As with the previous MM layout there is a pre-signal proposed on the SBL(N)

approach to assist buses turning right to the A38(SW). It is assumed that the

southbound BRT stop will be used exclusively by buses travelling straight-ahead

along the SBL; so this platform is shown in advance of the pre-signal to achieve a

position closer to the junction. The northbound platform stop could be moved closer

to the junction if it was subsequently decided to move the start of the bus lane further

south and reduce the merge length for general traffic. It is noted that consideration

will need to given to the position of bus stops on the A38 near this junction if the aim

of the platform stops here is to allow passenger interchange between services using

the A38 and SBL. As no controlled crossing is proposed on the A38(SW) arm these

stops would best be located on the A38(NE) arm. This consideration obviously

applies to Option 1 as well.


As with Option 1 the proposed normal mode of control would be Micro-processor

Optimised Vehicle Actuation (MOVA). All the right turns would require control with

a full phase so the most likely stage sequence would entail running the left/straight-

ahead movements on the two SBL arms first; followed by the two right turns, and

then repeating this control for the two A38 arms. However, within MOVA, there is

the opportunity to allow the controller to use alternative stages if, for example, there

was a demand for only the right turn from the SBL(N) but no demand present for the

right turn from the SBL(S) (SD Code-6MRR). As such the full number of stages

defined would be as follows:

• Stage 1: SBL(N) and SBL(S) approaches - Left and straight-ahead movements;

• Stage 2: SBL(N) and SBL(S) approaches - Right turn movements if BOTH

demanded;

• Stage 3: SBL(N) - Alternative to Stage 2; Demand for SBL(N) RT only;

• Stage 4: SBL(S) - Alternative to Stage 2; Demand for SBL(S) RT only;

• Stage 5: A38(SW) and A38(NE) approaches - Left and straight-ahead

movements;

• Stage 6: A38(SW) and A38(NE) approaches - Right turn movements if BOTH

demanded;

• Stage 7: A38(SW) - Alternative to Stage 6; Demand for A38(SW) RT only; and

• Stage 8: A38(NE) - Alternative to Stage 6; Demand for A38(NE) RT only.

The usual sequence appearing will comprise four stages. Given the expected volumes

of right turners in the weekday peak periods (2016) it is anticipated that the usual

stage sequence will be either 1-2-5-6 or 1-3-5-6. For modelling purposes the former

has been assumed; even though the right turn volume from the SBL(S) arm is

expected to be quite low.

Constraints

All the physical design constraints described when discussing Option 1 apply equally

to the Option 2 layout. As can be seen from Drawing No CTR-AEB-730 SK411 it will

be feasible to satisfy those relating to Castle Farm; although Winford Bridge will be

directly affected by a signalled crossroads layout. This is because the bridge will

inevitably lie under the junction; making it necessary to remove this structure and

provide a new culvert for Colliter’s Brook extending north-south between the SW and

NE corners of the junction. One advantage of the crossroads is that the reduced land-

take needed in the SE quadrant would allow the lime kilns to be retained in their

present location; although widening on the A38(NE) approach would bring the road

closer to these.


General




Option 1

The summary LINSIG results in Annex B show that this option would achieve a good

level of operational performance. In the AM peak hour the predicted Practical

Reserve Capacity (PRC) for the main roundabout (Controller Stream 1) is +31.1%

using a cycle time of 90 seconds. In the PM peak hour a PRC of +34.8% is predicted

using the same 90 second cycle time. With this level of PRC it is probable that MOVA

would operate with a lower cycle time than 90 seconds in practice. This is

advantageous to signalled roundabout operation as the risk of ‘excess’ internal

queuing is correspondingly reduced.

For modelling purposes a 1-4-5 sequence was assumed after examining initial results.

This indicated that the internal ‘clearance’ stage for the A38(SW) right turn (Stage 2)

would not generally be triggered as queuing would not reach the queue call loops. As

such the appearance of Stage 2 was not assumed. In addition; a cyclic appearance of

the southbound BRT bus phase (Stage 3) was not considered realistic.

Internal queuing associated with the right turn from the SBL(N) approach suggests

that this will extend through the upstream circulatory stop-line adjacent to the

A38(NE) entry during the main SBL stage (Stage 4); notably during the PM peak

hour. As such it is expected that the following ‘clearance’ stage (Stage 5) will always

generally be called by internal queue detector inputs. The results suggest that the

‘excess’ queue occurring during Stage 4 will generally be associated with right

turning traffic entering the roundabout from the outside right turn lane on the

SBL(N); namely because all vehicles are stopped. The internal queuing associated

with right turners choosing to use the central entry lane is not so great but may, on

occasion, impede straight-ahead drivers using the same lane and route through the

roundabout in Stage 4. This potential impedance could create a safety risk if these

drivers are tempted to make a late lane change to avoid being stopped. However, the

results suggest that this is only likely to be an issue in the PM peak and, even then,

not all the time. Furthermore, the use of a lower time than 90 seconds would further

assist in mitigating this.

Option 2

The summary LINSIG results in Annex B show that this junction would operate

marginally over-capacity in the AM peak hour; with a predicted PRC of -5.6% at the

maximum desirable cycle time of 120 seconds. The critical approaches in this period

are the A38(SW) and the SBL(S); although the right turn from the SBL(N) is also

predicted to be over-capacity. The performance in the PM peak hour is better; with a

PRC of +8.9% predicted with a cycle time of 90 seconds.

The results assume that use of the pedestrian crossings will be low; and that any

users will generally cross within the extended ‘green man’ time afforded by the walk-

with-traffic operation of these facilities. As such the variable clearance times for the

crossings are all assumed to run for the minimum time of 5 seconds. This could be

longer if, say, a pedestrian stepped onto a crossing just as the green man expired. In

this case on-crossing detection would extend the clearance period as required.

The results show that, despite the size of the layout, it would operate marginally

over-capacity during the weekday AM peak hour in the Opening Year (2016). Hence

there would be no potential to cater for any additional growth in traffic using the SBL

in this period beyond 2016. Given the expected demands for the different turning

movements there is little incentive in considering prohibiting turns to improve

capacity. Prohibiting the right turn from the SBL(S) might be considered as the

volume of traffic making this movement in both peak hours is expected to be low.

However, removing this right turn would have negligible impact in improving

capacity for the capacity-critical traffic movements. Only left and straight-ahead

traffic on the SBL(N) would potentially benefit.


This Technical Note has considered two different layout options for the SBL/A38

junction. Operational assessments have been undertaken using LINSIG and the

results presented in Annex B. Drawings showing the two options are included in

Annex A. The conclusions relating to the two options are as follows:

Option 1

• The signalled roundabout layout would operate well; with a PRC in excess of

+30% in both weekday peak hours using a 90 second cycle time. A lower cycle

time achievable given this level of reserve capacity would suit signalled

roundabout operation by helping to prevent ‘excess’ queuing on the circulating

sections of the roundabout. Single stream MOVA operation could be

considered for the roundabout (as modelled); or Linked MOVA using more

than one controller stream at the roundabout;

• The layout would allow the southbound bus lane on the SBL(N) approach to be

extended to a bus gate entry onto the roundabout. This would remove the need

for a remote pre-signal installation and potentially provide a higher level of

priority;

• Layout constraints associated with Castle Farm can be accommodated; whilst

accommodating Winford Bridge within the central island would avoid the

need to remove this structure. However, an existing open section of Colliter’s

Brook would need to be accommodated in culvert under the SW circulating

section of the roundabout. The existing culvert position north of the bridge

may also need to be reviewed and altered to fit with the SBL(N) alignment; and

• With the layout proposed in CTR-AEB-370 SK409 the old lime kilns just south

of the A38 would be affected and lost. Without a significant and costly re-

alignment of the A38 to the south it would not be possible to accommodate an

80 ICD roundabout in this location given the other constraints imposed by

Castle Farm and the position of Winford Bridge.

Option 2

• Despite the size of the junction and its multiple lane approaches an over-

capacity situation is still predicted during the AM peak hour in the Opening

Year (2016). In consequence there is no potential to accommodate further traffic

growth in this period. In the PM peak hour a PRC of around +9% is predicted

using a cycle time of 90 seconds;

• Layout constraints associated with Castle Farm can be accommodated as with

Option 1. Furthermore, the junction could be accommodated without

impacting upon the location of the lime kilns; and

• Winford Bridge will be directly affected by the signalled crossroads layout.

This is because the bridge will inevitably lie under the junction; making it

necessary to remove this structure and stabilise the ground to accommodate

the overlying pavement construction. A new culvert for Colliter’s Brook

extending north-south between the SW and NE corners of the junction would

also be needed.

On the basis of the above conclusions it is considered that the signalled roundabout

layout (Option 1) should be taken forward as the preferred layout. This is because it

offers far more in operational terms than the signalled crossroads; the predicted PRC

of circa +30% in both peak periods (2016) giving greater confidence that the junction

will work well. This level of spare capacity will also ensure that a degree of traffic

growth can be accommodated in the weekday peak hours beyond the opening year.

In contrast, the signal crossroads is predicted to be over-capacity during the AM peak

hour in 2016. Whilst banning movements might be considered to reduce conflicts this

is not really desirable. The right turn from the SBL(S) could be prohibited as the

expected demand for this movement is low; but the other three right turns are

predicted to attract a reasonable amount of traffic. Just prohibiting the right turn from

the SBL(S) would not materially affect the capacity available to the traffic movements

identified as critical.

Adopting the roundabout layout would allow Winford Bridge to be retained, albeit

redundant, within the central island. It is accepted, however, that this layout would

affect the location of the existing lime kilns. It is not the purpose of this Technical

Note to assess the value of these historic structures and, as such, whether losing them

is acceptable or not. However, it is clear that there is a conflict between achieving a

junction layout with a good PRC level on the line of the A38 and retaining these kiln

structures.

This is probably the ‘key’ junction on the SBL route and, as such, the wider

operational benefits of the road link will depend on how well this intersection

performs. For example, the ability of the scheme to remove traffic pressure from

Winterstoke Road and also the well used ‘rat run’ route through Barrow Gurney will

be highly dependent on how well the SBL section between the A370-A38 operates.

The LINSIG results suggest that the signalled crossroads layout would not provide a

sufficient level of spare capacity longer term and, as such, provides insufficient

‘comfort factor’ despite the size of the junction and its multi-lane approaches.

Technical note no. 4 addendum - A38 Bridgwater Road Junction: Option 1A Design / Assessment



Background


design of the South Bristol Link Road (SBL) from its proposed junction with the A370

Long Ashton Bypass to Cater Road Roundabout. As a significant length of the

proposed route lies within Bristol City Council’s (BCC) administrative boundary the






This Technical Note describes a variant arrangement (1A) for a proposed signalled

roundabout at the SBL/A38 junction; described in Technical Note 4 (TN4) as Option 1.

Section 2 - Option 1A

General Description

Drawing No CTR-AEB-730 SK409A in Annex A shows a signalled roundabout option

with an Inscribed Circle Diameter (ICD) of 80m as before. As discussed in TN4 for

Option 1; a diameter of this size is considered necessary to ensure that there is

sufficient internal circulatory space to accommodate a degree of queuing at internal

stop-lines without operation being unduly compromised by the need to keep these

queues very short to prevent exit blocking. On the southbound SBL approach the bus

lane is extended to the roundabout entry and separate signalling used to control bus

egress onto the circulating carriageway. This removes the need for a remote pre-

signal installation on this approach.

As before, Toucan crossings are proposed on the A38(NE) arm to provide controlled

crossing provision for the shared use footway/cycleway running along the east side

of the SBL. Additional controlled crossings are proposed on the northern SBL arm to

provide a pedestrian linkage to/from the northbound BRT platform stop. On the

southern SBL arm there is a need to maintain the footway route along the south side

of the A38. Non-signalised crossing points over the entry/exit carriageways are

shown to maintain this linkage; although these could be controlled if desired.

The proposed road marking layout allows the right turn from the southbound SBL

approach to be made using either the centre or outside lanes at entry. On the other

approaches right turners would be directed into the outside entry lane only.


The proposed normal mode of control would be Micro-processor Optimised Vehicle

Actuation (MOVA); which offers flexible adaptive control. The Method of Control

adopted could be ‘single stream’ MOVA; in effect treating the roundabout as a single

junction with rigid linking, or multi-stream ‘Linked’ MOVA. For modelling purposes

‘single stream’ MOVA operation has been modelled, with stages used to both control

the various entries, the bus gate and also provide ‘clearance’ events for removing

‘excess’ internal queues when required. The two right turns of significance are those

from the SBL(N) and the A38(SW); so the stages considered necessary with ‘single’

stream MOVA control for the roundabout are as follows:

• Stage 1: A38(SW) and A38(NE) approaches;

• Stage 2: A38(SW) RT ‘internal’ clearance and SBL(S). Called by queue detection

in front of the stop-line on the circulating section adjacent to the SBL(N) arm;

• Stage 3: SBL)N): BRT Bus Phase;

• Stage 4: SBL(N) and SBL(S) approaches; and

• Stage 5: SBL(N) RT ‘internal’ clearance and A38(SW). Called by queue

detection in front of the stop-line on the circulating section adjacent to the

A38(NE) arm;

Constraints

With the Option 1A layout the roundabout centre has been shifted to the east so that

the old lime kilns are now situated within the central island. With the former Option

1 layout these were adversely by the works associated with the A38 southbound

entry; and would have been lost. The displacement of the A38 from its current

alignment means that Winford Bridge; where Colliter’s Brook passes under the road,

would become redundant. To the south of the bridge it would be necessary to culvert

an existing open section of Colliter’s Brook to accommodate the re-aligned A38 SW

arm whilst, to the north of the bridge, it may be necessary to re-route the existing

culvert.

Other key constraints imposed by Castle Farm and its immediate curtilage to the west

of the proposed roundabout are met. In fact, Option 1A moves the proposed

roundabout further from Castle Farm. Following consultation with the owners the

layout constraints identified were to:

• Maintain the existing vehicular access onto the A38;

• Avoid any impact on the walled garden/courtyard in front of the main

farmhouse;

• Maintain the existing belt of trees screening the eastern side of the farm. With

the SBL in place this would also assist in reducing the noise and visual impact

of the new link; and

• Maintain the access track from the farm to the NE; this linking to a new track

and ‘cross-over’ point in order to maintain agricultural access to the fields on

the east side of the SBL.


General

The likely traffic operation of the signalled roundabout was described in TN4. The

revised Option 1A layout has maintained the same approach lane arrangement on the

four arms; and the same internal road marking layout. As such, the LINSIG results

will be the same as Option 1 but are included in Annex B.

The summary LINSIG results in Annex B show that this option would achieve a good

level of operational performance. In the AM peak hour the predicted Practical

Reserve Capacity (PRC) for the main roundabout (Controller Stream 1) is +31.1%

using a cycle time of 90 seconds. In the PM peak hour a PRC of +34.8% is predicted

using the same 90 second cycle time. With this level of PRC it is probable that MOVA

would operate with a lower cycle time than 90 seconds in practice. This is

advantageous to signalled roundabout operation as the risk of ‘excess’ internal

queuing is correspondingly reduced.

For modelling purposes a 1-4-5 sequence was assumed after examining initial results.

This indicated that the internal ‘clearance’ stage for the A38(SW) right turn (Stage 2)

would not generally be triggered as queuing would not reach the queue call loops. As

such the appearance of Stage 2 was not assumed. In addition; a cyclic appearance of

the southbound BRT bus phase (Stage 3) was not considered realistic.

Internal queuing associated with the right turn from the SBL(N) approach suggests

that this will extend through the upstream circulatory stop-line adjacent to the

A38(NE) entry during the main SBL stage (Stage 4); notably during the PM peak

hour. As such it is expected that the following ‘clearance’ stage (Stage 5) will always

generally be called by internal queue detector inputs. The results suggest that the

‘excess’ queue occurring during Stage 4 will generally be associated with right

turning traffic entering the roundabout from the outside right turn lane on the

SBL(N); namely because all vehicles are stopped. The internal queuing associated

with right turners choosing to use the central entry lane is not so great but may, on

occasion, impede straight-ahead drivers using the same lane and route through the

roundabout in Stage 4. This potential impedance could create a safety risk if these

drivers are tempted to make a late lane change to avoid being stopped. However, the

results suggest that this is only likely to be an issue in the PM peak and, even then,

not all the time. Furthermore, the use of a lower time than 90 seconds would further

assist in mitigating this.


This Addendum Technical Note has considered a variant position for a signalled

roundabout at the SBL/A38 junction. As with Option 1 considered in TN4 this

signalled roundabout layout would operate well; with a PRC in excess of +30% in

both weekday peak hours using a 90 second cycle time. A lower cycle time achievable

given this level of reserve capacity would suit signalled roundabout operation by

helping to prevent ‘excess’ queuing on the circulating sections of the roundabout.

Single stream MOVA operation could be considered for the roundabout (as

modelled); or Linked MOVA using more than one controller stream at the

roundabout.

With the Option 1A layout proposed in CTR-AEB-370 SK409A the old lime kilns just

south of the A38 would be retained by incorporating them within the central island of

the new roundabout. Winford Bridge is rendered redundant for traffic purposes by

the required shift in the alignment of the A38 (SW) arm; whilst the new layout is

beneficial in moving the junction away from Castle Farm and its immediate curtilage.

On the basis of the above conclusions it is considered that the signalled roundabout

layout (Option 1A) should be taken forward as the preferred layout. This layout

accommodates all local constraints; although a local realignment of the existing A38 is

necessary to prevent the loss of the lime kilns. In operating terms, the junction can be

expected to yield the same level of reserve capacity as Option 1.

B.5 Technical notes no. 5 – Brookgate junction

Technical note no. 5A – Brookgate junction location

Date: 29 August 2012


At a meeting held with the landowners of Ashton Vale Trading Estate in August

2012, it was agreed with North Somerset Council to assess an alternative arrangement

for the Brookgate junction in order to avoid the area used for storage by some

businesses on the trading estate.

A sketch (CTRAEB_730_SK010) has been produced for the alternate junction

arrangement to the trading estate. The alternative alignment follows Brookgate

northbound and cuts through an existing dense hedgerow at the end of a cul-de-sac

to connect to the South Bristol Link. This technical note compares the alternative

option and its associated implications against the proposed route by Mott MacDonald

(MM).

Assessment of options at Brookgate junction

The table below sets out the advantages and disadvantages of the two options under

consideration.

Advantages

Proposed (by MM) Alternative

• The route construction is 150m shorter than

the alternative making it a more cost

effective option.

• The route access to SBL is approximately

160m shorter northbound and 440m

shorter southbound from the Trading

Estate.

• The junction is located adjacent to the

Ashton Vale Trading Estate reducing the

visual impact.

• The route does not affect the storage

space area used by businesses on the

trading estate.

Disadvantages

Proposed (by MM) Alternative

• The alignment passes through an area used

for storage by some businesses of the

trading estate. An alternate location could

be identified to provide the necessary

storage.

• Access for some businesses on Brookgate

particularly the ones nearest to the route

(Units 15) may be difficult due to the

proximity of the route and the accesses. A

review of the alignment may be required in

order to resolve this.

• The alternative route includes a tight

radius bend in the alignment. Curve

widening will need to be introduced to

allow the passing of HGV’s.

• The alternative option is a much longer

route than the proposed.

• The route cuts through dense hedgerows

which may have environmental

protection and will negatively impact

fauna and flora.

• The alternative route crosses areas

allocated for Ashton Vale Village Green

as Exchange land for the Bristol Stadium.

Acquiring this land will mean providing

an alternative exchange land which will

be complicated and further add to the

costs of this option.

• The route is located within 30m from

residential properties on Ashton Drive.

This could require Part 1 compensation

due to visual, noise, vibrations and

possibly artificial lighting on nearby

residential properties.

• The junction which is likely to be light is

located in open space creates a wider

visual intrusion.

Ecological implications

An ecological constraints plan (Figure 1) prepared by Atkins Ltd is attached with this

document. The plan shows that the alternative route and junction layout passes

through Ashton Vale Fields SNCI, a locally designated wildlife site. Although this

SNCI receives no statutory protection, it will be protected through the planning

system. If this option was chosen, it would require strong justification, and

compensatory habitat creation (including a long-term commitment to management of

that land) would be required.

This option also passes through a thick mixed species hedgerow, likely to be of

ecological value.

Conclusion

By comparing the two junction’s advantages and disadvantages, the proposed route

by Mott MacDonald has less impact on the surrounding environment, better

alignment, shorter travel distance and less cost implications than the alternate

junction. Disruption to the storage area can be mitigated by providing an alternative

location.

It is recommended that the proposed route is taken forward and an alternative

storage area provided to affected businesses on the estate in order reduce the impact

for businesses.

Technical note no. 5B – Brookgate junction Option Design / Assessment



Background











as follows:





• Hareclive Road




Brookgate junction; and the results of operational assessments undertaken using

LINSIG. The flows used in each case have been provided by Atkins for the 2016




General

Two options have been considered for the Brookgate junction. The only difference is

the termination treatment proposed for the bus lane on the northbound SBL

approach; in order to assist buses in executing the right turn to the bus link

connection to Long Ashton Park & Ride. The layout drawings for each are included in

Annex A; and are as follows:



The following sub-sections describe the two options; and any potential issues in

respect of design standards or vehicle/pedestrian accessibility where they exist.

Operation in traffic terms in covered specifically in Section 3.

Option 1

Option 1 shown in Drawing No CTR-AEB-730 SK412 includes a pre-signal

termination for the bus lane on the northbound SBL approach. This is situated some

70m south of the main stop-line to minimise the risk of any queuing for the right turn

extending back to the pre-signal and impeding straight-ahead traffic. It will also

ensure that MOVA X detectors can be sited satisfactorily in both approach lanes (circa

40-45m from the stop-line) without the risk of these being clipped by vehicles

crossing between lanes.

The layout varies from the original concept layout developed by Mott MacDonald as

follows:

• The proposed pedestrian/cycle route is on the east side of the SBL; whereas in

the original scheme this was situated on the west side. This makes a controlled

crossing facility over the northern SBL arm unnecessary;

• On the northbound SBL approach the straight-ahead and right turn

movements are controlled by separate phases. This differs from the Mott

MacDonald approach where the right turn was treated as ‘opposed’; with no

subsequent filter arrow phase provided either to assist drivers either. This was

considered an unacceptable situation for BRT buses attempting to make the

right turn;

• The proposed approach flows predicted for the Brookgate arm suggested that

land take could be reduced by the removal of two lanes at the stop-line. It was

also felt that pedestrian and cyclists crossing Brookgate could be adequately

catered for with a non-signalised crossing facility over this arm. Adding a

controlled crossing was considered unnecessary; whilst it could only run in

conjunction with buses emerging from the bus link; and

• Lane markings on the southbound SBL approach have been altered to allow

straight-ahead traffic to use the short nearside lane as well. This was

previously signed for left turning vehicles only in the Mott MacDonald layout;

but would result in negligible utilisation of this short lane as this movement is

predicted to be very low. In order to accommodate straight-ahead traffic in this

lane the start of the bus lane on the southbound SBL exit has been moved to the

south to create a merging zone.

Option 2

Option 2 shown in Drawing No CTR-AEB-730 SK413 simply removes the pre-signal

on the northbound SBL approach. Buses requiring the right turn would therefore be

required to move to the offside lane from the bus lane without signalled assistance.

The need for the pre-signal shown in the Option 1 layout must be gauged against the

likely difficulty that buses would have in making the lane change for the right turn

unassisted. In all probability bus drivers will make an early move out of the bus lane

before the termination point when a suitable lane change ‘gap’ is available. At the

termination point other vehicles travelling ahead would be forced to yield to a bus in

executing the cross-over movement to the nearside lane. This would create the ‘gap

opportunity’ for the bus to move to the right turn lane. The Atkins forecasts show

that the expected volume of right turning traffic is low. In consequence, the queuing

in this lane can be expected to be negligible and is unlikely to impede a bus making a

lane change manoeuvre at the end of the bus lane. Assuming such buses will be ACIS

equipped a virtual loop or ‘trigger zone’ could be set covering both lanes some 20m

or so from the bus lane termination point. This would be used to ‘priority’ call Stage 2

controlling the right turn.


The likely traffic operation of the junction has been assessed using LINSIG. A

summary of the results is included in Annex B. The layout of the main junction does

not vary with two options and, as such, only one set of LINSIG results is discussed in

this Technical Note. Specific modelling of the pre-signal and its linking and co-

ordination with the traffic signals at the main stop-line has not been undertaken. This

could be ‘rigidly’ linked with a bus demand calling a specific stage including the bus

phase at the pre-signal. Alternatively, the pre-signal could be a separate controller

stream; and a priority linking input send via MOVA to force a particular stage at the

main junction upon receipt of a bus call.

The summary LINSIG results in Annex B show that both options would operate

satisfactorily in both peak hours in 2016. In the AM peak hour the predicted Practical


PM peak is +3.4% using a slightly longer cycle time of 110 seconds.


This Technical Note has considered two different layout options for the

SBL/Brookgate junction. An operational assessment has been undertaken using

LINSIG that can be applied to both options and the results presented in Annex B.

Drawings showing the two options are included in Annex A. The conclusions

relating to the two options are as follows:

• Whilst a pre-signal could be used at the end of the northbound bus lane the

delay-saving advantage this would give is considered negligible;

• Both layouts are likely to operate satisfactorily in the weekday peak hours

(2016); with a reasonable degree of reserve capacity (PRC) achievable in the

peak periods using a cycle time of 90-110 seconds; and

• Allowing straight-ahead traffic to use the short nearside lane on the SBL

southbound approach would be necessary to prevent an over-capacity

situation developing on this arm in the PM peak hour (2016).


Brookgate junction should be taken forward. At the bus lane termination point other

vehicles travelling ahead would be forced to yield to a bus in executing the cross-over

movement to the nearside lane. As such, this would create the ‘gap opportunity’

necessary for the bus to move to the right turn lane in which expected stop-line

queuing is expected to be negligible. It is accepted that using a pre-signal would

provide more positive, controlled assistance to buses moving from the bus lane to

execute the right turn. However, this increases the cost of the installation and

potentially the complexity of the signalling without a discernable delay saving benefit

to buses. The LINSIG results show that the longest queue on this approach is

expected to be circa 19pcu in the morning peak hour (2016); but the average delay of

only 14 seconds reflects the fact that the straight-ahead traffic is likely to have a green

signal most of the time. In view of this bus drivers are likely to elect to make a lane

change from the bus lane early when an opportunity exists and; in all probability, a

pre-signal if provided will be rendered redundant.

B.6 Technical note no. 6 - A370 roundabout



Background


design of the South Bristol Link Road, from its proposed junction with the A370 Long

Ashton Bypass to Cater Road Roundabout. Taking on board comments from the

public consultation and Council Officers on the scheme layout, this stage of the

commission is to produce a further developed preliminary design for inclusion in a

planning application.

This Technical Notes describes the various options considered for the A370

roundabout junction since the public consultation and is set out in 3 sections as listed

below:

• A370 roundabout – Location 1 (100m south west of the Park & Ride);

• A370 roundabout – Location 2 (RSA1 and NSC QA Review adjustments); and

• A370 roundabout – Location 3 (south of A370) – The Preferred Location

The pre-consultation design included a three arm roundabout for the junction with

the A370 located 300m west of Long Ashton Park & Ride.

Section 2 - A370 roundabout - Location 1 (100m south west of the Park & Ride)

Introduction

At the Initial Design Workshop on 2nd July 2012, it was agreed that the location of

the roundabout junction on the A370 would be reviewed following feedback from the

pre-planning application consultation. In particular, comments questioned whether

or not the junction could be moved closer to the Long Ashton Park & Ride facility.

This would move the junction further away from Long Ashton and would eliminate

the need for a farm accommodation bridge for cattle to cross the line of the SBL.

Pre planning application consultation assessment

In February 2012 a technical note was prepared by others, setting out the reasons for

the location of the roundabout. This section reviews the conclusions of the technical

note in the context of the consultation feedback and further advice from North

Somerset Council.

Assessment of the proposed roundabout location

Below is an extract from the technical note giving the reasons favouring the

roundabout in its pre-application consultation location:

1. ‘The current location of the A370 roundabout is some 300m south west of the Park &

Ride site.

2. Its location has been chosen to allow the various geometric standards to be incorporated

into the design and enable more innovative segregated lanes to ensure expedient flow of

traffic through the junction for particular flow movements.

3. Its location had been selected to allow adequate and safe as possible distance between

the roundabout and the junction serving the Park & Ride.

4. It also ensures that the South Bristol Link alignment remains as close as practically

possible to the urban boundary of Bristol, thus avoiding major impacts across the open

land south of Bristol and visible from Ashton Court.

5. One further constraint was the location of an existing farm access overbridge, over the

A370 and the junction position was selected to avoid costly impacts on that structure.’

Below is an extract from the February 2012 technical note by others giving the reasons

against the alternative location being discussed in this section. The alternative option

is located approximately 100m south west of the Park & Ride facility and situated

between the Park & Ride and the existing farm access over bridge.

1. The exit and entry lanes on the A370 Bristol arm would coincide with the deceleration

off-slip junction serving the Park & Ride and B3128 to Long Ashton. This could

potentially lead to adverse road safety conditions, particularly for vehicles exiting the

roundabout due to the close proximity of the two different junctions.

2. Siting the roundabout south westwards of the Park & Ride would see the junction

impact significantly on the existing farm access track overbridge. The structure would

obstruct forward visibility to and from the junction leading to road safety concerns

which could only be addressed by the costly relocation of the structure and access

tracks.

3. The South Bristol Link exit and entry arm and the associated main alignment towards

Brookgate Junction would be constructed across the Kennel Farm Landfill for a length

of nearly 300m. This would increase construction costs due to the need to either;

remediate or most likely dispose of landfill material to ensure suitable long life of the

new South Bristol Link road construction and reduce future maintenance liabilities.

4. The alternative South Bristol Link alignment resulting from the revised roundabout

location would cross land earmarked as Exchange Land for Town and Village Green

Land lost as a result of the Ashton Vale to Temple Meads Rapid Transit Scheme and

alternate equivalent exchange land would need to be found adding additional costs.’

Review of the alternative option

Halcrow is for the most part in agreement with the reasons stated in the technical

note to have the roundabout in its pre-application location (although with

reservations on the segregated lanes). However, following a review of the technical

note and consultation comments a schematic layout of an alternative option has been

prepared which would have some benefits.

The schematic layout of the alternative option approximately 100m south west of the

Park & Ride facility is shown on sketch no. CTRAEB/730/SK001 in Appendix 1.

Set out below are reasons that support that the roundabout junction closer to the

Long Ashton Park & Ride facility may be a viable option:

1. The option would avoid the conflict between the existing off-slip road (B3128)

from the A370 serving the Park & Ride facility and Long Ashton by providing

an exit from the alternative roundabout with direct access to the slip road. The

A370 would have an exit/entry of its own to the roundabout. This removes any

previous adverse road safety concerns with the A370 arm and the deceleration

lane of the B3128 off slip road coinciding. However, this option would require

further traffic analysis to ensure that there is adequate traffic capacity on the

A370 approach. No geometric design has been considered at this stage.

2. This option requires very little additional land take (refer to Appendix 1)

although an examination of the existing highway boundary would be

necessary to ascertain the exact land take required should this option be

progressed.

3. The issue of obstruction to forward visibility on the approach to the alternative

junction due to the farm overbridge structure can be addressed. TD 9/93

Chapter 1 (Design Speed), Para 1.26 (b) states that on the approaches to a

roundabout, the lengths of carriageway on the approach shall be 1.5 times the

Desirable Minimum Stopping Site Distance from the give way line. The A370 is

a 60mph single carriageway with an assumed 100kph Design Speed. From

Table 3 in TD 9/93, the Desirable Minimum Stopping Site Distance for a 100kph

Design Speed is 215m. Thus there is a requirement for stopping site distance of

215m to be provided over a 322.5m (1.5 x 215m) length on the approach to the

roundabout give way line. Preliminary calculations on the alternative option

indicate that this value is achievable for the north and southbound approach

without the need for compromising road safety or relocating the existing farm

overbridge structure.

4. The alternative option alignment would also avoid land identified as Exchange

Land for Town and Village Green Land lost as a result of the Ashton Vale to

Temple Meads Rapid Transit Scheme (refer to the sketch). This would

eliminate any additional cost implications of finding equivalent land for

another land exchange.

5. In addition to the above points, the alternative option would avoid additional

costs in constructing a farm accommodation overbridge for Parsonage Farm to

allow cattle to cross the South Bristol Link. Initial cost estimate of this bridge

will be in the region of £300k to £500k. This would reduce the overall

construction cost and avoid disruption to the farm’s activities.

6. The ground investigation survey at Kennel Farm Landfill site in 2011 provided

information from a single borehole. The findings of this investigation revealed

that the presence of contaminants at significant levels was thought to be

unlikely. The site was operated as an inert landfill site until February 2004. As

the extent of the landfill site cannot be verified by a single borehole, further

investigations would be required if this option is pursued to ascertain the exact

extents of the landfill site in order to make an informed assessment.

However, additional construction cost implications due to the need to dispose

of landfill material to ensure suitable foundation for the road construction

would likely offset the savings from avoiding the need for the farm

accommodation bridge for Parsonage Farm.

Further advice from North Somerset Council (NSC)

Kennel Farm Landfill located south west of the Long Ashton Park & Ride facility is

land reserved for future extension to the Park & Ride. The alternative option

alignment would cross this land which would limit the possible extension to the Park

& Ride facility.

At the Design Workshop on 3rd August 2012, it was agreed that Halcrow would

investigate aligning the SBL to the side of this landfill to ensure that future extension

of the existing Park & Ride would be possible. Subsequent to this, however, NSC

stated that the proposed A370 junction location put forward at the consultation

should be retained in order to ensure that possible future extension of the Long

Ashton Park & Ride is not compromised.

Conclusion

The comments made in the February 2012 technical note by others are largely valid

for the location of a roundabout 300m west of the Park & Ride site. However,

considering the issues raised at the pre-planning application consultation there might

be advantages of a possible alternative location closer to the Park & Ride and these

are set out below:

• The issues of conflict with the A370 and the B3128 off slip lane can be

addressed, although there would need to be verification of the impact on traffic

flows.

• The issue of obstruction to forward visibility on the approach to the junction

due to the overbridge structure can be accommodated without the need for

compromising road safety or relocating the existing farm overbridge structure

and tracks.

• The extent of Kennel Farm Landfill would require further investigation to

determine the extent of the site to be able to give a more informed view on the

cost implications to the scheme.

• The alternative option alignment would avoid land identified as Exchange

Land for Town and Village Green Land lost as a result of the Ashton Vale to

Temple Meads Rapid Transit Scheme.

• The alternative option would also avoid the need to construct a new farm

accommodation overbridge for Parsonage Farm to allow cattle to cross the

South Bristol Link route, reducing overall scheme costs.

However, as stated above NSC has confirmed that this alternative crosses land

reserved for future extension to Long Ashton Park & Ride and it was agreed that this

option should be discounted.

Section 3 - A370 roundabout - Location 2 (RSA1 and NSC QA Review adjustments)

Introduction

At the Design Workshop on 31st August 2012, it was agreed that the layout of the

roundabout junction on the A370 should be assessed to take account of comments

from the Stage 1 Road Safety Audit and from North Somerset Council’s initial QA

Review. The resulting layout is shown in Appendix 2 (Sketch CTRAEB/730/SK015).

Revised junction design (North of A370)

The design and location of this revised layout takes into account the

recommendations from the Stage 1 Road Safety Audit and NSC’s QA Review. The

design removes the dedicated lane northbound on the A370 as this was undesirable,

having the potential to lead to differential speeds/accidents as vehicles exit the

roundabout. The revised layout complies with Figure 2/8 from TD 51/03, reducing the

length of the southbound segregated lane onto the SBL and additional land take. The

circulatory carriageway diameter has been increased from 50m to 60m to improve

traffic flow and capacity.

Due to the location of the existing farm access overbridge on the A370, the revised

junction has been moved approximately 60m north-west of the original design in

order to meet current DMRB design standards with regards to the Desirable

Minimum SSD. A Desirable Minimum SSD of 215m over a 322.5m length on the

approach to the roundabout is achieved with this revised layout.

The RSA1 comments and NSC QA queries are addressed in the tables below.

Stage 1 Road Safety Audit

The following table sets out how the issues raised have been addressed.

Summary of issues from audit How these issued have been addressed

3.2.1: Summary: Headlight glare from

oncoming vehicles.

The layout of the traffic islands is such that vehicles

proceeding southbound on the A370 may be

The revised layout has been moved

westward as recommended in the audit.

Additionally, the splitter islands have been

extended to provide a physical separation

between the two opposing traffic streams.

affected by headlight glare (during hours of

darkness). This is attributed to the layout of the

islands on the southbound approach to the

roundabout. It is considered that drivers may be

confused by the layout presented and travel on the

wrong side of the carriageway, resulting in head-on

collisions.

This should prevent/reduce any headlight

glare from oncoming vehicles.

3.2.2: Summary: Potential for southbound

vehicles to travel on wrong side of road

Allied to Item 3.2.1 above, the layout of the traffic

islands is such that vehicles proceeding southbound

on the A370 may not immediately recognise the

intended vehicle path (particularly with the

channelised lane northbound). This may lead to

potentially severe conflicts between oncoming

vehicles.

This may be a particular problem at night and there

is a presumption that the junction will be lit. The

alignment of street lighting columns will be critical

in providing the correct vehicle path through the

junction

The alignment has been revised and splitter

islands have been extended to provide a

physical separation between the two

opposing traffic streams. The revised layout

will remove the chance of vehicles going

down the wrong side of the carriageway.

The junction will be lit.

3.2.3: Summary: Reverse curve on

northbound entry

The proposed roundabout is constructed on-line

and as such, uses a reverse curve to achieve the

desired deflection for the right-turn movement from

the A370 onto South Bristol Link Road. Reverse

curves in this configuration should be avoided

where possible. This can result in loss of control

accidents on approaches to junctions.

The revised layout has been moved

westward in order to eliminate the reverse

curve. There is still adequate deflection for

the right turn movement from the A370 onto

the South Bristol Link.

3.2.4: Summary: Separation of traffic on

channelised left turn from A370 southbound

onto South Bristol Link Road

The merge length for the channelised left turn from

the A370 onto the South Bristol Link Road is

approximately 120m long, however, there is little or

no separation between the two eastbound traffic

lanes. This may result in vehicles from the

channelised left-turn seeking to merge straight

away once they pass the end of the traffic island,

which in turn is likely to result in differential

vehicle speeds and consequently shunt or side-

swipe accidents are likely to result.

The layout with the merge has been replaced

with a ‘give way’ on the exit of the

segregated lane to avoid the risk of

differential speeds merging and shunt or

side-swipe accidents.

3.2.5: Summary: Carriageway lane lines from

the roundabout onto the A370 southbound

are not suitable.

The proposed layout shows the discontinuation of

the A370 southbound lanes (situated south of the

roundabout).

The existing A370, south of the roundabout, is two-

lane two-way for a few hundred metres before it

The revised layout will tie-in to the 3 lanes

south of the roundabout on the A370 and the

lane lines will be changed to reflect this.

reaches the viaduct. The development of the

channelised ahead movement on the A370

(northbound) will require a three-lane cross section

on the south side of the junction. The lane lines

shown do not reflect this.

Problem 3.2.6: Summary: No diverge for the

channelised ahead lane (Lane1)

The channelised ahead lane is not separated from

the main northbound approach to the roundabout.

Therefore under the current design, queuing traffic

for the South Bristol Link Road may block the ahead

lane, creating an increased risk of shunt type

and/or late lane changing accidents.

The channelised ahead lane has been

removed in the revised layout provides a

3lane entry to the roundabout to reduce the

risk of SBL traffic blocking the northbound

flow.

NSC QA review

The following table sets out how the issues raised have been addressed.

Comments from QA review How these issued have been addressed

NSC_QA_15:

New roundabout jct between SBL and A370 – The

initial decision to have a roundabout, instead of a

signalised junction, with segregated lane for the

inbound traffic into Bristol was a politically

motivated decision, most likely made to avoid any

extra delay to the volume of traffic on this route

during the AM peak. Halcrow will be scrutinising

the decision-making and options at this location to

ensure that a roundabout is the most suitable

design option. There is a strong safety concern with

the A370 NB segregated lane as the design would

force fast traffic on the NB A370 to give way/merge

with the slow right-turners from the SBL. The

A370 is a very long straight section of carriageway

with existing speed limit 60 mph.

The revised layout removes the segregation

lane northbound and so safety concerns

regarding the conflict with fast and slow

moving traffic northbound are eliminated.

NSC_QA_52:

Major traffic flows appear to run between the SBL

and Bristol. Does this need to be a signalised

junction to prevent A370 south bound traffic

causing queues on SBL westbound?

No. The traffic modelling of the junction

indicates that the roundabout performs

without traffic signals.

NSC_QA_53:

Not possible to determine proposed layout of lanes

at junction. It would appear that there is:

• A single northbound lane for Bristol bypassing

the roundabout;

• Two circulatory lanes;

• Two lanes on the link between the circulatory and

the A370 north bound;

• I have concerns that two lanes of traffic on the

circulatory appear to be merging into one lane

then joining the A370 south bound over a very

short distance. This is likely to cause congestion

The revised layout has been designed to

eliminate the need for the segregated A370

northbound

There are two circulatory lanes in this layout

northbound- the first leads to the B3128 exit

slip road to Long Ashton and the Park &

Ride facility and the second lane continues

northbound to Bristol. The two lanes will not

merge into one.

and accidents at the junction.

NSC_QA_54:

If you have dedicated left turn lanes do you require

two lanes on the roundabout approach?

With the dedicated left turn lane it is still

considered appropriate to have the two lane

entry.

NSC_QA_55:

Are the dedicated left turn lanes formed as lane

drops or diverges? Has the length of these been

considered to avoid congestion and accidents?

The dedicated left turn lane will be a diverge

and the roundabout has been checked for

capacity using ARCADY.

NSC_QA_56:

A370 south bound approach to roundabout- The

splitter island may need to be lengthened

northwards to discourage motorists from heading

in a straight line and past the wrong side of the

splitter island. You can almost drive in a straight

line through the wrong side of the roundabout to

continue along the A370.

The roundabout layout has been altered to

avoid this issue.

NSC_QA_57:

Can we provide a splitter island for the PROW

crossing, near chainage 100.000? They are required

to cross two lanes with high traffic flow from the

roundabout where drivers will not be expecting

them.

The original PROW crossing near chainage

100 has been diverted to the proposed farm

accommodation bridge near chainage 360.

NSC_QA_58:

Do we require a hard shoulder for the dedicated left

turn lanes to allow space for broken down cars and

prevent blockage of the road network?

The segregated lane width will have enough

space for a broken down vehicle to pull over

and to allow vehicles to pass.

Further Considerations

Halcrow’s revised layout for the A370 roundabout design meets the design criteria

for visibility from TD 9/93 Chapter 1. Para 1.26 (b). The required Desirable Minimum

Stopping Site Distance (SSD) of 215m over a 322.5m length on the approach to the

roundabout is achievable with this revised layout for southbound traffic.

The pre-consultation design did not achieve the required SSD and measured

approximately 174m from the Give Way due to the obstruction of the agricultural

farm overbridge abutments.

The revised layout extends beyond the existing highway boundary and will require

additional land purchase. The layout will have to consider the recently completed

Festival Way cycle track.

Conclusion

The proposed layout shown in Appendix 2 has advantages in that it addresses the

issues raised in the RSA1 and NSC QA review but impinges on land north of the

existing A370. It is therefore closer to Long Ashton and likely more visually intrusive

to Ashton Court. It was agreed at the design workshop held on 10th October 2012 that

this option should be reviewed to consider a location closer to the original

roundabout position. The results of this review are set out in the next section below.

Section 4 - A370 roundabout – Location 3 (South of A370)

Introduction

At the Third Design Workshop on 10th October 2012, there were some concerns raised

by North Somerset Council (NSC) about the revised location of the A370 junction

which was moved approximately 60m north-west of the pre-consultation design

location to meet SSD requirements. It was thought that the revised junction location

would result in objections from English Heritage and the residents of Long Ashton.

This section reviews the implications of re-locating the A370 junction closer to the

position shown at pre-planning application consultation while addressing some of

the issued raised at the time. The revised layout of this option is shown in Appendix

3.

Original junction design

The original roundabout design had an ICD of 50m, a dedicated lane northbound and

another segregated left lane southbound that linked to the SBL. Due to the presence

of the farm access overbridge, the required Desirable Minimum Stopping Site

Distance (SSD) could not be achieved for the south bound approach to the

roundabout.

TD 9/93 Chapter 1, Para 1.26 (b) states that on the approaches to a roundabout, the

lengths of carriageway on the approach shall be 1.5 times the Desirable Minimum

SSD from the Give Way line. Assuming a 100kph Design Speed based on the existing

60mph speed limit, the Desirable Minimum SSD required for a 100kph Design Speed

is 215m (table 3 in TD 9/93). Thus there is a requirement for stopping site distance of

215m to be provided over a 322.5m (1.5 x 215m) length on the approach to the

roundabout give way line.

The Desirable Minimum SSD measures approximately 167m to the give way line on

the roundabout. This value is below the DMRB’s requirements and is a departure

from standards.

Relocated junction design (South of A370)

The relocated junction has a SSD similar to that of the pre-consultation layout and

hence this would be a departure from standards with reference to TD9.

North Somerset Council use the Highways Agency standards as guidelines. As the

supervising organisation, in circumstances where environmental impact and costs

saving are significant, they have authority to agree Departures where safety is not

unduly compromised.

As a means to mitigate the impacts of the reduced SSD, it is proposed that the posted

speed be reduced from 60mph to 40mph on the southbound approach and

northbound exit. The speed limit currently changes on the Bristol side of the

proposed roundabout and it is proposed to move this change in speed limit to the

west of the roundabout. Clear advance signage on the southbound approach will also

be required to advise motorists of the roundabout.

Analysis of the Capacity of the Roundabout

Following a design review in March 2013 the layout of the A370 roundabout was

adjusted to ensure the required Entry Path Curvature (EPC) on the approaches could

be achieved without recourse to using a subsidiary raised over-run area adjacent to

the main central island. This made it necessary to increase the roundabout ICD from

60m to 70m. Following concerns raised by the Stage 1 Road Safety Audit about the

terminal ‘give way’ at the end of the segregated left turn lane (SLTL) southbound

onto the SBL; this was also replaced by a short ‘lane gain’ on exit; with a subsequent

merge to one lane at the end of this two lane section. Whilst the former ‘give way’

termination was compliant with TD51/03 it was considered that a short lane gain

would offer a safer solution, with less risk of shunts caused by drivers not expecting

to yield and braking sharply.

The revised layout has been tested using ARCADY with the 2016 Opening Year

forecasts. Results are included in Appendix 4 and a short summary on the operating

performance provided below.

An option incorporating a segregated left turning lane (SLTL) on the A370

southbound approach is preferred; as otherwise the operation of this arm is likely to

be problematic in the 2016 PM Peak Hour. The Level of Service for this arm in the PM

peak without the SLTL was found to be ‘C’ - ‘Stable Flow’ in earlier testing work; but

the maximum RFC in the nearside lane was predicted to reach 96%. Hence it was

considered appropriate to include the segregated lane in the final ‘preferred’ layout

design.

In the AM peak hour the arm under most pressure will be the A370 northbound arm

(to Bristol). This is because there is a very high conflicting circulatory right turn from

the SBL in this period; the traffic modelling predicts 950veh/hr so the lane

arrangements on the SBL arm allow for this movement to be made in both entry

lanes. The SLTL on the A370 southbound arm has no benefit in this period; as

analysis shows that a simple two lane entry would suffice. The A370 northbound arm

is predicted to reach a maximum lane RFC of 84% in the nearside/centre lanes (LOS-

B) -‘reasonable free flowing’. This suggests that the roundabout will not have much

residual spare capacity in the scheme Opening Year (2016); but will function.

Longer term it may be necessary to signalise the roundabout and, as such, the islands

and lane arrangements have been designed to facilitate this change in the future. This

would allow the SBL and A370 northbound arms to be better managed and

controlled in the AM peak hour. The Method of Control for a 3-arm signalled

roundabout would result in only right turn traffic from the A370(S) to SBL needing to

queue within the circulating carriageway at any point in the cycle. The traffic figures

suggest this movement is low in both peak periods; so subsequent signal control

would be viable as a means of extending the design life.

In the PM peak hour the ARCADY results show that the approach lanes on the A370

southbound approach (from Bristol) are expected to reach a maximum RFC of 58%;

this assuming most of the left turning traffic (90%) uses the filter lane to bypass the

main give-way line. The highest lane RFC of circa 67% occurs in the nearside lane on

the SBL approach. The maximum RFC predicted are all well below the desirable

design maximum of 85%; indicating that in this period the roundabout will operate

well within capacity in the 2016 Opening Year.

Conclusion

The revised location south of the A370, shown in Appendix 3, has advantages in that

it addresses the concerns raised by NSC at the Design Workshop of 10th October 2012

regarding the proximity to Long Ashton and the visual intrusive for Ashton Court.

These could potentially have led to objections from the residents of Long Ashton and

English Heritage. Although having a reduced SSD on the approach on the A370

southbound, it is considered that with the proposed change to the posted speed limit

and the use of appropriate advance signage, the operational safety of the roundabout

will not be unduly compromised.

For details of your nearest Halcrow office, visit our website halcrow.com

Options Appraisal (Part 2) Engineering Options · 1/3/2014 · Options Appraisal (Part 2) –...

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