Chiltern and South Bucks Local Plan
Buckinghamshire County Council
Phase 3B Modelling Report
BRJ10103\FMRP3B | 3
8 June 2018
Phas e 3B Modelling R eport Bucki nghamshire C ounty C ouncil
Phase 3B Modelling Report
Chiltern and South Bucks Local Plan
Project No: BRJ10103 Document Title: Phase 3B Modelling Report Document No.: BRJ10103\FMRP3B Revision: Final Date: 8 June 2018 Client Name: Buckinghamshire County Council Client No: C&SBLP\FMRP3B Project Manager: Tom Withey Author: Catherine Hill File Name: M:\Transport Modelling\B12798E1 Bucks Local Plans\Technical Work\3. Transport Planning\Phase 3\Report\Chiltern and South Bucks Local Plan Modelling Support Phase_3b_Final_080618.docx
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Document history and status
Revision Date Description By Review Approved
0 21/03/18 First Draft CH TW TW
1 01/05/18 Second draft following client comment CH TW TW
2 03/05/18 Third draft following client comment CH TW TW
3 08/06/18 Fourth draft following client comment CH TW TW
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Contents 1. Introduction ...... 1 1.1 Background ...... 1 1.2 Scope of study ...... 1 1.3 Development scenarios ...... 2 1.4 Mitigation options ...... 2 1.5 Structure of report ...... 2 2. Modelling methodology ...... 4 2.1 Overview ...... 4 2.2 Forecast model updates ...... 4 2.2.1 M25 smart motorway scheme (Junction 10 to 16) ...... 4 2.2.2 Site access points ...... 4 2.2.3 Development of the DM forecast scenario ...... 4 2.2.4 Development of the DS forecast scenario ...... 4 2.2.5 Adjustment for Crossrail ...... 5 2.3 Modelling the mitigation options ...... 5 2.3.1 Highway schemes ...... 5 3. Development scenarios ...... 6 3.1 Overview ...... 6 3.2 Development summary...... 6 3.2.1 Do Minimum ...... 10 3.2.2 Do Something ...... 10 3.2.3 Comparison with National Trip End Model (NTEM) ...... 10 4. Mitigation options ...... 12 4.1 Overview ...... 12 4.2 Option generation ...... 12 4.3 Option sifting ...... 12 4.4 Options for appraisal ...... 12 5. Results ...... 16 5.1 Overview ...... 16 5.1.1 Congestion ratio ...... 16 5.1.2 Change in travel time ...... 16 5.1.3 Development select link plots ...... 17 5.2 Summary of Impacts ...... 17 5.2.1 Great Missenden ...... 17 5.2.2 Prestwood ...... 17 5.2.3 Chesham ...... 17 5.2.4 Amersham ...... 21 5.2.5 Little Chalfont ...... 25 5.2.6 Holmer Green (and the Hazelmere Crossroads) ...... 27 5.2.7 Chalfont St Giles ...... 29 5.2.8 Chalfont St Peter ...... 29
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5.2.9 Beaconsfield ...... 31 5.2.10 Gerrards Cross ...... 34 5.2.11 Denham area (including the Denham Roundabout) ...... 34 5.2.12 Iver and Iver Heath ...... 37 5.2.13 Middle Green ...... 41 5.2.14 Stoke Poges ...... 41 5.2.15 Farnham Common and Farnham Royal ...... 41 5.2.16 Burnham ...... 42 5.2.17 Taplow ...... 45 6. Summary and conclusion ...... 46 6.1 Summary of results ...... 46 6.2 Conclusions ...... 51
Figure 3-A Chiltern and South Bucks Districts forecast housing growth assumptions ...... 8 Figure 3-B Chiltern and South Bucks Districts forecast employment growth assumptions...... 9 Figure 5-A AM 2036 DS - AM 2036 DM Change in Travel Time- Chesham ...... 19 Figure 5-B AM 2036 DS Select Link Plot – Local Plan developments in Chesham ...... 20 Figure 5-C AM 2036 DS Mitigation – AM 2036 DM change in travel time – Chesham ...... 21 Figure 5-D 2036 DS – AM 2036 DM change in travel time – Amersham ...... 22 Figure 5-E AM 2036 DS Select Link Plot – Area South of Amersham (Between A413 and A355) and Area South of Amersham (Land at Crown Farm) Option Sites 4 and 5 ...... 23 Figure 5-F AM 2036 DS Select Link Plot – Area South of Little Chalfont Green Belt option (6) ...... 24 Figure 5-G AM 2036 DS Mitigation – AM 2036 DM change in travel time - Amersham ...... 25 Figure 5-H PM 2036 DS – PM 2036 DM change in travel time – Little Chalfont ...... 26 Figure 5-I AM 2036 DS Mitigation- AM 2036 DM change in travel time- Little Chalfont ...... 27 Figure 5-J AM 2036 DS – AM 2036 DM change in travel time – Hazelmere Crossroads ...... 28 Figure 5-K AM 2036 DS Mitigation- AM 2026 DM change in travel time- Hazelmere Crossroads ...... 29 Figure 5-L AM 2036 DS – AM 2036 DM change in travel time – Chalfont St Peter ...... 30 Figure 5-M AM 2036 DS Mitigation – AM 2036 DS change in travel time – Chalfont St Peter ...... 31 Figure 5-N AM 2036 DS – AM 2036 DM change in travel time – Beaconsfield ...... 32 Figure 5-O AM 2036 DS Select Link Plot – Local Plan Developments in South of Beaconsfield ...... 33 Figure 5-P AM 2036 DS Mitigation – AM 2036 DM change in travel time – Beaconsfield ...... 34 Figure 5-Q AM 2036 DS – AM 2036 DM change in travel time – Denham area...... 36 Figure 5-R AM 2036 DS Select Link Plot – Land North of Denham Roundabout Green Belt option (10) ...... 37 Figure 5-S AM 2036 DS – AM 2036 DM change in travel time – Iver ...... 39 Figure 5-T AM 2036 DS Select Link Plot – Green Belt options 13 and 14 ...... 40 Figure 5-U AM 2036 DS Mitigation – AM 2036 DM change in travel time – Iver ...... 41 Figure 5-V AM 2036 DS – AM 2036 DM change in travel time – Burnham ...... 43 Figure 5-W AM 2036 DS Select Link Plot – Land Adjacent to Taplow Station Green Belt option (15) ...... 44 Figure 5-X AM 2036 DS Mitigation – AM 2036 DM change in travel time – Burnham ...... 45
Table 3-A Revised forecast scenarios ...... 7 Table 3-B Do Minimum Phase 3 growth ...... 10 Table 3-C Do Something Phase 3 growth ...... 10 Table 3-D 2036 modelled scenario growth and NTEM growth ...... 11 Table 4-A List of mitigation options which have been included in the DS with mitigation scenario ...... 15 Table 5-A Congestion ratio criteria ...... 16 Table 6-A RAG rating description ...... 46 Table 6-B Impact summary table...... 51
Appendix A. Local Plan Modelling Phase 2 Development Summary Appendix B. Green Belt Options tested in Phase 3B Appendix C. Congestion Ratio
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Appendix D. Change in travel time
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1. Introduction
1.1 Background
Jacobs is framework consultant to the Transport for Buckinghamshire Alliance (TfB) between Ringway Jacobs and Buckinghamshire County Council (BCC). Under the terms of this contract, Jacobs is commissioned to undertake transport planning, modelling and appraisal projects on behalf of BCC.
Jacobs has been commissioned to assess the transport impact of the joint Local Plan proposals of Chiltern (CDC) and South Bucks (SBDC) District Councils. The joint Local Plan proposal is comprised of the latest development quantum for the Local Plan preferred Green Belt options and the council’s draft Housing and Economic Land Availability Assessment (HELAA). For the purpose of this report, the districts of Chiltern and South Bucks will hereafter be referred to as ‘the districts’. This report has been prepared in conjunction with Buckinghamshire County Council's Transport Strategy Team which has advised on the methodology and content of the report from the perspective of the Local Highway Authority . In addition, Buckinghamshire County Council’s Highways Development Management Team and Transport for Buckinghamshire were involved in the development of the mitigation options.
Jacobs has already completed the first and second phase of the modelling support work, which assessed the transport impacts of a number of potential Local Plan development growth scenarios, with varying levels of future year employment and housing growth up to 2036. Further details of the previous phases of work can be found in the respective phase one and phase two modelling reports.
This report documents the results of the phase three traffic modelling work, and outlines the development of the revised phase three Local Plan growth scenario and revised committed development scenario. This involved testing the Local Plan growth scenario based on updated information for the Green Belt Preferred Options, sites from the draft Housing and Economic Land Availability Assessment May 2017 (HELAA) with the latest quantum of development provided by the districts, as well as revising the phase two committed development scenario to include new planning completions, and planning permissions granted, since 2015.
As with phase two, an additional Local Plan growth scenario with mitigation has also been developed, which includes the same development growth assumptions as the without mitigation scenario, but with the addition of a selection of potential mitigation schemes designed to mitigate the impacts of the housing and employment growth. This comprises a number of schemes, some of which are carried over from the previous phases of work and some of which are new options for phase three.
In parallel with this work, Jacobs has also completed a similar exercise combining the Local Plan assessments for all four districts within Buckinghamshire; known as the ‘Countywide Local Plan Modelling Phase 3’. Further details can be found in the Phase 3 Forecast Modelling Report 1.
Where the districts are concerned, the Countywide Local Plan Modelling overlaps with this joint Local Plan assessment; however, as the two Local Plan assessments have been created for different purposes, using a different modelling methodology, it is expected that the model outputs may differ in some areas. It is also important to note that the local modelling constitutes a more detailed assessment (with a higher degree of validation) than the Countywide Local Plan Modelling work where the districts of Chiltern and South Bucks are concerned and therefore supersedes the Countywide Local Plan Modelling.
1.2 Scope of study
This report covers the third phase of modelling work, which primarily focusses on identifying and mitigating the impacts of the preferred Green Belt option and draft HELAA sites on traffic conditions within the districts. The scenario also takes into account a single housing development at Hazelmere, known as the site adjoining Holmer Green. The site is situated on the boundary between Wycombe and Chiltern Districts, forming part of Wycombe District Council’s emerging Local Plan. Impacts on motorway junctions are being assessed
1 Jacobs. 2017. Countywide Local Plan Modelling: Phase 3 Forecast Modelling Report.
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separately from the work detailed in this report and the results can be found in the Chiltern and South Bucks Local Plan Modelling Support Phase 3a report 2.
The objectives of the project are summarised below: • To model the impact of the Local Plan growth scenario across the districts and the surrounding network, to identify areas of the highway network which are adversely impacted by the proposed development. • To model the impact of a number of highway schemes in the Local Plan growth scenario, to identify which schemes successfully mitigate any identified impacts. • To produce model outputs which illustrate the impacts of the Local Plan growth scenario with and without mitigation compared with the Do Minimum.
1.3 Development scenarios
A number of different forecast scenarios were developed during the first and second phases of work in cooperation with BCC and the districts. This comprised of a Do Minimum (DM) scenario, which included only committed developments up to 2036, and several different Do Something (DS) scenarios which tested a variety of different growth scenarios for the joint Local Plan proposals, on top of committed development in the DM.
For phase three, the land use assumptions in the DM development scenario have been updated to include any new planning completions, as well as additional planning permissions granted, since the phase two work was completed. It should be noted that the development quantum for Wycombe and Aylesbury remain unchanged from phase two.
The DS scenario for this phase has also been updated to reflect the latest development quantum for the Local Plan preferred Green Belt options and the council’s draft HELAA. Further details of the forecast scenarios are provided in Table 3-A in section 3. As with the previous phases of work, a DS with mitigation scenario has also been developed which uses the same development quantum as the phase three DS scenario.
By comparing the outputs from the DM and DS scenarios, the impacts of HELAA and Local Plan preferred Green Belt options, can be ascertained. And by comparing those scenarios against the DS with mitigation scenario, the extent to which they can be mitigated, can be ascertained.
1.4 Mitigation options
For phase three, a package of mitigation options has been put together in cooperation with the districts to mitigate the impacts of the Local Plan growth scenario. This comprises of a mixture of options carried over from phase two and supplemented by additional schemes brought forward for phase three.
Section 4 provides further detail of the options included for this phase of work.
1.5 Structure of report The structure of the report is as follows:
1. Introduction – Outlines the purpose and background of the report. 2. Modelling methodology – Describes the development of the revised forecast scenarios and the modelling of the mitigation options. 3. Development scenarios – Summarises the development land use changes in the revised DS forecast scenario. 4. Mitigation options – Summarises the option sifting process and mitigation options taken forward for testing and appraisal.
2 Jacobs. 2018. Chiltern and South Bucks Local Plan Modelling Support Phase 3a Report
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5. Results – Presents the results of the revised DS forecast scenario and the DS with mitigation forecast scenario. 6. Summary and conclusion – Summarises the results of the phase three modelling support work.
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2. Modelling methodology
2.1 Overview
The model used to test the development impacts has a base year of 2015 and a forecast year of 2036. This section sets out the updates made to the forecast year networks and matrices as part of the phase three work, as well as the modelling process for the mitigation options in the DS with mitigation scenario.
For phase three both the DM and DS scenarios have been revised, as set out below. It is worth noting that some minor revisions to the model network have also been included.
2.2 Forecast model updates
2.2.1 M25 smart motorway scheme (Junction 10 to 16)
Following the completion of the majority of modelling work for phase two, BCC requested that Jacobs test the M25 smart motorway scheme, which had not previously been considered in the project scope. This was carried out as an addendum to the existing modelling work which had already been completed at that time.
For phase three, the DM network has been updated to take account of this scheme as it is now planned to be in place by 2036.
2.2.2 Site access points
For this phase of work, the site access points were based on a high level assessment of potential access solutions with BCC Highways DM. As a result, the loading points have been updated to reflect these indicative access points. However, they are indicative points and are not necessarily the points which would be utilised in any finalised scheme.
2.2.3 Development of the DM forecast scenario
The land use assumptions for the DM scenario have been updated from the previous phase of work. In this phase, planning completions and employment and housing commitments have been updated for Chiltern District and South Bucks District. The DM land use assumptions for Aylesbury Vale and Wycombe District remain unchanged from phase two.
Therefore, the trips assumed within the DM matrices for this phase of work have been updated from the phase two work, to reflect the revised land use assumptions for Chiltern District and South Bucks District.
2.2.4 Development of the DS forecast scenario
A number of updates have been made to the development growth assumptions in the DS to reflect the district’s latest Local Plan growth scenario. This includes revisions to both the preferred Green Belt options as well as the draft HELAA sites. Further details of the employment and housing development quantum can be found in section 3 of this report.
The trip generation and distribution for the additional DS development has been derived from TRICS trip rates and Census Journey to Work data, respectively. This exercise was carried out during the phase two modelling work and for the most part this remains unchanged for phase three. However, at the request of Highways England, the TRICS trip rate used for the office employment sites has been revised for this phase of work so that they reflect the rates more likely to be generated from the sites. It should be noted that at one site, the National Epilepsy Centre, there is a specialist accommodation for the elderly which is likely to have lower residential trip rates than have been assumed in the modelling.
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2.2.5 Adjustment for Crossrail
In addition to the above model updates, the impacts of Crossrail have been modelled in the DM and DS forecast scenarios in the form of a reduction in car journeys (to represent a mode shift from car to rail) in impacted areas. The extent of the reduction in car journeys has been derived using the following assumptions: • Only car journeys which start or end within 1,500m of a Crossrail station are considered (for stations that fall within the London zones, all car journeys have been considered). • 35% of these journeys will switch from car to rail in relation to Crossrail. The Crossrail trip reduction for zones within the Chiltern and South Bucks Model has previously been calculated as part of the Countywide Local Plan Modelling work1. Therefore, these trip reductions have been carried over to this piece of work.
The assumed percentage reductions and radii were calibrated such that the outturn reduction in car trips approximated the reductions calculated by separate third party modelling of those schemes. This was to ensure that the modelling assumptions/trip impacts were consistent across the different modelling exercises for business case development for these national strategic infrastructure schemes. This modelling data was provided by Transport for London.
2.3 Modelling the mitigation options
A number of mitigation options have been shortlisted to be included in the DS forecast scenario. Further detail of the sifting process and the options identified can be found in section 4 of this report. The following subsection summarises the methodology adopted to model the mitigation options.
2.3.1 Highway schemes
A number of highway schemes have been added to the DS scenario in consultation with the districts; where possible scheme coding has been carried over for phase two. Any new schemes included have been coded based on detailed information provided by BCC and the districts.
However, at this stage, a number of the mitigation schemes are conceptual only, and are designed to identify locations where some form of mitigation in the future would likely reduce some of the negative impacts of the DS development on the highway network.
A full list of the mitigation options included in the DS with mitigation scenario can be found in Table 4-A. This includes a description of all changes made to the model.
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3. Development scenarios
3.1 Overview
This section sets out the revisions made to the DM and DS forecast scenario, in line with the updated land use information provided by the districts. This represents the latest Local Plan preferred Green Belt option scenario and committed development scenario.
The committed development scenario (DM) consists of:
• Planning completions from 2015 to 2017
• Latest employment and housing commitment data up to 2036 within the districts
The Local Plan growth scenario (DS) consists of:
• DM completions and commitments as above
• 35 draft HELAA housing sites in Chiltern District
• 15 draft HELAA housing sites in South Bucks District
• 4 draft HELAA employment sites in Chiltern District
• 1 draft HELAA employment site in South Bucks district
• 14 Green Belt sites across the districts (1 employment, 1 specialist housing, 7 housing and 5 housing and employment)
• 1 Green Belt housing site at Hazlemere in Wycombe District, adjoining Holmer Green in Chiltern District.
For both the DM and DS development scenario, forecast housing and employment growth has been added to the existing phase two 2015 base land use information to generate a new development quantum.
3.2 Development summary
Table 3-A provides a summary of the employment and housing figures included in the phase three DM and DS scenario. Details of phase 2 inclusions can be found in Appendix A.
Future scenario (2036) Summary details
• As phase two but; • An additional 594 houses and 796 jobs in Chiltern District • An additional 598 houses and 473 jobs in South Do Minimum (DM) committed Bucks District development scenario • No change from phase two in Aylesbury Vale and Wycombe District • Growth capped to NTEM levels outside of Buckinghamshire
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Future scenario (2036) Summary details
• As phase two but; • A reduction of 800 houses and an additional 2,188 jobs in Chiltern District Do Something (DS) Local Plan growth • A reduction of 882 houses and an additional 4,057 scenario jobs in South Bucks District • An additional 30 houses (on top of the 320 assumed in phase 2) in Wycombe District at the Hazlemere site
Table 3-A Revised forecast scenarios
Compared with the phase two development scenario, there has been a reduction of 1,652 houses for phase three. However, there are an additional 6,245 jobs in the DS phase 3 forecast across Chiltern and South Bucks district. There has been no change in either housing or jobs to Aylesbury Vale in the DS for this phase.
Figure 3-A and Figure 3-B show the distribution of draft HELAA sites and the preferred Green Belt options across the different settlement areas in the DS scenario. Appendix B also sets out the locations of the preferred Green Belt options.
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DM : Outstanding housing commitments
A: Draft HELAA housing sites – CDC & DM 415 SBDC A 393 B 115 DM 8 B: Preferred Green Belt housing sites
A 7 DM 67 B 0 A 69 B 0 DM 205
A 141
Chesham B 50
G. Missenden Prestwood DM 39 DM 114 A 18 A 135 Chesham Bois B 0 DM 0 B 700
A 9 Amersham B 0 L. Chalfont Holmer Green DM 0 A 0 DM 0 B 0 A 0 Penn B 650 Chalfont St. Giles DM 650
Knotty Green A 105 Seer Green DM 40 Chalfont B 410
A 0 Beaconsfield St. Peter B 0 DM 180 DM 400 A 0 A 30 Gerrards Denham B 0 DM 149 B 0 Cross Green DM 0
A 463 DM 96 A 0 Denham B 1,000 DM 38 B 0 A 0 Farnham A 11 B 0 New Denham Common Stoke B 0
Poges Burnham Iver Heath DM 246 Taplow DM 16 A 0 Iver A 0 B 0 Middle Green B 430 DM 59 Richings A 0 DM 124 Park DM 80 B 0 DM 10 DM 216 A 26 A 35 A 0 DM 0 A 0 B 0 B 0 B 0 A 0 B 850
B 0
Growth areas/ existing settlements
Figure 3-A Chiltern and South Bucks Districts forecast housing growth assumptions
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DM : Outstanding Employment commitments DM 635
A: Draft HELAA Employment sites – CDC & A -317 SBDC B 0 DM 0 B: Preferred Green Belt Employment sites A 0 DM 0 B 0
A 0 B 0 DM 0
A 271
Chesham B 0
G. Missenden Prestwood DM 0 DM 0 A 833 A 571 Chesham Bois B 0 DM 0 B 556
A 0 Amersham B 0 L. Chalfont Holmer Green DM 0 A 0 DM 0 B 0 A 0 Penn B 0 Chalfont St. Giles DM 162 Knotty Green A 0 Seer Green DM 0 Chalfont B 0
A 0 Beaconsfield St. Peter B 0 DM 0 DM 0 A 2083 A 0 Gerrards Denham B 0 DM 45 B 0 Cross Green DM 0
A 0 DM 0 A 0 Denham B 1667 A 0 B 0 DM 170 Farnham B 0 New Denham A 0 Common Stoke B 1567 Poges
Burnham Iver Heath DM 28 Taplow DM 1619 A 0 Iver A 0 B 1233 Middle Green B 1044 DM 0 Richings A 0 DM 0 Park DM 0 B 0 DM 0 DM 231 A 0 A 0 A 0 DM 0 A 0 B 0 B 0 B 0 A 0 B 2567
B 0
Growth areas/ existing settlements
Figure 3-B Chiltern and South Bucks Districts forecast employment growth assumptions
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3.2.1 Do Minimum
Table 3-B displays the additional houses and jobs that are included in the DM land use scenario from 2015 to 2036.
Location Totals
Chiltern District • 1,547 houses and 796 jobs
South Bucks District • 1,614 houses and 2,092 jobs
Wycombe District • As phase 2
Aylesbury Value District • As phase 2
Table 3-B Do Minimum Phase 3 growth
3.2.2 Do Something
Within the county, the DS phase 3 scenario contains the DM land use plus the Local Plan growth scenario. Table 3-C provides a summary of the DS scenario for Chiltern, South Bucks and Wycombe district.
Location Totals
Chiltern District • DM commitment plus 2,452 houses and 1,914 jobs
South Bucks District • DM commitment plus 2,845 houses and 10,161 jobs
Wycombe District • DM commitment plus 350 houses
Aylesbury Vale District • DM commitment
Table 3-C Do Something Phase 3 growth
3.2.3 Comparison with National Trip End Model (NTEM)
NTEM is an industry standard tool provided by the Department for Transport for calculating future trip growth and is used in the majority of transport scheme business cases and for other transport planning purposes. Table 3-D provides a summary of the total household and job growth for both of the 2036 forecast scenarios. Alongside these scenarios, the table also includes NTEM growth figures for the period 2013 to 2036. This is from version 6.2 of the dataset and is being used for comparative purposes.
Consistency with NTEM growth figures is a requirement for all WebTAG compliant models to be used for major scheme business cases. This modelling is for a Local Plan assessment rather than a business case, and therefore it is not necessary to constrain growth to NTEM. Indeed, because the Local Plan growth is generally in excess of NTEM levels (particularly in South Bucks), it was decided that constraining to NTEM growth would not be necessary at this stage.
A comparison of the model against NTEM is useful as it helps to identify the scale of difference between NTEM and the Local Plan assumptions, and thereby understand how the districts’ Local Plan growth differs from the levels of growth mandated by the Department for Transport for use in transport scheme business cases.
NTEM DM DS District HH Jobs HH Jobs HH Jobs
Chiltern 4,748 2,747 1,547 796 3,999 2,710
South Bucks 609 2,382 1,614 2,092 4,459 12,253
Total 5,357 5,129 3,161 2,888 8,458 14,963
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Table 3-D 2036 modelled scenario growth and NTEM growth
Across all forecast scenarios growth in households is higher in South Bucks compared with Chiltern; however, the opposite trend is observed in the NTEM growth data. This reflects the higher proportion of outstanding housing and employment commitments, draft HELAA sites and preferred Green Belt options which fall within South Bucks District in the forecast scenarios.
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4. Mitigation options
4.1 Overview
This section presents the option generation and sifting process in relation to the mitigation options which have been included in the DS with mitigation forecast scenario. A full list of the options selected for appraisal can be found in Table 4-A.
4.2 Option generation
In collaboration with the districts a long list of mitigation options were collated. This included a number of mitigation options that had already been modelled as part of the Countywide Local Plan Modelling work 1, as well as a variety of other suggestions from a number of different sources. In most cases, the options were designed to mitigate the anticipated rise in congestion and delay in the vicinity of the Local Plan sites and preferred Green Belt options. However, in some cases more general mitigation ideas were also included.
The long list included a variety of highway improvements such as signal optimisation, traffic calming measures, development access improvements, general junction improvements, relief roads and carriageway widening.
4.3 Option sifting
A mitigation teleconference workshop was held on the 13th th February 2018 with the districts and BCC to discuss the long list of mitigation options in relation to the model outputs produced from the DS forecast scenario. It was decided that the primary aim of the mitigation options should be to reduce the traffic impact of the Local Plan growth scenario on the local road network. As a result of this meeting and a subsequent meeting on the 20 th February 2018, a select number of schemes from the long list, which were considered to meet this criterion, were brought forward for further appraisal and modelling.
4.4 Options for appraisal Table 4-A below outlines the mitigation options taken forward for appraisal and modelling in the DS with mitigation forecast scenario. Mitigation measures were those chosen after the sifting process was completed. This represents then a strategic appraisal of measures considered at this stage to be appropriate.
Scheme name Scheme description
Taplow Station Additional eastbound traffic lane on Bath Road and a right turn ban into Access and Berry Hill. Improvements
A new single carriageway relief road south of Iver connecting with Iver Relief Road Thorney Lane north and Mansion Lane via two new priority junctions. (Option 3b)
This scheme includes traffic calming measures on several roads in Beaconsfield Beaconsfield including Wattleton Road, Burkes Lane, Holtspur Top Lane, Transport Gregories Road and Candlemass Lane. It also includes a ban of right Strategy turns at the A40/ Broad Lane junction.
Berry Hill Signal timing optimisation to reduce queuing at this junction. Junction, Taplow
Gore Hill Capacity improvements at the junction including a new left turn slip from Roundabout the A355 south. Improvements
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Scheme name Scheme description
Additional A second access point to the development on the A355 Relief road, north development of the existing junction for easier access from the north for development access point for traffic. This consists of a new three arm roundabout with single lane the Area East of approaches. Beaconsfield site
Signalisation of The existing priority junction will be upgraded to a signalised junction to the A413 / mitigate the additional traffic from the new developments in Chalfont St Gravel Hill Peter. The existing junction geometry will be maintained. Junction, Chalfont St Peter
Additional The existing roundabout will be upgraded to include an additional lane on capacity at A413 approach from the eastern arm to alleviate congestion arising from the / Joiners Lane developments to the east. Roundabout, Chalfont St Peter
Ledborough The existing staggered priority junctions will be replaced with a four arm Lane/ roundabout with two lane approaches and a two lane circulatory. The Longbottom purpose of this scheme is to divert traffic to the east of Beaconsfield to Lane reduce congestion through the town centre. Roundabout, Beaconsfield
Capacity The upgrades to the existing roundabout include a new left turn filter lane improvements at from A40E to A335S, an additional lane on approach from A40W and an Pyebush additional lane on approach from A355 relief road. The purpose of the Roundabout, scheme is to manage the additional traffic moving through this junction Beaconsfield with the development in place.
White Lion Road/ The junction improvements comprise widening of the traffic lanes on the Stanley Hill approach from A404 Stanley Hill, to ease the congestion arising from the Roundabout Area South of Little Chalfont preferred Green Belt options. Improvements, Amersham
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Scheme name Scheme description
A413 / Stanley The improvements comprise of widening the approach lanes from the Hill / A355 A404 Stanley Hill to ease congestion resulting from the additional Roundabout development traffic heading southbound from the new developments in Improvements, Amersham and from the Area South of Little Chalfont preferred Green Amersham Belt options.
Signalisation of The existing roundabout will be fully signalised to manage the additional the A416/ Park development traffic from the north of Chesham. Signal timings will be Rd Roundabout, optimised within the model as no signal timings are available for Chesham inclusion.
Widening of A4 The existing carriageway between Huntercombe Lane/ Bath Road Bath Road and junction and the Bath Road/ M4 junction 7 roundabout will be widened to signal two lanes in both directions. In addition, the signal timings at the optimisation of Huntercombe Lane/ A4 Bath Road junction will be improved to A4/ accommodate the additional development traffic passing through this Huntercombe area. Note that these proposed mitigations are in Slough. Lane Junction, Burnham
Capacity The existing double roundabout will be upgraded to include two lane improvements at approaches on all arms to reduce queuing during the peak hours. the Hazlemere Crossroads
Additional The existing roundabout will be upgraded to include an additional lane on capacity at A413 the approach from the northern arm to alleviate congestion arising from / Joiners Lane the developments to the east. Roundabout, Chalfont St Peter
Ledborough The new roundabout will also include a left turn filter lane from Lane/ Ledborough Lane to the A355, in order to remove a proportion of vehicles Longbottom from the western approach to the roundabout. Lane Roundabout, Beaconsfield
Signalisation of The existing priority junction with the A40 will be upgraded to a signalised Potkiln Lane, junction. Currently it is difficult for traffic to turn out of this junction onto Beaconsfield the A40 which results in queuing on the approach from Potkiln Lane. Signalising the junction should allow more traffic to egress.
Signalisation of The existing roundabout will be fully signalised to manage the additional the A416/ Broad development traffic from the north of Chesham. Signal timings will be St Roundabout, optimised within the model as no signal timings are available at this time. Chesham
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Scheme name Scheme description
Additional Further enhancements to the existing roundabout to include an additional capacity lane on approach from A355 S. The purpose of the scheme is to manage improvements at the additional traffic moving through this junction with the development in Pyebush place and other mitigations relieving congestion on the other three arms. Roundabout, Beaconsfield
All movements A new access junction to facilitate access to development in place of the priority junction DS modelled access on Hedgerley Lane. on A40 between Burnham Avenue and Pyebush
Queenway Link A new roundabout to replace the existing Eastern Dene/A404 junction, and Eastern combined with the Queensway link. As a package, this will provide a Dene bypass of Hazlemere Crossroads for certain movements. Improvements
Burton Lane A mini roundabout in place of the current priority junction at Burton Junction Lane/A404. Improvements
Table 4-A List of mitigation options which have been included in the DS with mitigation scenario
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5. Results
5.1 Overview
The purpose of this section is to present the modelling outputs for the DM, DS and DS with mitigation forecast scenarios. As with phase one and two, a set of model outputs have been produced to illustrate the impacts of the DS forecast scenario compared with the DM. However, in addition to this the same set of outputs have been generated comparing the DS ‘with mitigation’ against both the DS ‘without mitigation’ and the DM.
All of the plots are available to view in Appendix C and Appendix D.
5.1.1 Congestion ratio
The congestion ratio plots show the ratio of the congested travel time to the free flow travel time on each modelled link. An increase in the congested travel time on a link is not only affected by increases in flow, but also delays at the downstream junction. As a result it is possible, where junctions are constrained, to see congestion on a particular link, without any significant increase in demand flow.
These plots have been generated for all scenarios (DM, DS, DS mitigation) and time periods (AM, IP, PM).
Links are plotted according to the following criteria:
Colour of the band Congestion ratio Interpretation
Transparent 100% Link experiences free flow conditions
Travel times are up to 50% greater than in the uncongested Green <=150% situation
Travel times are one and a half (150%) to two times (200%) Yellow <=200% higher than in the uncongested situation
Travel times are two (200%) to four times (400%) higher than in Orange <=400% the uncongested situation
Travel times are more than four times (>400%) higher than in Red >400% the uncongested situation
Table 5-A Congestion ratio criteria
5.1.2 Change in travel time
The change in travel time plots show the difference in congested link travel times between an altered and comparison scenario (for example DS and DM) as a percentage. The congested link travel time is the same as that used for the congestion ratio. It is worth noting that where an area is already congested in the comparison model, travel times will be more sensitive to smaller increases in trips.
In addition, the % change is only shown when the congested travel time is more than twice the free flow time in either scenario. This ensures that only those areas which experience relatively high levels of congestion are shown. Plots have been produced comparing the DS with the DM, the DS with mitigation with the DM, and the DS with mitigation with the DS.
The DS with mitigation change in travel time plots compared with the DM show the impact of the development plus the mitigation over the DM case. The DS with mitigation change in travel time plots compared with the DS show the impact of the mitigation on the network over the DS case i.e. with development but no mitigation.
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5.1.3 Development select link plots
The select link plots show the hourly demand volume (in vehicles) travelling to and from a specific preferred Green Belt option. Demand flow is the ‘unmetered’ traffic flow, i.e. it shows the demand for travel on a link within the modelled hour, as opposed to the amount of flow that can actually use the link given upstream capacity constraint and queuing.
The location of each of the ’preferred’ Green Belt options is shown in Appendix B.
5.2 Summary of Impacts
To aid interpretation of the model outputs, a description of the impacts has been provided below for each settlement area described in the phase one report. Where necessary, plots of congestion ratio and/or change in travel time have been provided. In some cases this also includes development select link plots. It should be noted that comments regarding the impacts on the strategic road network (M4, M25 and M40) are provided in the phase 3a report3.
5.2.1 Great Missenden
The inclusion of the Local Plan growth scenario has only a slight impact on the Great Missenden area, with limited increases of less than 5% in travel time on a handful of links. These small increases are due to a slight increase in demand flow in this area resulting from the additional development across the districts.
This is to be expected as there are no Green Belt sites in the near vicinity, and an increase of less than 100 houses from the draft HELAA sites.
5.2.2 Prestwood
As with Great Missenden, the inclusion of the Local Plan growth scenario has a slight impact on the area in terms of increases in travel time and congestion. There are some slight increases in travel time on the A4128 through the settlement of up to 0.5%. This is the result of small increases in demand flow in this area resulting from the additional development across the districts.
This is to be expected as there are no Green Belt sites in the near vicinity, and an increase of less than ten houses from the draft HELAA sites.
5.2.3 Chesham
There are significant increases in travel time in the AM peak with the inclusion of the Local Plan growth developments. The main cause of the observed impacts in the town centre can be attributed to demand flow increases through the A416/ Park Road Roundabout and A416/ White Hill Roundabout, both of which are over capacity on the approach to their respective northern arms.
The impact of the increased traffic results in vehicles queuing back from the A416/ Park Road Roundabout, through the A416/ White Hill Roundabout, and onto the surrounding road network. Roads and junctions particularly affected in the AM peak, compared with the DM, include A416 Broad Street (up to 300%), A416/ Park Road Roundabout and A416/ White Hill Roundabout. Figure 5-A illustrates the observed impacts in the AM peak in Chesham town centre.
Away from the town centre in the AM peak, there are also significant travel time increases on the approach to the B485/ Wey Lane/ Church Street junction (up to 725%), and significant travel time increases on B485 Missenden Road eastbound (up to 700%), compared with the DM. The travel time increase on the B485 Missenden Road is the result of increased queuing back from the B485/ A416 priority junction (which is already
3 Chiltern and South Bucks Local Plan Modelling Support, Local Plan Impacts on the Strategic Road Network. March 2018
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significant in the DM scenario), where capacity for left turning traffic has decreased because of an increase in northbound traffic on the A416.
There is much less of an impact in the PM peak with only Pednor Road showing significant adverse impacts; the cause of the impact is the same as with the AM peak, however flows on the B485, exiting at the A416 junction are lower than in the AM.
The traffic increases on the A416, which are the overall cause of the impacts seen in the DS scenario, are as a result of the combined effect of traffic increases from the draft HELAA sites in Chesham and the Area North East of Chesham Green Belt option (1). This is illustrated in Figure 5-B, which shows the traffic flows from those sites.
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The observed increases in travel time are the result of delay at the highlighted junctions, due to increase in demand flow on the A416.
A416/ White Hill Rbt
A416
A416/ Park Road Rbt
Figure 5-A AM 2036 DS - AM 2036 DM Change in Travel Time- Chesham
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The Green belt and HELAA sites result in approximately 130 additional two-ways trips on the A416.
A416/ White Hill Rbt
A416/ Park Road Rbt
Figure 5-B AM 2036 DS Select Link Plot – Local Plan developments in Chesham
Two mitigation schemes have been tested at this location. This includes the signalisation of the A416/ Park Road roundabout and A416/ White Hill Roundabout to prioritise traffic on the A416, and reduce the congestion that occurs with the Local Plan growth scenario in place. Figure 5-C illustrates the impact of the mitigation in terms of travel time changes in the AM peak compared with the DM situation.
With the mitigation in place, there are travel time improvements on the A416 itself, but there are significant travel time increases on the side roads joining the A416. This is due to the signalisation of the aforementioned junctions, which encourages more through traffic on the A416 which subsequently reduces the capacities for vehicles on the side roads.
It should be noted that with signalisation at the two roundabouts, there is scope to revise the signal timings to better manage the congestion levels in the future, e.g. to provide more capacity to side roads and less to the A416, in order to reach a better balance between conflicting movements. That exercise has not taken place as part of the Local Plan Modelling, however some form of detailed junction modelling may be required as mitigation for relevant planning applications or as part of a wider mitigation scheme for the area and would need to be considered in the context of the air quality management area.
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The inclusion of the signalised junctions reduces the impact that the additional development has on the A416 compared with the DS without mitigation scenario.
A416/ White Hill Rbt Cameron Rd
White Hill
A416
A416/ Park Road Rbt
Figure 5-C AM 2036 DS Mitigation – AM 2036 DM change in travel time – Chesham
5.2.4 Amersham
With the addition of the preferred Green Belt options (to the south and east of Amersham) and draft HELAA sites there are some significant increases in travel time localised to the east and south of Amersham, as a result of demand flow increases on the main corridors through the town. These impacts are observed particularly in the AM peak, but also to an extent in the PM peak, compared with the respective DM scenario.
Impacted roads and junctions which experience significant travel time increases include the westbound A404 White Lion Road (up to 110%) in the AM and PM peak, on the westbound side of A404/ White Lion Rd Roundabout. This roundabout is over capacity as result of the demand flow increases on the A404 arising from the draft HELAA sites and the Area South of Little Chalfont (West of Lodge Lane) Green Belt site to the east. Although the demand flow increases are relatively small, the already congested nature of the junction in the DM scenario leads to proportionally much higher impacts than the demand flows would suggest.
At the A404/ A413/ London Rd Roundabout, there are significant observed increases in travel time on A404 Stanley Hill (up to 180%) and A355 London Road West (up to 40%). This occurs as a result of increased west- east strategic traffic through the junction from the direction of A416 Station Road and High Street, reducing the capacity for left turners from Stanley Hill.
Figure 5-D illustrates the travel time increases at the key locations described previously in the AM peak, compared with the DM. Figure 5-E shows the additional traffic generated by the Green Belt sites to the south and west of Amersham. Figure 5-F shows the additional traffic generated by the Area South of Little Chalfont (West of Lodge Lane) Green Belt site in the AM, on the Amersham area.
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The observed increases in travel time are the result of delay at the highlighted junctions, due to increases in demand flow on the main strategic corridors to the south and east of the town.
A404/ White Lion Rd Rbt
A404/ A413/ London Rd Rbt
Gore Hill Rbt
Figure 5-D 2036 DS – AM 2036 DM change in travel time – Amersham
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The two green belt option sites to the south and west of Amersham contribute to increased demand flow at the highlighted constrained junctions in the DS scenario.
A404/ White Lion Rd Rbt
The Broadway/ A355 E/ A335 S Rbt
A404/ A413/ London Rd Rbt
Gore Hill Rbt
Figure 5-E AM 2036 DS Select Link Plot – Area South of Amersham (Between A413 and A355) and Area South of Amersham (Land at Crown Farm) Option Sites 4 and 5
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The Area South of Little Chalfont green belt option site to the east of Amersham contributes to increased demand flow at the highlighted constrained junctions in the DS scenario.
A404/ White Lion Rd Rbt
A404/ A413/ London Rd Rbt Gore Hill Rbt
Figure 5-F AM 2036 DS Select Link Plot – Area South of Little Chalfont Green Belt option (6)
Several mitigation schemes have been tested in the Amersham area to address the impacts described above in relation to the additional development. This includes capacity increases at the A404/ White Lion Road Roundabout, Gore Hill Roundabout and A404/ London Road Roundabout. Figure 5-G illustrates the impact that the mitigation schemes have on changes in travel time in the AM peak compared with the DM.
The capacity increase at the A404/ White Lion Road eliminates the constraint at the junction and removes the travel time increases on the A404, leading to travel time improvements compared with the DM situation. The capacity increase at the A404/ A413/ London Road Roundabout has a similar affect, relieving the congestion on the northern approach to the junction.
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The observed improvements in travel time as a result of the mitigation schemes are significant at the highlighted junctions.
A404/ White Lion Rd Rbt
A404/ A413/ London Rd Rbt
Figure 5-G AM 2036 DS Mitigation – AM 2036 DM change in travel time - Amersham
5.2.5 Little Chalfont
As with, Amersham, the inclusion of the draft HELAA sites and the Area South of Little Chalfont (West of Lodge Lane) Green Belt site (and to a lesser extent the two other sites south of Amersham), results in moderate increases in travel time on a number of roads and junctions in the Little Chalfont area. The increases in travel time are greater in the PM peak, compared with the AM peak at these locations.
Overall, there are travel time increases on the approaches to the A404 White Lion Rd/ B4442 Cokes Ln Roundabout in both the AM and PM peak (up to 400%), and on the approaches from minor roads which join the A404, including Burtons Lane, Stony Lane, Chalfont Avenue, Oakington Avenue and Chessfield Park (up to 125% in the AM and PM peaks).
These travel time increases occur as a result of the increased demand flow on the A404 and B4442 arising from the additional development included in the DS scenario. The increased demand flow along the B4442 and A404 reduces the capacity for vehicles turning out of the minor roads into the main traffic flow, and vehicles turning into the roundabout circulatory flow at the A404 White Lion Rd/ B4442 Cokes Ln Roundabout junction.
Figure 5-H shows the travel time increases in the PM peak in the Little Chalfont area, as described in the previous paragraphs. The AM peak increases are similar albeit with more congestion on Burtons Lane and a little less on Church Grove and Stony Lane.
.
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The Burtons Lane mini roundabout tested as part of the Mitigation Scenario helps to relieve congestion on Burtons Lane. Furthermore, the A404 White Lion Rd/ B4442 Cokes Ln Roundabout experiences a reduction in travel time. Figure 5-I shows the impact of the mitigation scheme in the AM peak.
The observed travel time increases on the minor roads are the result of increased demand flow on the A404 and B4442 arising from the greenbelt options and HELAA sites in the local area.
Stony Lane
A404 White Lion Rd/
B4442 Cokes Ln Rbt Chalfont Ave Church Grove
A404 A404
B4442 Burtons Ln
Figure 5-H PM 2036 DS – PM 2036 DM change in travel time – Little Chalfont
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The new Burton Lane mini roundabout contributes to improving travel time increases resulting from the Local Plan developments.
Stony Lane
Chalfont Ave A404 White Lion Rd/ B4442 Cokes Ln Rbt A404
Burtons Ln B4442
Figure 5-I AM 2036 DS Mitigation- AM 2036 DM change in travel time- Little Chalfont
5.2.6 Holmer Green (and the Hazelmere Crossroads)
There are no draft HELAA sites in the immediate vicinity of Holmer Green, the nearest site is Area South of Holmer Green (Skimmers Orchard) Green Belt option. As a result, there are no notable increases in travel time in centre of Holmer Green however there are impacts on the outskirts, notably Amersham Road. The greatest impact of the Green Belt site is felt to the southeast, at the Hazelmere Crossroads, in Wycombe District.
At the Hazelmere Crossroads, there are moderate increases in travel time in the AM peak on the approach to the junction from Amersham Road (up to 120%) and on Park Lane (up to 50%). These travel time increases occur in part due to the additional demand flow generated from the Area South of Holmer Green (Skimmers Orchard) Green Belt option, and resulting reassignment of traffic on local roads.
Figure 5-J shows the change in travel time impacts at the Hazelmere Crossroads (and surrounding local roads) with the Local Plan growth scenario in place compared with the DM.
Two mitigation schemes were tested at the Hazelmere Crossroads to provide extra capacity at the junction on the constrained approaches and a new roundabout to replace the Eastern Dene/ A404 junction complemented by the Queensway link, providing a bypass for Hazelmere Crossroads. Figure 5-K shows the change in travel time impacts at the Hazelmere Crossroads with the mitigation schemes. The mitigation schemes provide significant travel time improvements at Hazelmere Crossroads and on the wider network surrounding it. Notable improvements in the AM peak can be seen on A404 Amersham Road in directions towards both High Wycombe and Amersham. In addition to this, the B474, Park Lane and Eastern Dene (Southbound) illustrate positive changes in travel time.
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With the preferred green belt option growth in place there are travel time increases on Amersham Road and Park Lane, as a result of traffic reassignment and the additional demand flow generated by the new development.
Eastern Dene A404
Hazelmere Crossroads Park Lane
Amersham Rd
A404 B474
Figure 5-J AM 2036 DS – AM 2036 DM change in travel time – Hazelmere Crossroads
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Eastern Dene
Hazelmere Crossroads Park Lane Amersham Rd
Queensway Link
B474 A404
Figure 5-K AM 2036 DS Mitigation- AM 2026 DM change in travel time- Hazelmere Crossroads
5.2.7 Chalfont St Giles
Within the Chalfont St Giles area there are no preferred Green Belt options and only a small increase in development (18 houses) from the draft HELAA sites.
As a result, there are only slight travel time increases experienced on isolated links in the DS scenario across all time periods, compared with the DM.
5.2.8 Chalfont St Peter
There are some significant travel time increases experienced in the Chalfont St Peter area in the DS scenario as a result of the additional housing development included as part of the draft HELAA sites and particularly from the Area South East of Chalfont St Peter (Winkers) Green Belt option. Collectively these sites contribute an additional 515 houses in the area. It should be noted that the National Epilepsy Centre, Chalfont St Peter preferred Green Belt option does not contribute much additional traffic to the network as the site is used for specialist accommodation and not general housing.
The majority of travel time increases are felt in the AM peak, particularly as a result of additional traffic leaving the Area South East of Chalfont St Peter (Winkers) Green Belt site to access the A413. This results in significant queuing at the A413/ Gravel Hill junction, as the additional development traffic turning out from the minor arm is constrained by the main flow of traffic on the A413 (which has also increased compared to the DM). This results in travel time increases of up to 540% on the approaches to the junction.
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In addition, a number of minor local roads in the immediate vicinity of the Green Belt option experience additional development traffic. These include Joiners Lane and Denham Lane (Both North and South of the junction with Joiners Lane).
The travel time increases experienced in the AM peak are not present in the PM peak. This is because the majority of vehicles are returning to the development rather than departing the development in the PM peak. This development traffic is leaving the A413 at either the A413/ Joiners Lane Roundabout or at the A413/ Gravel Hill junction, and at both locations this movement is not constrained by the conflicting traffic flow.
Figure 5-L illustrates the AM peak travel time increases at the constrained locations in Chalfont St Peter with the Local Plan growth scenario in place.
Rickmansworth Ln A413/ Gravel Hill junction
Copthall Ln
A413 Joiners Ln
A413/ Joiners Ln Rbt Market Place
Figure 5-L AM 2036 DS – AM 2036 DM change in travel time – Chalfont St Peter
Two mitigation schemes have been tested in Chalfont St Peter to address the observed impacts in the Local Plan growth scenario. The existing A413/ Gravel Hill priority junction has been replaced with a signalised arrangement, and the A413/ Joiners lane Roundabout has been tested with a capacity increase on the approach from the northern and eastern arms. The inclusion of the mitigation reduces the travel time increases at the junctions compared with the DS ‘without’ mitigation scenario, as shown in Figure 5-M.
Queuing on the approaches to A413/ Gravel Hill Junction from Gravel Hill is reduced as the signals provide sufficient capacity for traffic turning out of the minor arm. However, as expected travel time on the A413 increases in both directions compared with the ‘without’ mitigation scenario, as signals add delay to the major flow. Queuing on the approach to A413/ Joiners Lane Roundabout is significantly reduced, resulting in an overall reduction of travel time compared with the DM on Joiners Lane.
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The inclusion of the mitigation reduces the travel time increases at the two constrained junctions with the preferred green belt option growth in place.
Rickmansworth Ln
A413/ Gravel Hill Denham Ln junction West Hyde Ln
Copthall Ln
A413 Joiners Ln
A413/ Joiners Ln Rbt
Figure 5-M AM 2036 DS Mitigation – AM 2036 DS change in travel time – Chalfont St Peter
5.2.9 Beaconsfield
There are significant travel time increases in the Beaconsfield area in both the AM and PM peak periods with the inclusion of the Local Plan growth scenario. The impacts in this location are, in part, the result of the additional draft HELAA sites, and also the Area East of Beaconsfield Green Belt option. This contributes a moderate amount of additional traffic to the network generated by the new employment and housing development.
In the AM peak, the B474 through Beaconsfield town centre, and a number of minor roads connecting with this through route, experience significant increases in travel time due to additional development traffic from the preferred Green Belt options heading to the A355 and M40 (dominate flow in the AM peak) as well as to High Wycombe and Amersham. A proportion of these additional trips are also to and from the Area East of Beaconsfield Green Belt option. Note that the Beaconsfield link road is assumed in place in the DM.
The additional development traffic on the B474 results in travel time increases on the approaches to the B474/ Reynolds Road/ Baring Road Roundabout and Gregories Road/ Burkes Road/ B474 Roundabout, as the junctions are already over capacity in the AM DM scenario. This results in the increased travel times observed on the B474 itself (up to 90%), through the town centre.
In addition, the increased traffic on the B474 also reduces the capacity at the minor arms of junctions along the route, making it more difficult for traffic to turn out into the main flow and resulting in travel time increases. Impacted minor roads include Ledborough Lane (up to 77%), Reynolds Road, (up to 400%), Baring Road (up to 325%), Warwick Road (up to 77%) and Gregories Road (up to 120%).
In the PM peak the increases here are far less significant with percentage increases in the order of 5-20%.
In both the AM and PM peak the Pyebush Roundabout is significantly constrained on all approaches due to additional traffic demand through the junction. This results in significant congestion and travel time increases at
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the junction. There are travel time increases of up to 20% on the northern arm, up to 650% on the eastern arm, up to 40% on the western arm and up to 40% on the southern arm (PM peak only).
The Area East of Beaconsfield Green Belt option contributes a large amount of additional traffic at the Pyebush Roundabout due to its close proximity to the junction, particularly on the approach from the north. However, a proportion of the increase in traffic is also generated from other Local Plan growth development sites across the two districts, as the Pyebush Roundabout is a key junction for strategic traffic in this area, in part because it provides access to the M40.
Figure 5-N shows the travel time increases in the Beaconsfield area during the AM peak with the Local Plan growth scenario in place, compared with the DM. Figure 5-O shows the increases in traffic flow across the Beaconsfield area in the AM peak which arise from the inclusion of the Area East of Beaconsfield Green Belt option in the DS scenario.
The observed travel time increases through Beaconsfield town centre and at the Pyebush Roundabout arise because of additional traffic generated from the Local Plan growth scenario.
B474
Reynolds Rd Ledborough Ln Longbottom Ln
Baring Rd A355 Warwick Rd
Gregories Rd
Pyebush Rbt
M40 J2
Figure 5-N AM 2036 DS – AM 2036 DM change in travel time – Beaconsfield
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Baring Rd A355 Phase 3B Modelling Report
The Local Plan developments in the South of Beaconsfield contribute toward an increase in traffic through the Pyebush Roundabout in the AM peak which results in significant congestion.
Pyebush Rbt Gregories Rd
M40 J2
Figure 5-O AM 2036 DS Select Link Plot – Local Plan Developments in South of Beaconsfield
A number of mitigation schemes have been tested in the Beaconsfield area to address the travel time increases through the town centre, at the Pyebush Roundabout and at the Ledborough Lane/ Longbottom Lane junctions. This includes upgrading the Ledborough Lane/ Longbottom lane junctions to a single roundabout, various capacity increases at the Pyebush Roundabout, and the signalisation of the Potkiln Lane/ A40 Oxford Road junction. Figure 5-P shows the travel time changes in the AM DS with mitigation scenario compared against the DM.
The Ledborough Lane/ Longbottom Lane roundabout has the desired effect of diverting traffic from the B474 through the town centre, to Ledborough Lane and the A355 via the new junction. This significantly reduces the travel time increases experienced in the AM peak through the town centre in the DS ‘without’ mitigation scenario. However, there are now travel time increases on Ledborough Lane itself as a result of the additional traffic using the road, particularly in the AM peak (up to 80%).
At the Pyebush Roundabout, the capacity improvements tested on all four arms alleviates all queuing at the junction, and as a result there are travel time reductions on all arms in both peak periods, compared with the DM. However, these improvements also result in a greater amount of traffic reaching the M40 J2, leading to travel time increases on the approach to the junction from the A355 in both the AM and PM peak (up to 60% in the AM peak). As well as this, congestion and associated travel time increases occur to the west of junction on the A40, as the improvements at the Pyebush Roundabout draws additional traffic to this corridor.
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The signalisation of Potkiln Lane also significantly reduces congestion on the approach to the junction with the A40. This leads to a reduction in travel time compared with the DM scenario along the length of the road as far back as Jordans in both the AM and PM peak periods.
An additional scheme included in the mitigation scenario is the priority junction to the west of Pyebush Roundabout on the A40; the addition of this priority junction has no discernible impact on the mainline A40.
B474 Ledborough Ln The inclusion of the mitigation reduces congestion Longbottom Ln through the town centre and at the Pyebush Roundabout compared against the DM. However, it also leads to travel time increases on Ledborough Lane, M40 J2 and A40 corridor.
Potkiln Lane
Pyebush Rbt
M40 J2
Figure 5-P AM 2036 DS Mitigation – AM 2036 DM change in travel time – Beaconsfield
5.2.10 Gerrards Cross
The Gerrards Cross area is only slightly impacted in terms of travel time increases with the Local Plan growth scenario in place. This is to be expected as there is a relatively small increase in housing from the draft HELAA sites compared with other areas, and no preferred Green Belt options in the settlement.
5.2.11 Denham area (including the Denham Roundabout)
There are moderate travel time impacts in the Denham area as a result of the Local Plan growth scenario. These impacts are felt at Denham Green and mainly at the Denham Roundabout during both the AM and PM peak periods.
There are no preferred Green Belt options located in Denham Green, however, there are a number of additional draft HELAA sites which contribute an additional 180 houses in the area. This results in additional traffic turning out from Denham Green Lane, onto the A412. The increase in traffic on Denham Green Lane combined with the increase of traffic on the A412 (arising from the additional development across the two districts), results in slight
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travel time increases on the approach to the junction on Denham Green Lane (up to 60%) in the AM and PM peaks. There are similar increases in travel time on Old Rectory Lane.
The Land North of Denham Roundabout Green Belt option contributes a moderate amount of additional traffic onto the network in both the AM and PM peaks at the Denham Roundabout. The access to the development is via Denham Court Drive which connects to Denham Roundabout. This, combined with other Local Plan development across the district which travels through the Denham Area to use the Strategic Road Network results in moderate increases in traffic flow on links around Denham Roundabout.
Figure 5-Q illustrates the AM peak travel time increases which occur with the Local Plan growth scenario in place at Denham Roundabout. Figure 5-R shows the traffic arriving and departing the Land North of Denham Roundabout Green Belt option during the AM peak, and the additional traffic passing through the Denham Roundabout because of this.
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The additional traffic generated from the Land North of Denham Roundabout greenbelt option site results in slight congestion, notably on Old Mill Road
Old Mill Rd
A40 Oxford Rd
A40 Oxford Rd/ Old Mill Rd Jct
M40
M40
A412 Denham Rd
Figure 5-Q AM 2036 DS – AM 2036 DM change in travel time – Denham area
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The additional traffic generated from the Land North of Denham Roundabout greenbelt option site results in moderate congestion at the Denham Roundabout
Figure 5-R AM 2036 DS Select Link Plot – Land North of Denham Roundabout Green Belt option (10)
There are no mitigation measures proposed for the Denham area and as such the small increases in travel time seen in the DS scenario are relatively unchanged in the DS with mitigation scenario,
5.2.12 Iver and Iver Heath
Iver Heath itself is only slightly impacted with the addition of two preferred Green Belt options which are in the near vicinity. This includes the Land North of Iver Heath, and Area West of Iver Heath developments. There are no significant impacts in the AM peak whilst in the PM peak, the only significant travel time increases occur to the north east of Iver Heath on the A412, due to additional trip generation from development, and trips from the Denham area.
Iver experiences significant travel time increases in both the AM and PM peaks, as a result of additional traffic on the network generated by the Area North of Iver Station and Area to East of Ridgeway Business Park, Iver Green Belt options. This results in travel time increases on the approaches to the B470 High Street/ Thorney
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Lane North (up to 150%) and B470 High Street/ Bangors Road South (up to 250%) Roundabouts, where the approaches from the minor arms are over capacity due to the increase in traffic demand at these locations.
Figure 5-S illustrates the travel time increases in Iver with the Local Plan growth scenario in place in the AM peak. Figure 5-T shows the additional traffic demand generated from the two preferred Green Belt options to the south of Iver, and the impact this has on the B470 High Street and roads to the south of Richings Park (Thorney Mill Road/North Park).
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The additional traffic generated from the two preferred greenbelt option sites to the south of Iver, results in additional congestion at the highlighted junctions
Bangors Rd S
B470/ Thorney Ln N Rbt
B470 High St
B470/ Bangors Rd S Rbt
Thorney Ln N
Figure 5-S AM 2036 DS – AM 2036 DM change in travel time – Iver
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The additional traffic generated from the two preferred greenbelt option sites to the south of Iver, results in additional congestion at the highlighted junctions
B470/ Thorney Ln N Rbt
B470/ Bangors Rd S Rbt
Thorney Ln N
Figure 5-T AM 2036 DS Select Link Plot – Green Belt options 13 and 14
Mitigation tested in the Iver area includes the Iver Relief Road. It should be noted that the main purpose of the relief road is to provide environmental benefits through the rerouting of HGV traffic away from Iver High Street as documented in the Iver Transport Study.
The inclusion of the Iver Relief Road has the effect of reducing some of the through traffic on the B470 High Street; and results in lessening of the impacts around Iver. In the PM peak adverse impacts are almost all eliminated, however in the AM peak there remains some travel time increases around the Bangors Road South roundabout., this is illustrated in Figure 5-U.
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The Iver Relief Road has some effect of reducing through traffic on the B470 High Street, however, there remains some significant travel times increases in the AM.
B470 High St
Thorney Ln N
Iver Relief Road
Figure 5-U AM 2036 DS Mitigation – AM 2036 DM change in travel time – Iver
5.2.13 Middle Green
The Middle Green area is only slightly impacted by the inclusion of the Local Plan growth scenario. This is expected as the location is not in the vicinity of any preferred Green Belt options or draft HELAA sites.
5.2.14 Stoke Poges
Stoke Poges, like other areas is also only slightly impacted by the inclusion of the Local Plan growth option scenario. This is expected as this location does not include any preferred Green Belt options or draft HELAA sites in the vicinity.
5.2.15 Farnham Common and Farnham Royal
There are 26 new dwellings from HELAA sites in the near vicinity of either Farnham Common or Farnham Royal. As a result, Farnham Common is relatively unaffected by the Local Plan growth scenario compared to other areas. However, there are some moderate travel time increases in the AM and PM peak in the local area of Farnham Royal, as a result of increases in strategic traffic on the A355 and B416 arising from the additional development in the wider area.
The majority of these travel time increases are experienced on the immediate approaches to the A355 Beaconsfield Road/ B416 Park Road and A355 Beaconsfield Road/ Farnham Lane roundabouts (up to 50% in the AM peak). These two junctions are over capacity in the DM scenario, and with the additional development in place in the DS scenario these junctions are further constrained. It should be noted however that the area of
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impact is localised to just these two junctions, with the majority of Farnham Royal experiencing only slight increases in travel time.
In addition, this study has not considered the environmental impacts arising from the Local Plan growth scenario on protected areas such as Burnham Beeches in this area. Further information regarding this is provided in Section 6.2.
5.2.16 Burnham
The Burnham area experiences significant increases in travel time in the AM and PM peak with the addition of the draft HELAA sites and in particular the Land Adjacent to Taplow Station Green Belt option. The majority of travel time increases are felt on the A4 Bath Road in both peak hours, as well as the M4 in the AM peak.
In both the AM and PM peak, all the observed travel time impacts are the directly linked to increases in traffic flow on the A4 Bath Road as a result of the additional development, particular arising from the Land Adjacent to Taplow Station Green Belt option. The additional traffic leads to significant congestion on the approaches to the Huntercombe Lane/ A4 Bath Road signalised junction, which already exceeds capacity in the DM.
As a result, queuing traffic extends as far back as A4 Bath Road/ Lake End Road/ Lent Rise Road Roundabout to the west, and the A4 Bath Road/ Station Road/ Elmshott Lane Roundabout to the east, resulting in significant travel time increases (up to 1500%). To the north, the queuing traffic extends back on Huntercombe Lane North with traffic time increases off the joining roads, resulting in travel time increases (up to 1300% in the AM peak). In addition, traffic is also queuing back onto the M4 west at junction 7, which results in significant travel time increases in the AM peak (up to 60%).
Figure 5-V shows the travel time increases experienced in the Burnham area as a result of the inclusion of the Local Plan growth scenario, compared with the DM. Figure 5-W illustrates the additional traffic generated by the Land Adjacent to Taplow Station Green Belt option on the local network and in particular its impact through the Huntercombe Lane/ A4 Bath Road junction and at A4 Bath Rd/ Lake End Rd/ Lent Rise Rd Roundabout.
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There are significant increases in travel time across Burnham which occur due to increasing congestion at the Huntercombe Lane/ A4 Bath Road Junction and A4 Bath Rd/ Lake End Rd/ Lent Rise Rd Rbt
Huntercombe Ln N
Huntercombe Ln/ A4 Bath Rd Jct
A4 Bath Road
A4 Bath Rd/ Lake End Rd/ Lent Rise Rd Rbt. A4 Bath Rd/ Station Rd/ Elmshott Ln Rbt.
M4 J7
Figure 5-V AM 2036 DS – AM 2036 DM change in travel time – Burnham
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Huntercombe Ln N
A4 Bath Road
A4 Bath Rd/ Huntercombe Ln Jct
M4 J7
The Land Adjacent to Taplow Station greenbelt option contributes to approximately 115 extra vehicles through the Huntercombe Lane/ A4 Bath Road junction during the AM peak.
Figure 5-W AM 2036 DS Select Link Plot – Land Adjacent to Taplow Station Green Belt option (15)
Two mitigation schemes have been tested in the Burnham area. This includes signal timing improvements at the A4 Bath Road/ Huntercombe Lane junction and the widening of the existing A4 Bath Road carriageway between A4 Bath Road/ Huntercombe Lane junction and M4 Junction 7/ A4 Bath Road junction.
The inclusion of the two schemes significantly reduces congestion on the approaches to the A4 Bath Road/ Huntercombe Lane junction in the AM and PM peak, with travel times reducing to levels lower than in the DM scenario. However, the improvements provided by the mitigation result in adverse impacts at the A4/Lent Rise Road roundabout downstream of the Hunterconbe Lane junction, due to the increased capacity at the Huntercombe Lane junction allowing more traffic to reach the Lent Rise Road junction. Figure 5-X shows the improvements in travel time which occur in the Burnham area in the AM peak with the mitigation in place, compared with the DM.
However, with respect to the mitigation scheme which proposes widening the A4, only a small stretch on the A4 falls within BCC control, therefore, the mitigations would need to be discussed through Duty to Co-operate engagement and consultations with the relevant Highway Authorities.
.
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There are widespread improvements in travel time across the Burnham area with the mitigation in place.
Huntercombe Ln N
Huntercombe Ln/ A4 Bath Rd Jct
A4 Bath Road
A4 Bath Rd/ Lake End Rd/ Lent Rise Rd Rbt. A4 Bath Rd/ Station Rd/ Elmshott Ln Rbt.
M4 J7
Figure 5-X AM 2036 DS Mitigation – AM 2036 DM change in travel time – Burnham
5.2.17 Taplow
The Taplow area is relatively unaffected by development increases in the DS scenario, with the only significant impacts being those occurring at Burnham as described above.
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6. Summary and conclusion
This section summarises the results of the modelling work in terms of the overall impact of the Local Plan growth scenario on Chiltern and South Bucks Districts, and the effectiveness of the schemes tested to mitigate the observed impacts of development growth.
6.1 Summary of results
As with the previous stage, a RAG (red, amber or green) rating has been applied to each settlement area discussed in the previous section, based on a purely qualitative assessment of the overall impact of the Local Plan growth scenario in terms of increased travel time and congestion; red represents a significant impact, amber a moderate impact and green a slight impact. In addition, a second RAG rating has also been applied based on a qualitative assessment of the overall improvement, if any, the DS with mitigation scenario provides.
The RAG rating is qualitative and reflects impacts at each settlement as a whole; in some cases very large travel time percentage increases can occur on minor or lightly trafficked roads. In other cases, whilst the percentage increases are very high the absolute numbers, in terms of additional seconds of travel time, are relatively low. For these reasons, an apparently large percentage increase in travel time on selected links may not be expected to lead to a moderate or significant in travel time at the settlement as a whole. The RAG rating is identified cognisant of those points
Table 6-A outlines a broad definition of each qualitative category. This rating is based only on outputs produced as part of this modelling work.
RAG Description rating
Significant impact in terms of travel time increases and congestion on a number of links and junctions over a wide area.
Moderate impact in terms of travel time increases and congestion on a number of key links and junctions in the local area.
Slight impact in terms of travel time increases and congestion on a number of key links and junctions in isolated areas.
Table 6-A RAG rating description
Table 6-B summarises the results of the modelling for the settlement areas previously described. It is important to note that the table highlights the extent to which the mitigation has been successful at reducing the impacts observed in the DS scenario across the geographic area. In some cases, more detailed local modelling may be required at the planning application stage for individual proposals to determine the exact scale of development impacts, and whether the currently proposed schemes will be effective.
Note that Table 6-B does not refer to the impacts on motorways; these can be found in the Local Plan Modelling Support Phase 3a report.
DS+ Settlement DS Key links and junctions Comments mitigation
Only slight increases in travel time observed in the DS scenario. Great Minimal impact is expected as there None Missenden are no Green Belt sites in the near vicinity and only small increases in houses from the draft HELAA sites.
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DS+ Settlement DS Key links and junctions Comments mitigation
No mitigation was tested at this settlement.
Only slight increases in travel time observed in the DS scenario. Minimal impact is expected as there are no Green Belt sites in the near Prestwood None vicinity and only small increases in houses from the draft HELAA sites. No mitigation was tested at this settlement.
Significant increases in travel time on some links in Chesham arising Significant increases in travel from the Green Belt option and draft time at: HELAA sites. • A416 St. Mary’s Way The inclusion of the signalised • Chesham B485 Church Street junctions on the A416 removes the • A416/ Park Rd Rbt travel time increases on the A416 • A416/ White Hill Rbt and at the two roundabouts. However, there are still travel time • B485 Missenden Road increases on minor roads connecting with the A416.
Increases in travel time to the south and east of Amersham. The Area South of Little Chalfont (West of Significant increases in travel Lodge Lane) Green Belt site time at: contributes a moderate amount of Amersham • A404/ A413/ London additional traffic demand in the Rd Rbt area. • A404/ White Lion Rbt The inclusion of the mitigation schemes at the three junctions has reduced the travel time increases experienced in this area.
Moderate increases in travel time on a number of minor roads joining Moderate travel time increases the A404 and at the A404/ B4442 at: roundabout. Impacts are the result • A404 White Lion Rd/ of increased demand flow on the B4442 Cokes Ln Rbt A404 and B4442 from the draft Little • Burtons Lane HELAA sites and Green Belt options (particularly the Area South Chalfont • Stony Lane of Little Chalfont (West of Lodge • Chalfont Avenue Lane)). • Oakington Avenue Mitigation tested was a mini • Chessfield Park roundabout at Burtons Lane this improved the situation at Burtons lane, but not more widely.
Holmer Moderate travel time increases There are no moderate or Green & at: significant impacts in terms of travel Hazlemere • A404 Amersham Road time increases within Holmer Green itself. However, there are moderate Crossroads
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DS+ Settlement DS Key links and junctions Comments mitigation
impacts to the area South of Holmer Green (Skimmers Orchard). Impacts are also felt to the southeast at the Hazelmere Crossroads, and on the A404 on Amersham Road. Two mitigation schemes were tested at the Hazelmere Crossroads, both of which contribute to improvements in travel time on the A404.
There are no moderate or significant impacts in this area. This is expected as there is only minimal Chalfont St development included in the Local None. Giles Plan growth scenario at this location. No mitigation was tested at this settlement.
There are significant travel time increases experienced at a small number of locations in the AM peak which arise because of the additional draft HELAA sites and Significant travel time increases the Area South East of Chalfont St at: Peter (Winkers) Green Belt option • A413/ Joiners Ln Rbt site. Chalfont St • A413/ Gravel Hill Jct Peter The mitigation schemes significantly • Market Place reduce the travel time increases at • Rickmansworth Lane this settlement. However, the improvement is offset slightly by the • Copthall Lane travel time increases on the A413 on the approaches to the A413/ Gravel Hill Jct, although delay here is to be expected as the junction has been signalised .
Significant travel time increases There are significant travel time at: increases across the Beaconsfield • Reynolds Rd area in both the AM and PM peak periods with the Local Plan growth • Baring Rd scenario in place. • Gregories Rd Additional traffic demand generated • Warwick Rd Beaconsfield from the Area East of Beaconsfield • B474 Green Belt option site is partially • Pyebush Rbt responsible for this. However, additional traffic generation also • Ledborough Ln/ arises from development traffic Longbottom Ln Jct across both districts due to the • Ledborough Ln strategic nature of the A355 and B474.
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DS+ Settlement DS Key links and junctions Comments mitigation
The mitigation has removed the travel time increases through the town centre and at the Pyebush Roundabout. However, it has also lead to travel time increases on the A40, to the south of Beaconsfield, Ledborough Lane and at the M40 J2 roundabout.
The Gerrards Cross area is only slightly impacted in terms of travel time increases as there are relatively few additional houses at Gerrards this location from the draft HELAA n/a Cross site allocations and no preferred Green Belt option sites in the near vicinity. No mitigation was tested at this settlement.
There are moderate travel time increases at the Denham Roundabout which arise because of Moderate travel time increases the addition of the Land North of at: Denham Roundabout Green Belt • Old Mill Rd option site. Denham • A40 Oxford Rd/ Old Mill There are also some slight travel area Rd Jct time increases in Denham Green, • Denham Rbt which arise in part due to the draft HELAA sites in the area but also • A412 Denham Rd because of increases in traffic on • Denham Green Ln the A412.
No mitigation was tested here.
Significant travel time increase Only slight increase in travel time in at: Iver Heath and significant increase • Bangors Rd S/ B470 in Iver as a result of the four Rbt preferred Green Belt sites in the immediate vicinity of the area. Iver and Iver • Thorney Ln N/ B470 Heath Rbt In Iver the inclusion of the Iver Relief Road improves conditions in • Thorney Lane North the PM peak however there are still • Bangors Road some residual effects in the AM • Iver High Street peak.
Only slight changes in travel time in this area as it does not include any additional draft HELAA sites or Middle Green n/a preferred Green Belt option sites. No mitigation was tested at this settlement.
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DS+ Settlement DS Key links and junctions Comments mitigation
Only slight changes in travel time in this area as it does not include any Slight increases in travel time on additional draft HELAA sites or Stoke Poges the Hollybush Hill preferred Green Belt option sites. No mitigation was tested at this settlement.
There are 26 new dwellings in the near vicinity to either Farnham Common or Farnham Royal.
Generally slight impacts in most Farnham Common is only slightly
places, however, moderate impacted by the inclusion of the Farnham travel time increases at: Local Plan growth scenario. Common and • A355 Beaconsfield Rd/ Generally, the Farnham Royal area Farnham B416 Park Rd Rbt. as a whole is only slightly impacted Royal by the additional development. • A355 Beaconsfield Rd/ However, there are moderate Farnham Ln Rbt. impacts at two specific junctions in Farnham Royal. No mitigation was tested at this settlement.
There are significant increases in travel time across the Burnham area as a result of the increased Significant travel time increases traffic flows through the at: Huntercombe Lane/ A4 Bath Road • Huntercombe Ln N/ signalised junction in the AM and Stomp Rd Rbt PM peak. • Huntercombe Ln N The Land Adjacent to Taplow • Huntercombe Ln/ A4 Station Green Belt option site Bath Rd Jct contributes 200 extra two-way trips Burnham approximately through the junction • A4 Bath Rd during the AM peak and is therefore • M4 J7 largely responsible for the travel • A4 Bath Rd/ Station time increases at this location. Rd/ Elmshott Ln Rbt The inclusion of mitigation in Slough • A4 Bath Rd/ Lake End at the Huntercombe Lane/ A4 Bath Rd/ Lent Rise Rd Rbt Road junction significantly reduces travel time increases across the area such that only a small section of the A4 remains adversely affected.
The Taplow area is only slightly impacted by the inclusion of the Local Plan growth development. The majority of the impact in this Taplow n/a area is felt to the east in Burnham as a result of the Land Adjacent to Taplow Station Green Belt option site.
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DS+ Settlement DS Key links and junctions Comments mitigation
No mitigation was tested at this settlement.
Table 6-B Impact summary table
6.2 Conclusions
The Chiltern and South Bucks Model has been used to indicate where the Local Plan growth scenario for Chiltern and South Bucks is likely to result in “pinch points” on the highway network, in terms of increased journey times and congestion. The model has also been used to indicate the extent to which proposed transport improvements are likely to mitigate the impacts of the Local Plan growth development. The extent to which the mitigation measures have been successful varies across the different settlement areas.
The results from the model indicate that there are a number of settlement areas which experience moderate to significant increases in travel time with the Local Plan growth scenario in place. This includes Chesham, Amersham, Little Chalfont, Holmer Green, Chalfont St Peter, Beaconsfield, Denham area, Iver and Burnham. The majority of these settlements are the focus of the proposed mitigation schemes which have been tested as part of this work.
There are also a number of settlements which experience only slight increases in travel time, which is on the most part due to there being no relatively large draft HELAA sites or preferred Green Belt option sites in the near vicinity of the settlements. This includes Great Missenden, Prestwood,), Chalfont St Giles, Gerrards Cross, Middle Green, Stoke Poges, Farnham Common and Taplow.
In general, all of the mitigation schemes tested provide a benefit in terms of reduced congestion and journey times. However, in Burnham the mitigation improves conditions in the locality of the scheme, but this has the consequence of allowing additional traffic through to other junctions which are close to capacity in the DM scenario. This means that additional junctions, not previously identified as congested in the DS ‘without’ mitigation scenario, are becoming constrained with the mitigation in place. However, for Burnham as a whole, the net effect is overwhelmingly positive in terms of congestion reduction.
It should be noted that when assessing impacts and the extent to which they are mitigated, there is no universal definition of how to define an impact, and what impacts are considered “acceptable” and “unacceptable”. The model has been used to provide a relatively high level indication of the potential impacts of the Local Plan and proposed mitigations, commensurate with the requirements of a Local Plan evidence base. A RAG analysis of the potential impacts has been provided in different areas, which is appropriate given the nature of the strategic model, but the quantification of the scale of impact based on the model should be avoided. It is expected that as various sites come forwards for planning permissions, the normal development management processes would require further more detailed assessment to assess the possible impacts within each area and impacts on the network for example, as part of a transport assessment accompanying a planning application.
To accord with the scope of the Local Plan this study has not considered the impact of the proposed Heathrow expansion. The Burnham Beeches Special Area of Conservation, which is of European – wide importance for biodiversity, is located within the Local Plan area. This report and its plots provide information on the projected traffic flows in the vicinity of the area in the year 2036. The impact of traffic and in particular increased nitrogen deposition in the SAC, is a critical issue and will need to be assessed by BCC and the districts in conjunction with Natural England.
BCC has produced a draft high level analysis of traffic flows in the vicinity of the SAC. This assessment is based on the issued methodology from Natural England. It is entitled “Impact on Burnham Beeches SAC of Chiltern and South Bucks Local Plan modelled growth”. This initial assessment provides a starting point for the Council’s
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consultation and discussion with Natural England, which is continuing as the Local Plan proceeds through its subsequent stages.
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Appendix A. Local Plan Modelling Phase 2 Development Summary
Future scenario (2033) Summary details
• As phase one: • 953 houses and 0 jobs in Chiltern District • 1,016 houses and 1,619 jobs in South Bucks District Do Minimum (DM) • 9,416 houses and 24,265 jobs in Aylesbury Vale District • 2,180 houses and 850 jobs in Wycombe District • Growth capped to NTEM levels outside of Buckinghamshire
• DM commitment plus: • An additional 3,846 houses and 522 jobs in Chiltern District Do Something (DS) Local Plan growth • scenario An additional 4,325 houses and 6,578 jobs in South Bucks District • An additional 320 houses in Wycombe District at one site
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Appendix B. Green Belt Options tested in Phase 3B
Option Number Green Belt option
1 Area North East of Chesham (Lye Green Road Area) 2 Area South of Holmer Green (Skimmers Orchard) 4 Area South of Amersham (Between the A413 and A355) 5 Area South of Amersham (Land at Crown Farm / Whielden Street) 6 Area South of Little Chalfont (West of Lodge Lane) 7 National Epilepsy Centre, Chalfont St Peter 8 Area South East of Chalfont St Peter (Winkers) 9 Area East of Beaconsfield 10 Land North of Denham Roundabout 11 Land North of Iver Heath, South East of Pinewood 12 Area West of Iver Heath 13 Area North of Iver Station 14 Area to East of Ridgeway Business Park, Iver 15 Land Adjacent to Taplow Station
List of Green Belt options tested in phase 3B
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1 Chesham
Great Missenden Prestwood
Chesham Bois Little Amersham Chalfont Holmer Green 6
2 5 4
Chalfont St Giles 7
Beaconsfield 8 Chalfont St Peter 9
Gerrards Cross Denham Green
10 Farnham Common
New Denham Farnham Royal Stoke Poges 11 Iver Heath 12 Burnham Taplow Iver
15 14 13
Richings Park
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Phase 3B Modelling Report
Appendix C. Congestion Ratio
This has been provided as a separate PDF.
BRJ10103\FMRP3B Phase 3B Modelling Report
Appendix D. Change in travel time
This has been provided as a separate PDF.
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