Junction 17 - M4

Enhancements to capacity at the junction are required to accommodate planned and emerging growth within Wiltshire and increasing strategic attraction to the A350 corridor for routes between the Midlands and the South Coast

Traffic problems on the A350 have been of concern for many years. The following key transport problems at the junction have been identified:

• the strategic role of the A350, part of the Major Road Network (MRN), is threatened by increasing congestion, with potential negative connectivity and economic impacts for West Wiltshire
• operational and safety performance of the M4, part of the Strategic Road Network (SRN), threatened by growth in demand at M4 Junction 17
• capacity at M4 Junction 17 will constrain planned and future housing and employment growth in the Chippenham area and the A350
• junction improvements are needed to ensure the overall success of the A350 MRN package

There is the potential to deliver many strategic and local benefits including:

• creating a more reliable, less congested, and better-connected transport network that works for the users who rely on it
• providing a well-connected, reliable and resilient transport system to support economic and planned development growth at key locations
• supporting and helping to improve the vitality, viability and resilience of Wiltshire's economy
• providing transport infrastructure to support new housing in the area
• assisting the efficient and sustainable distribution of freight in Wiltshire to build stronger, more balanced economy by enhancing productivity and responding to local growth priorities
• reducing the level of air pollutant and climate change emissions from transport
• improving safety for all road users and reducing the number of casualties on Wiltshire's roads

Various options were considered in the Options Assessment Report (OAR) of July 2019 which examined six options across three themes: public transport, improvements to the existing highway network and delivering additions to highway network.

Three options were short listed, all relating to delivering improvements to the highway at M4 junction 17:

• Option A: Widen A429 and B4122 approaches to M4 Junction 17 and deliver full signalisation.
• Option B: Widen all approaches to M4 Junction 17 and deliver full signalisation. The M4 off-slips, A429, B4122 and A350 will be widened to provide additional capacity, while the northern and southern edges of the gyratory will be widened to accommodate additional lanes. The eastbound M4 on-slip will be widened to provide a longer merge.
• Option C: Widen overbridges at M4 Junction 17. Widen approaches to M4 Junction 17 and deliver full signalisation. Option C will build upon Option B but will widen the full gyratory to accommodate additional lanes.

Option B was the best performing option, providing a higher level of capacity increase over Option A, while having a lower cost than Option C which would require significant investment to widen the overbridges.

Option B has since been amended to include three narrow lanes on the overbridge elements of the gyratory, which will further increase capacity.

The Outline Business Case has now been submitted to the Department of Transport.

If it is approved, it is anticipated that funding will be made available for the detailed design of the scheme, and for obtaining prices from contractors to undertake the work.

Once the detailed design is completed and the costs and benefits have been confirmed the scheme construction will start, which could be in autumn 2026 and will take about a year to complete.

In March 2020, the Government awarded Major Road Network funding to develop the case for the junction improvements, having considered an initial submission made in July 2019. Subsequent stages of DfT funding will be required to deliver the scheme, alongside a local contribution.

If approved, and depending on the final details, the scheme could cost in the region of £27 million. 

The proposals have been assessed in accordance with the Department for Transport's methodology which compares the costs with the value of the benefits, especially in terms of journey time savings, reduced vehicle operating costs and accidents. The current proposals are considered to be the best option for attracting investment.

Not improving the junction would result in increased delays and congestion at the junction as traffic increases, with consequent increased costs in terms of journey times and vehicle operating costs.

The estimated scheme costs will be refined as the proposals are developed in more detail. At this early stage, a risk allowance is included in the initial estimated scheme cost to provide a contingency item and to allow for unknown costs.

The scheme costs are estimated using a price base, which is then adjusted according to the latest predictions of inflation for future years, when the expenditure is likely to be incurred.

The scheme will be assessed in accordance with the Department for Transport (DfT) Transport Analysis Guidance (TAG). Further information is available at:

https://www.gov.uk/guidance/transport-analysis-guidance-tag (opens new window) 

It is aimed for the works to be completed within existing highway land and hence may be completed as Permitted Development. Clarification will be sought form the Local Planning Authority if needed on this matter.

Some traffic regulation orders and other legal processes may need to be followed, especially in connection with installing traffic signals, any changes to speed limits or other traffic management measures connected to the scheme.

During construction temporary traffic regulations may be required for safety reasons, including temporary speed limits.

The requirements for the scheme will be considered when it has been designed in more detail. The environmental factors will be described and assessed in the Outline Business Case. 

Consideration of carbon reduction requirements will be reviewed as guidance is updated. 

As part of the Outline Business Case preparation, a Carbon Management Plan will be developed and considered in accordance with DfT requirements.

A Walking, Cycling and Horse-riding Assessment Report is being developed with the proposals at the junction, in order to identify and maximise the potential for pedestrians and cyclists to cross the M4 corridor in a safe and convenient fashion.

The business case for the scheme will need to consider a range of traffic growth scenarios for both high and low growth. It is anticipated that the Government will revise future traffic and economic growth figures in view of recent events, and the scheme will be assessed based on any revised Government predictions as they are published. 

The scheme has the potential to improve road safety and reduce the number killed and seriously injured on our roads. The removal of traffic delays could reduce traffic noise and air pollution with consequent health benefits. The Outline Business Case for the scheme will take these factors into consideration. 

The scheme proposals are modelled utilising National Highways latest traffic model for the junction, which incorporates counts from 2018. The use of the National Highways model allows for regular comparison of recorded traffic flows along the motorway corridor.

Future growth factors have been considered and these will be revised and assessed as emerging proposals for the Local Plan Review are better established.

The assessment of scheme in connection with the Outline Business Case will consider all environmental aspects of the proposed scheme and any potential mitigation measures. The current consultations and information being collected will help inform the design of the proposals to reduce the impact of the scheme. 

 Yes. The Outline Business Case, which is currently being developed, and the supporting documentation will be available to the public. The following draft OBC documents have been submitted to DfT:-

M4 Junction 17 (Major Road Network Fund) OBC (PDF) [5MB] (opens new window)

M4 Junction 17 Appraisal Toolkit (PDF) [320KB] (opens new window)

Appendix A - Strategic Appendices 

Appendix A1 - Options Assessment Report (PDF) [3MB] (opens new window)

Appendix A2 - WCHAR (PDF) [13MB] (opens new window)

Appendix A3-1 - Location Map (PDF) [1MB] (opens new window)

Appendix A3-2 - General Arrangements (PDF) [876KB] (opens new window)

Appendix A3-3 - Potential Cycle Route (PDF) [1MB] (opens new window)

Appendix B - Economic Appendices

Appendix B1 - Local Model Validation Report (SATURN) (PDF) [46MB] (opens new window)

Appendix B2 - Traffic Forecasting Report (SATURN) (PDF) [5MB] (opens new window)

Appendix B3 - Local Model Validation Report (VISSIM) (PDF) [252KB] (opens new window)

Appendix B4 - Traffic Forecasting Report (VISSIM) (PDF) [1MB] (opens new window)

Appendix B5-1 - Appraisal Specification Report (PDF) [6MB] (opens new window)

Appendix B5-2 - Appraisal Specification Report - Addendum (PDF) [397KB] (opens new window)

Appendix B6 - Economic Appraisal Report (PDF) [2MB] (opens new window)

Appendix B7 - Appraisal Summary Table (PDF) [82KB] (opens new window)

Appendix B8-1 - Economic Efficiency of the Transport System (TEE) (PDF) [87KB] (opens new window)

Appendix B8-2 - Analysis of Monetised Costs and Benefits (AMCB) (PDF) [73KB] (opens new window)

Appendix B8-3 - Public Accounts (PA) (PDF) [66KB] (opens new window)

Appendix B9 - Cost Proforma (PDF) [72KB] (opens new window)

Appendix B10 - Social and Distributional Impacts Report (PDF) [2MB] (opens new window)

Appendix B11 - Environmental Appraisal Report (PDF) [1MB] (opens new window)

Appendix B11-1 - Air Quality (PDF) [81KB] (opens new window)

Appendix B11-2 - Greenhouse Gases (PDF) [79KB] (opens new window)

Appendix B11-3 - Noise (PDF) [67KB] (opens new window)

Appendix B11-4 - Biodiversity (PDF) [88KB] (opens new window)

Appendix B11-5 - Water (PDF) [98KB] (opens new window)

Appendix B11-6 - Landscape (PDF) [83KB] (opens new window)

Appendix B11-7 - Heritage (PDF) [69KB] (opens new window)

Appendix C - Commercial Appendices

Appendix C1 - Procurement Strategy (PDF) [393KB] (opens new window)

Appendix D - Financial Appendices

Appendix D1 - Quantitative Cost Risk Analysis (PDF) [831KB] (opens new window)

Appendix E - Management Appendices

Appendix E1 - Project Delivery Experience (PDF) [2MB] (opens new window)

Appendix E2 - Project Governance (PDF) [322KB] (opens new window)

Appendix E3 - Stakeholder Engagement and Communications Plan (PDF) [421KB] (opens new window)

Appendix E4 - Delivery Programme (PDF) [1MB] (opens new window)

Appendix E5 - Risk Management Plan (PDF) [2MB] (opens new window)

Appendix E6 - Risk Register (PDF) [126KB] (opens new window)

Appendix E7 - Benefits Realisation, Monitoring and Evaluation Plan (PDF) [1MB] (opens new window)

Appendix E8 - Carbon Management Plan (PDF) [909KB] (opens new window)

The effects of traffic flows as a result of the scheme will be considered and will form part of the assessment in the Outline Business Case.