Junction 17 - M4
M4 junction 17 is a key junction providing access from the M4 towards the South coast. The junction provides the vital link between the motorway network and the A350 connecting the towns in west Wiltshire, including Chippenham, Melksham and Trowbridge; it provides a link to Malmesbury and the A429 in the north, and also allows for local access via the B4122.
Funding for enhancements at M4 Jn17 has been sought through Major Road Network Funding, and a scheme at this junction was identified as a priority by the Western Gateway Sub-national Transport Body (STB).
The scheme will build upon the success of the previous improvement works undertaken in 2018, which introduced partial traffic signalisation at the motorway off-slips. The scheme will facilitate further improvements, to allow for envisaged future traffic growth.
The scheme is being developed as a single package of improvements to maximise and optimise capacity of the junction within the bounds of the existing Highway, without the need for land take, and which would be delivered as a single project. It is anticipated that this approach will help avoid the potential need for progressive and sequential improvement projects at the junction over many years as individual developments come forward.
The scheme proposals include:-
- Introduction of traffic signals to all approaches to the roundabout i.e. completion of the full signalisation of the junction;
- Carriageway widening and additional traffic capacity on all approaches to the junction (M4 off slips, A350, A429 and B4122);
- Increase in the number of traffic lanes across the motorway bridges from two to three;
- Widening of the circulatory carriageway and introduction of additional traffic lanes and capacity around the junction;
- Repositioning of the layby along the A429 to allow for carriageway widening work.
Timeline for delivery
The current anticipated high level timeline for the scheme is as follows:-
- August 2022 - Outline Business Case Submission to DfT for scheme development funding
- Summer 2024* - Full Business Case submission to DfT for Detailed Design funding (*subject to progress from previous stage)
- Autumn 2024 - Construction commences
- Late 2025 / early 2026 - Scheme Opening
Opportunity for comment
Wiltshire Council would welcome any comments that members of the public, local organisations or businesses may have on the scheme proposals. If you have any comments or queries please contact us at the following e-mail address:-
Alternatively, please write to us at:-
Major Highway Projects Team
Frequently Asked Questions (FAQ's)
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 will be submitted to the Department of Transport shortly.
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 2023 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:
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.
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.