Junction 17 - M4
Scheme background
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).
Scheme proposals
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 outline business case (OBC) was submitted to the Department for Transport (DfT) in Summer 2022 with an updated copy being sent in Summer 2023. In May 2024, Wiltshire Council was granted approval from the DfT to take forward plans and develop the full business case (FBC). The notional timelines indicated below are subject to confirmation and funding from the Department of Transport and the successful completion of the previous stage.
- Summer 2024 - Start Design Stage
- Spring 2026 - Procurement
- Summer 2026 - Full Business Case Submission
- Spring 2027 - Construction Commences
- Spring 2028 - Scheme Opening
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