M4 Junctions 3-12: Smart Motorway

Project status:   Planned

Start date:   Post 2015
End date:  Post 2021

Type:   Major Scheme
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Major Scheme

Part of our programme of improvements (value >10m)

Cost:  £614m to £862m


Development Consent Order

We have classed this scheme a Nationally  Significant Infrastructure Project, which means we will be seeking a Development Consent Order from the Planning Inspectorate. Please see the publications page for our recent presentation to the Planning Inspectorate on our proposals.

Public Information Exhibitions

Public Information Exhibitions were held between the 18 March and 29 March 2014. To view the exhibition boards please go to the publications page.

The feedback report, based on the responses from the Public Information Exhibitions in March, has now been published and is available on the publications page.

What is happening?

We propose to improve the M4 by making it a "smart motorway" (previously known as managed motorways) between junctions 3 and 12. Smart Motorways help relieve congestion by using technology to vary speed limits. They also allow the hard shoulder to be used as a running lane at all times to create additional capacity. They deliver these benefits at a significantly lower cost than conventional motorway widening and with less impact on the environment during construction. Find out more about Smart Motorways.

When and where is this happening?

On 8th May 2012, the Roads Minister Mike Penning, announced that funding would be provided to develop the M4 J3-12 Smart Motorway scheme, to ensure a "pipeline" of future Highways Agency major infrastructure improvements will be maintained, contributing to future economic growth, and supporting Government's National Infrastructure Plan. By developing the scheme now, it will be in a good position to be considered for delivery in the early years of the next spending review period (post 2015).

It is important to note that by advancing this scheme through its development phase, this does not guarantee its construction; this decision will be taken at the next spending review. For more information visit our Future Spending Review page.

This project is located on 31 miles of the M4 Motorway between the A312 at Heathrow Airport to Theale in Berkshire.

Why is this happening and what will it cost?

The strategic case for providing additonal capacity on the M4 within Thames Valley was examined in the Thames Valley Multi-Modal Study, which recommended against widening the motorway prior to 2016 in favour of demand management measures.  The Secretary of State endorsed these recommendations in 2003.

However the March 2008 'Advanced motorway signalling and traffic management feasibility study' identified this motorway link as a priority for the provision of additional capacity and Ministers agreed that hard shoulder running should be investigated as an alternative to widening.

The estimated cost of this scheme is in the range of £614 million and £862 million.

How will the scheme be carried out?

A number of cameras, information signs and signals on gantries will be installed as part of the improvement scheme. As the scheme design progresses we will provide more information on the location of these features.

The feasibility study identified a single Option which will allow vehicles to occupy the Hard Shoulder, as a running lane, on M4 Junctions 3 to 12, with Through Junction Running at junctions 4, 5, 6, 7, 8/9 & 11.

The specification and assessment will be based on the Smart Motorways Implementation Guidance, which sets out latest best practice for the design and operation of all-lane and through junction running.

The scheme will comprise the following elements:

  • MIDAS Queue protection technology
  • CCTV coverage
  • MS4 information signs
  • Lane specific speed signals at certain locations
  • Variable speed limits
  • Speed enforcement
  • Hard Shoulder usage as a running lane
  • Emergency refuge areas with emergency telephones

What are the benefits?

  • The improvements will contribute to all three Eddington Priorities, ie
    • Gateway - Improved reliability to Heathrow
    • Inter-urban - more reliable journeys from London to Reading, Bristol & West Country
    • Urban - Supports agglomeration in Thames Valley
  • To reduce congestion and improve resilience of the strategic road network
  • To improve journey time reliability on the strategic road network
  • To improve road safety on the strategic road network (including road workers)
  • To minimise environmental impact, enhancing the environment where appropriate
  • To provide better information for drivers using the strategic road network
  • Promote innovation to obtain better value for money
  • Optimise the use of technology and minimise whole life costs

How do I find out more information ?

More information will be posted on this project page as it becomes available. You can subscribe to be alerted when updates are made.

If you have any queries about this project you should contact the Highways Agency Information Line by emailing ha_​info@​highways.​gsi.​gov.​uk or calling 0300 123 5000.

Business Case


M4 Junctions 3 to 12 Smart Motorway
M4 Junctions 3 to 12 Smart Motorway . Red Line Drawings for the purpose of scoping - July 2014


M4 Junctions 3-12 Smart Motorway - Planning Inspectorate Introductory Meeting
M4 Junctions 3-12 Smart Motorway - Planning Inspectorate Introductory Meeting - February 2014


M4 Junctions 3-12 Smart Motorway – Public Exhibition Information
Information from the M4 Junctions 3-12 Smart Motorway Public Exhibition held in March 2014


M4 Junctions 3-12 Smart Motorway – Feedback Report
M4 Junctions 3-12 Feedback Report presenting the findings of the initial stakeholder consultation.