M4 junctions 3-12

We’ve made improvements on the M4 between junctions 3 and 12 by adding an additional lane for traffic to help increase capacity and reduce congestion. We’ve also put in place new technology to smooth traffic flows and manage incidents along this stretch.

Start date 2018
End date 2022
Cost £848 million

Latest updates

  • 05 December 2022

    Upgraded motorway between junctions 3 and 8/9 now open

    The national speed limit has been restored between junctions 4 and 8/9. This marks the lifting of the final restriction on the upgraded M4 between junctions 4 and 12.

    A section between junctions 3 and 4 in West London will remain at 60mph to help manage air quality in the area, and to manage traffic into London; our upgrade of the road and calibration of the technology here is complete.

    You’ll have seen between junctions 3 and 8/9 all lanes of the upgraded carriageway were open and the speed limit was 60mph. Drivers were kept at a lower maximum speed limit while we adjusted the technology which detects stopped vehicles. We have now increased the maximum speed on the upgraded section but we continue to make small adjustment to the technology and routinely check the system.  

    If drivers get into difficulty between junctions 3 and 8/9, they will be able to use one of 16 new emergency areas, bringing the total of emergency areas on the M4 between junctions 3 and 12 to 34.  In total there are 59 places to stop in an emergency, including retained sections of hard shoulder between junctions and on junction slip roads, every 1.3 miles on average.

    We use a system of inter-related features and technology on upgraded motorways which include:

    • Variable speed limits to help keep traffic moving, reducing frustrating stop-start traffic and making journeys quicker 
    • Clearly signed and orange-coloured emergency areas set back from the road and with telephones linking directly to our control rooms 
    • Detection systems to monitor traffic for changes in flows 
    • Stopped Vehicle Detection (SVD) technology which can identify a stationary vehicle, typically within 20 seconds
    • CCTV cameras that our operators are able to move and zoom to monitor and manage congestion and incidents, where notified. The system has the ability to see 100% of the carriageway
    • Signs and signals which provide drivers with information that alerts them to hazards ahead
    • Red X signals to close lanes to other traffic when a stopped vehicle is identified
    • Enforcement cameras to deter the minority who break speed limits and ignore Red X signals 

    Some lane and slip road closures will take place overnight for the next few weeks while we remove temporary traffic management associated equipment.

    This is the final section of this major upgrade to open on one of the busiest stretches of motorway in the country. We’re looking forward to real improvements it will bring to more than 130,000 journeys every day. We would like to thank you for your co-operation and understanding during this phase and during the roadworks that we started in 2018.

    If you’d like to know more about the main features of smart motorways, and advice and guidance on safer driving and what to do in an emergency, visit our ‘Driving on motorways’ hub. 

See more updates

Project information

Overview

Once our work is completed this stretch of the M4 will use the latest technology to monitor traffic flow and set speed limits. This will help keep traffic moving smoothly, instead of continually stopping and starting. 

Between junctions 3 and 12 the hard shoulder will be converted to a traffic lane, so there will be four lanes available for use by road users. Between junctions 4 and 4b, there will be five lanes. There will also be five lanes on the eastbound approach to Junction 4b. 

Where a hard shoulder does not currently exist, the motorway will be adapted to create a minimum of four lanes, including under the 11 bridges which are being replaced as part of the project. The barrier in the central reservation will be replaced by a new concrete barrier. 

The M4 is the main strategic route between London, the West of England and Wales. It connects people, communities and businesses, carrying on average 130,000 vehicles per day. 

The changes we’re making will:  

  • reduce congestion 
  • smooth the flow of traffic to improve journey times 
  • make journeys more reliable 
  • support economic growth within the region, by providing much needed capacity on the motorway  
  • continue to deliver a high level of safety performance on the network using smart motorway techniques and minimise environmental impacts of the scheme

The DCO process was established by the Planning Act 2008 and is used for certain large and complex schemes (including highway improvements) that have been designated as a Nationally Significant Infrastructure Project (NSIP) by the Government.

The benefits of the DCO process include extensive pre-application consultation, detailed analysis, including a full Environmental Impact Assessment, and examination by an independent inspector before the final decision is made.

Development Consent Order – requirements register

We made a commitment to publish and update a register listing the requirements to be completed as part of the Development Consent Order (specified under the Register of Requirements in Schedule 2). This sets out:

  • each requirement
  • whether the requirement needs approval by the Secretary of State (or other duty holder)
  • whether any approval has been applied for or given

All of the DCO requirements needed to start work were discharged in summer 2018.

Development Consent Order

isit our M4 J3-12 feedback tool to have your say. The tool includes an interactive map. You can click on a location to make a specific comment, or give us your general feedback.

ECHO is a one-way feedback tool, so if you’d like a response to your feedback you can email the project team at: M4J3to12smartmotorways@nationalhighways.co.uk

Documents

Traffic information

Information about scheduled roadworks and events on our motorways and major roads.

Contact us

info@nationalhighways.co.uk

0300 123 5000

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