M4 junctions 3-12: smart motorway


M4 junctions 3-12: smart motorway

Making the M4 a ‘smart motorway’ between junctions 3 and 12.

Scheme stats

Region: South East
Start date: March 2017
Cost: £586.4 to £862.4 million
Type: Major Scheme
End date: March 2022
Project status

M4 junctions 3-12: smart motorway

Making the M4 a ‘smart motorway’ between junctions 3 and 12.

Progress report

The Planning Inspectorate issued a report of recommendation to the Secretary of State on 3 June 2016.

What next?

The Secretary of State has until 3 September 2016 to make and announce a formal final decision on this scheme, based on the report of recommendation.


  • Advanced motorway signalling and traffic management feasibility study identified this motorway
  • link as a priority for additional capacity: March 2008
  • Department for Transport announces a programme of smart motorways, including this scheme: 2009
  • Scheme development commenced: 2010
  • Roads Minister announces funding for this scheme: 2013
  • Public consultation: November 2014 - December 2015
  • Speed limit consultation: January-February 2015
  • Planning application submitted: 30 March 2015
  • Planning application accepted for examination: 27 April 2015
  • Notification of acceptance under the Planning Act: 28 May 2015
  • CH2M and Arcadis appointed to develop detailed scheme design: July 2015
  • Balfour Beatty and VINCI appointed to develop methodology and programme: July 2015
  • Deadline for Secretary of State to announce final decision: 3 September 2016
  • Start of works: by end March 2017
  • End of works: end March 2022

Why we need this scheme

The M4 motorway (M4) is the main strategic route between London, the west of England, and Wales. It connects directly to the M25 and Heathrow Airport. The M4 carries over 130,000 vehicles per day and currently suffers from heavy congestion making journey times unreliable. Traffic flows on the M4 are forecast to increase to an average of 160,000 vehicles per day over the next 20 years. This will result in even more congestion if nothing is done.

The scheme in detail

This scheme will use the latest technology to improve journeys by monitoring traffic flow and setting speed limits accordingly. This helps to keep traffic moving smoothly, instead of continually stopping and starting. Information about road conditions and speed limits will be displayed to drivers on electronic road signs. The proposal also involves converting the hard shoulder permanently to a traffic lane. This will create much needed extra capacity necessary to support economic growth. The conversion of the hard shoulder will be continuous through junctions unless there is an operational reason not to do so. We propose that the new lane created from the hard shoulder would continue through junctions 4, 5, 6, 7, 8/9 and 11. We aren’t proposing this for junctions 3 and 12 or at junctions 4b and 10, as these are motorway to motorway links. To enable the provision of a smart motorway along the whole length of the proposed scheme, it will be necessary to widen or replace a number of bridges where there is currently no hard shoulder.


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

Environment impact

Smart motorways typically have a lesser environmental impact than traditional motorway widening schemes as there is less physical work. This minimises the potential impact on watercourses, habitats, landscape etc. Additionally, the technology used for Smart motorways allows traffic flows to be regulated; reducing congestion and therefore reducing air quality issues. We’ve undertaken an environmental assessment including:
  • air quality
  • cultural heritage
  • landscape
  • nature conservation
  • geology and soils
  • materials and noise
  • vibration
The results of this environmental assessment will be published . This will include any mitigation measures needed such as speed limit reductions and noise barriers.

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