M4 junctions 3-12: smart motorway

We’re improving the M4 between junction 3 at Hayes and junction 12 at Theale by upgrading it to a smart motorway. This means there will be an additional lane for traffic which will increase capacity and reduce congestion, more technology on the road to smooth traffic flows and manage incidents and more reliable journey times.

Start date 2018

End date 2022

Cost £848 million

M4 junctions 3-12 smart motorway

Latest updates

16 June 2020

Project reaches halfway point

In June 2020 the project hit the halfway point with the work to extend the New Thames Bridge at Bray that carries the M4 over the river. This bridge - started in 1937, delayed by World War II, and completed as part of the new M4 motorway that opened in 1961 – is being widened on the north side by 8 metres to accommodate the extra lanes required for the new smart motorway.

We are continuing our steady progress on the western section between junctions 8 and 9 to junction 12 which started in July 2018. We expect to finish most of the work on this section by the latter part of 2020, before opening it as a smart motorway during 2021. The eastern section is more complex due to a significant amount of bridge work and will open in 2022.

We are following Government policy which is that construction activity should continue where it can be done safely. All our sites have strict safeguarding measures to prevent the spread of COVID-19 and none of our sites are open to the public. With traffic volumes on our roads still lower than usual, we are continuing to work before traffic flows eventually return to normal. We plan our work carefully in order to minimise travel disruption.

See more updates

Project information

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 x bridges which are being replaced as part of the project. The barrier in the central reservation will be replaced by a new concrete barrier.

Through-junction running will be in place at There are several junctions on this stretch of the M4 which we will be changing so there’s a consistent number of lanes to pass through the junction. This reduces the need to change lane for vehicles staying on the motorway.

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

Statutory Instrument Consultation

Secretary of State decision

Development Consent Order

Speed limit consultation

Pre-application consultation

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.