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M25 alignment and junctions

The construction of a new north west runway will extend the airport to the west. The proposed runway will cross the M25 between Junctions 14 and 15 (J14 and J15) and will affect the operation of J14 and J14a.

The M25 is one of the busiest roads in the UK and we will ensure that our proposals do not result in disruption. We are working with Highways England, who are responsible for the operation, maintenance and improvement of the motorways and trunk roads in England, to ensure the project meets their design and safety standards. We demonstrated our track record in changes to the M25 when we built J14a as part of the Terminal 5 project.

Tunneling the M25

We have looked at options to bridge the runway over the M25, to tunnel the M25 under the runway (our previous design concept to the Airports Commission), or divert the M25 around the west end of the runway. The new level of the M25 will be determined by design standards so that it works with the existing J15 which we are not planning to change.

Our current thinking is to re-position the M25 carriageway approximately 150 metres to the west, lower it by approximately 7 metres into a tunnel and raise the runway height by 3 to 5 metres so that it passes over the M25 between J14a and J15. The motorway will then re-join its current route.

Crossing a motorway with a runway or taxiway has been done successfully at other airports around the world such as Paris (Charles de Gaulle) and Atlanta.

J15 will not be affected but some minor works may need to take place to tie the main carriageway and link roads into the junction. We believe this approach is the most deliverable as it would allow construction to proceed while the existing M25 motorway remains in operation. This minimises impacts to road users and has the least overall impacts on communities during construction and long-term operation.

We are also considering whether there would be benefits in introducing collector-distributor roads alongside the M25 that improve journey times for both airport and non-airport traffic. Such roads are however complex to build, require more land and are more costly.

M25 junctions

The expansion of the airport and realignment of the M25 will mean that we have to make some alterations to Junction 14 (J14) and Junction 14a (J14a).

An expansion of the airport to the north west, and a need to alter some local roads (see Local roads), is likely to mean more traffic will want to pass through these junctions and so we will need to improve their capacity. We are currently considering two ‘families’ of options for these junctions:

  • Family 1 – both J14 and J14a are retained: J14 will need to be redeveloped to accommodate the displaced traffic movements from surrounding areas and maintain good connections to nearby communities. J14a may need to be altered to fit with the westerly expansion of the airport. Both junctions may need altering to accommodate collector-distributor roads.
  • Family 2 – J14a is closed: J14a currently provides direct access to T5, but if T5 and the western apron is expanded (see section 2.2), the junction may have to be permanently closed. In those circumstances, J14 would need to be redeveloped to accommodate the displaced road traffic from J14a. More space and road adjustments may also be required for collector distributor roads.

Both options require the redevelopment of J14. Our preference is for options that provide short journey times for airport and non-airport road traffic, provide good connections to nearby communities whilst minimising property loss and construction effects on Stanwell Moor and Poyle.

Further information

For more information on both the alignment of the M25 and the motorway junctions can be found in section 6 of our "Scheme Development Report" and section 7 of "Our Emerging Plans". Both documents are available in the Documents & Resources section of this website.
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