Heathrow video shows M25 realignment if proposed third runway approved

Heathrow Airport has released details of how it will deliver a brand-new tunnel and bridges for the M25 while keeping the motorway live during construction of the proposed third runway.

image: Heathrow Airport (click image to see video)

Central to the project, which involves extending the new runway over the M25, is building a new section of motorway largely in cut and cover tunnel 130m to the west of today’s motorway, keeping construction separate from daily commuters. Traffic will then be switched from the old route to the new.

The airport says that once the new tunnel is complete motorists will benefit from a wider and safer stretch of motorway, 100% privately financed by Heathrow. It will include new link roads that will separate the passing M25 traffic from those leaving and joining the M4, eliminating weaving movements and offering safer journeys.

​Since proposlas for expanding Heathrow were submitted to Government, Heathrow says the issue of how to deal with the realignment of the M25 has been raised as one of the most common questions. Revealing its conceptual masterplan in a new video, Heathrow says the idea has been shaped over years of engagement with National Highways.

Alternative options to place a new runway over the M4 spur were evaluated but discarded by both Heathrow and the Airports Commission (which looked into the third runway over a decade ago) for having fewer benefits and causing more disruption.

A design that avoids the M25, a number of airlines have indicated, would result in a runway too short to accommodate all long-haul flights. Building over the M4 spur would require the demolition of thousands more homes and expose more residents to noise once operational.

The M4 spur also runs over a crucial rail tunnel that carries Heathrow Express and Elizabeth Line services in and out of Heathrow. With no space to build ‘offline’, it is unclear how construction could take place on this constrained stretch of road without significant disruption to both rail and road users for months on end.

Heathrow’s Surface Access Director, Sophie Chapman, said, “Heathrow expansion is a once in a generation opportunity to upgrade our connectivity in the air and on the ground. One of the legacies of our plan will be a 100% privately financed upgrade to one of the busiest stretches of the M25. We have a tested plan for building next to the current motorway that will keep motorists on the move during construction. We have already looked at the alternatives and they simply create more impacts for less benefits so we’re confident we have the best plan available.”

Heathrow submitted its proposals to Government this summer saying its vision is the only plan on the table ready to deliver the economic growth the country needs.

Building a new runway over a motorway has been successfully delivered at other airports including Frankfurt and Charles de Gaulle, Paris. Heathrow will continue to work closely with National Highways to finalise the details of its plans.