One option is to build a tunnel between junction 2 and airport exit

The M4 Flyover at Brentford . Picture: Google Street View

A multi-billion pound scheme to widen the elevated section of the M4 between junction 2 in Brentford and the exit to Heathrow airport is being considered as part of the planned improvements to infrastructure needed for the third runway.

The project which could cost over £2 billion was part of recommendations originally made by the Airports Commission and came to light recently with the publication of an assessment into the cost and viability by Highways England which was requested by the Department of Transport. Highways England have described the scheme as ‘very challenging’.

The M4 currently starts as a two lane dual carriageway with no hard shoulder from Chiswick Roundabout which is also junction 1. The report makes no mention of any plans for this section of the motorway which includes the Chiswick Flyover. The options that were assessed include a widening of the elevated section through Brentford to three lanes on each carriageway or putting the motorway into a tunnel starting from the west of junction 2.

The report looks at estimated costs and delivery timescales to the Strategic Road Network (SRN) proposed for surface access to the airport. It includes changes to be made to the junction of the M25 which have been requested by Heathrow but the M4 widening is a proposal made by the Airports Commission and has not been requested by the owners of Heathrow.

The total project cost would be in an indicative range or £1.32bn to £2.83bn with the tunnelled option being the most expensive option. One solution is for the elevated section to be retained westbound with the eastbound carriageway put underground.

Substantial land acquisition of residential and commercial properties would be needed along the length of the motorway as well as a significant realignment of the existing slip roads and an upgrade of the drainage system along the entire length.

The report describes the proposal as ‘an area of concern’ saying that there is a significant risk of cost overruns and states, “Providing an additional lane of capacity in both directions between M4 J2-3 is likely to impact the surrounding road network at J2 and may require substantial works in this heavily built-up location. An assessment of the impact of proposals on the surrounding road network at M4 J2 would require wider validation of the Commission’s traffic modelling and forecasting. This is outside our scope but we consider there is a risk of increased scope of works required to cater for the additional capacity at M4 J2 in particular.”

Highways England have developed a proposed solution that they consider feasible including a bored tunnel for eastbound traffic to J2. However, they warn that there are numerous unknowns in relation to costs such as required capacity, ground conditions, impacts on the surrounding network at Junction 2, lands costs and the internal condition of the elevated sections of the M4.

They also warn that with the significant number of infrastructure schemes that would be scheduled to start at similar times there would be a significant strain on the construction industries resources within the south of England particularly. We asked Highways England for more specifics and they sold us no detailed plans have been drawn up at this stage.

John Holland-Kaye, Heathrow Airports Chief Executive Officer told a parliamentary select committee in 2013 that he did not anticipate that a third runway would increase the number of vehicle journeys to the airport because they believed they could expand without increasing traffic due to the number of associated rail projects that would increase public transport access.

The issue was raised by Ruth Cadbury in the House of Commons on 17 November when she asked the Transport Secretary, “Deep in the Airport Commission’s papers is the hugely costly and disruptive proposal to double the capacity of the M4 at its London end with a tunnel coming up in Brentford or Chiswick. Will the Secretary of State confirm the Government’s estimate of the cost of service transport infrastructure needed for a third runway at Heathrow, and what proportion of that will Heathrow airport be required to pay?”

Chris Grayling replied, “There are two separate issues here. Improvements are needed to local roads in west London, and the M4 is one of those where plans are afoot now to deliver improvements way before we have a new runway in place. Heathrow airport will be expected to pay for the infrastructure improvements directly linked to the new runway. There are of course other improvements, such as M4 improvements, that are not directly linked and that have for some while been envisaged as part of the ongoing road improvement programme this Government are pursuing. My commitment is that where a transport improvement is required to make the third runway possible, that will be met by Heathrow airport.”

November 25, 2016