The flyover on foot: walking the A4/M4 corridor

Hounslow’s Great West Road, constructed to bypass Brentford High Street en route to west England and ease local congestion, opened to royal fanfare in 1925. King George V cut the ribbon at the road’s opening ceremony, ushering in its first wave of cars. In 1959, a similar scene: American actress Jayne Mansfield debuted the Chiswick flyover, built to ease congestion on Chiswick roundabout. The flyover was extended five years later, straddling the Great West Road (part of the A4) for two kilometres and forming part of the M4.

Despite celebrity endorsements and the best intentions of previous generations of planners, today the A4/M4 poses a policy conundrum. Economically, the corridor is home to thousands of jobs and shifts goods in and out of the city. But socially, its imposing interlocking concrete ramps bisect Hounslow, making it difficult and unpleasant for residents to cross without a car. There are also public health concerns arising from poor air quality and road safety. On 26 February, in the first field trip for our 2018 programme on severance1, candidates on FoL’s Future London Leaders course visited Brentford to learn about the impact of the A4/M4 on local communities and how different stakeholders are addressing this.

A4/M4 corridor
The multi-plane construction of the A4/M4 corridor was one of the first road engineering schemes of this kind.

The A4/M4 in context

Following a brief history of the corridor from Gareth James, Senior Transport Project Officer at LB Hounslow, his colleague Harris Vallianatos, Sustainable Transport Projects Officer, shared traffic statistics about the A4/M4. Data from 2016 counted 40,000 vehicles per day on the A4 and 86,000 per day on the M4; while these numbers have been stable over the last 20 years, the composition of traffic has changed.

The roads fall under the council’s proposed Great West Corridor Plan. Gareth noted that the plan identifies potential for 5,200 homes and at least 14,000 jobs, along with public realm and connectivity improvements along the A4/M4. The council also hopes to improve access to Gunnersbury Park and stitching together Carville Hall Park, which is divided by the A4/M4.

However, multi-agency ownership complicates change: Highways England manages the M4, TfL the A4, LB Hounslow the intersecting local roads, and several landowners abut the corridor. Each has different, and sometimes clashing, priorities.

Impact on communities and local ideas for change

Harris pointed out that Brentford has one of Hounslow’s largest concentrations of schools. The field trip coincided with the end of the school day, sending waves of adults and children on their return journeys over the A4. However, the environment—poorly lit, polluted, noisy—and crossings with limited pedestrian walking time make the journey unpleasant and dangerous. Some people continued to rush across the road after the three-second, blink-and-you’ll-miss-it green walk symbol.

Parents of children in local schools who accompanied us on the field trip hoped for better lighting and crossing points (both along the A4/M4 and side roads with limited provision) as well as reduced traffic speeds and volumes. Local councillor Melvin Collins suggested smaller scale improvements like painting, regular cleaning and planting would make the corridor feel maintained and less oppressive.

The afternoon school run at Clayponds Avenue along the A4/M4.
The afternoon school run at Clayponds Avenue along the A4/M4.

Local residents Nicola, Martin, and Victoria from Air Quality Brentford had further recommendations aimed specifically at tackling Brentford’s high pollution.2 Not only is Brentford affected by the A4/M4, its eastern boundary is the North Circular; the group is campaigning to expand Sadiq Khan’s proposed Ultra-Low Emission Zone beyond the North and South Circulars to all of London to avoid diverting traffic through Brentford, further worsening its air quality. They would also like a 30 mph speed limit where the A4 passes residential areas and a ban on idling at red lights.

Travel behaviour

Timothy Mackay, Principal Sponsor at TfL, pointed out that addressing driver behaviour needs to be part of any solution: drivers know they’ll contend with pedestrians in central London but expect more unencumbered travel in outer London, especially on A-roads and motorways. His colleague Faith Martin, Principal Technical Specialist, added that drivers need to know where the A4 intersects with smaller, local roads which are more likely to have pedestrians trying to cross. With limitations on how much they can change the A4/M4 structure, TfL is considering ways to signal to drivers that they are entering a built-up area.

Local resident behaviour is also affected. Dr Paulo Anciaes has been developing UCL’s Street Mobility Toolkit which offers a variety of methods to understand how roads impact people’s perceptions of an area and travel behaviour. In interviews with residents along the A4 west of Boston Manor Road, he found that when amenities are on the other side of the road, residents are more inclined to drive to them than to walk, compounding traffic and pollution. The toolkit generates a ‘severance index’ for an area; in this case, it’s a very high 84%.

Paulo’s research indicated that the most impactful measures to reduce severance along the A4 would be to add crossings (causing the severance index to plunge to just 7%) and/or reduce traffic volumes (49%). The former requires hard infrastructure and the latter requires policy and behaviour change, but neither comes with an overwhelming price tag. As the toolkit can be used by policymakers and community groups alike, it is a useful starting point to a deeper understanding of the impact of roads on neighbourhoods and possible solutions.

This map shows major roads (in grey: A4/M4 horizontally and North Circular vertically), corridor crossing points (orange circles with walk symbol), schools (blue circles with pair of children), shopping areas (in pink: South Ealing high street and Brentford high street), and other amenities (purple circles for supermarkets, train station, library).



(1) This field trip was arranged as part of a Future London Leaders course and was not a public event. Our series of public field trips on the topic of Solving Severance: Overcoming London’s Barriers begins in spring. Join our mailing list to receive invitations to the public events.

(2) From the Great West Corridor Plan: “[…] Along the combined A4/ M4 corridor and at Kew Bridge […] levels of >60 μg/m3 NO2 are can be found. Along M4 (west) and parts of east of the Great West Corridor (Brentford East) similarly high levels 50 – 60 μg/m3 NO2 can be found. The North and South Circular roads also suffer from high levels of pollution with levels varying between 50 – 60 μg/m3 NO2.” In 2017, the London mean roadside NO2 was 50.9 μg/m3 NO2.