M4 Chiswick-Langley

The M4 between Chiswick and Langley (J1 to J5) was built in its entirety and can be travelled today. It opened in 1964.

While it might not be particularly mysterious, its story is interesting in several ways. It was not part of earlier plans for London’s road network, and was conceived as a new route in the 1950s, leapfrogging a large number of much older proposals to reach construction very quickly. It is home to one of London’s most substantial lengths of elevated road. And it was considered an experimental route when it first opened to traffic, being home to a considerable number of innovative features and notable firsts.

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M4 Chiswick-Langley
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Genesis

Unlike many of the roads that make up the wider Ringways plan, the M4 between Chiswick and Langley is not descended from proposals made by either Bressey or Abercrombie. No road on this line, or even approximating it, appears in their plans.

Bressey and Abercrombie each had their own reasons to ignore this corridor.

For Bressey, carrying out his survey of London’s roads in the mid-1930s and drawing up his proposals in 1937, the A4 in this area had just been improved to a good standard and, unlike most of London’s other main roads, did not require further improvement.

Between Chiswick and Henlys Corner, the Great West Road bypassed the old Bath Road through Brentford and Hounslow, and was either dual carriageway or a wide three-lane single carriageway, offering travel as fast as any road in the south east. In the 1930s it could be described as state of the art.

From Henlys Corner onwards, the Bath Road was reasonably straight and wide. Colnbrook, the next settlement along the road, had been bypassed in 1929. On the map accompanying the Highway Development Survey, Bressey was content to mark the whole length as an “existing arterial road”1.

West from Langley, Bressey drew in Route 57, a southerly bypass of Slough.

For Abercrombie, who re-examined London’s main roads just six or seven years later, there was still no need for action. Like Bressey, he regarded the A4 between Brentford and Langley as a good main road already, but his plan also re-cast the main roads in west London in a way that would sideline the Great West Road.

Abercrombie’s Greater London Plan of 1944 indicated bypasses of Slough and Maidenhead that are essentially the same as the present-day M4, and clearly are refinements of Bressey’s Route 572. However, he envisaged the road splitting into two branches as it approached London. One branch, travelling north from the Slough Bypass at Burnham, would curve north-east to join Western Avenue. The other would reach Langley and then travel south-east to the Great Chertsey Road. With those two routes handling through traffic bound for Reading, Bristol and South Wales there was no need for further upgrades to the A4.

South Wales Radial Road

The situation appeared to change during the late 1940s or early 1950s, at which time several developments together required a new approach.

One was the abandonment of Abercrombie’s two branches, one to Western Avenue and the other to the Great Chertsey Road. Nothing has yet been found relating to this decision but it is evident that, at some point, there arose a new policy that the Slough Bypass would feed traffic in to the A4 towards London and not into the adjacent radial routes. Possibly anticipated traffic levels meant that it was no longer considered feasible to ask the A40 and A316 corridors to absorb this extra traffic load.

Another was the development of London Airport (now Heathrow), which opened for commercial flights in 1946. The suitability of the site for a major airport had been noted by Abercrombie. The traffic generated by an airport – and the need for much of that traffic to reach Central London – meant that the A4 and the Great West Road were now expected to handle much greater volumes of traffic.

The third was the start of planning work on the national motorway network, which commenced in the 1950s. One of the routes under consideration was Motorway No. 7, the London-South Wales Motorway. The alignment of this new road was settled around Maidenhead and Slough, where existing plans for bypasses of those towns (and earthworks for the latter, started before the outbreak of the Second World War) could be incorporated.

But the Slough Bypass ended at Langley. This was too far from London for the terminus of the motorway, but the only available corridor, which was the existing A4, made up of the Colnbrook Bypass, Bath Road and Great West Road, could not be upgraded to a motorway. As a result, and for the first time, a new alignment was required between Chiswick and Langley.

Design and route options

The Ministry of Transport engaged Sir Alexander Gibb and Partners in the late 1950s3 to examine route options and produce detailed designs for the London-South Wales Motorway between Chiswick and Langley.

From the outset the intention was to terminate the motorway at Chiswick, since this would bypass the entirety of the Great West Road, allow a connection to the North and South Circular Roads, and permit traffic bound for Central London to flow directly into the new Cromwell Road. That initial section, westwards from Chiswick Roundabout, was seemingly settled as an elevated viaduct at an early stage.

This can be seen in an Engineers’ Report produced by Gibb and Partners in November 1957. An imperative of the project was to provide a connection to London Airport, and the report considers options for running the motorway on a viaduct above the A4 Great West Road all the way to Heathrow. Consideration was also given to the width of the viaduct, including a discussion about whether it should carry a dual two-lane or single three-lane road. The report rules out the latter because it would lead to dangerous overtaking on a high speed road, would preclude classification as a motorway, and would be “regretted in a very few years”4.

Viaduct width

By November 1958 the consultants had evidently settled on the concept of a viaduct above the Great West Road from Chiswick to Osterley, supported on a row of piers in the central reservation, with the deck cantilevered out to each side – essentially the structure that was built. The road would then branch off the Great West Road to pass through Osterley Park and follow a corridor of mostly open land to Langley.

Their design was for a dual two-lane road on the viaduct, and this was evidently a cause of some concern at the Ministry: reading between the lines, the width of the viaduct must have been raised on multiple occasions. One civil servant wrote to another:

“I have again this morning spoken to the consulting engineers about the additional cost of providing dual three lane carriageways on the viaduct and they confirm the previous figure of £2M. to £2½M. as the additional cost of providing the three lane facility on the present cantilever design.”

Minute from JDW Jeffery to JG Smith, 19 November 19585

Jeffery went on to explain that a “more normal portal frame” structure would have a lower additional cost – about £½m more – but its foundations would be in the verges of the road, which would necessitate the relocation of mains pipework and cabling, adding a minimum of another £½m to the cost, and probably much more. The consultants warned against such a design on the basis that it would be extremely difficult to keep traffic moving on the A4 during construction works and that the piers would obstruct the view of traffic on side roads attempting to emerge onto the main road. They stated that two lanes of traffic each way could be sustained during the works if the recommended cantilever design was used.

Anxiety about the width of the viaduct plainly persisted at the Ministry, however, because the issue was raised again just two days later at another meeting with the consultants.

Mr Lander, on behalf of Gibb and Partners, brought to the meeting three methods of providing a dual three-lane viaduct, seemingly as a way of demonstrating why the consultants were arguing against the idea. They were as follows.

  1. Carriageways side by side supported by two lines of piers 60ft apart. One line would be in the central reservation of the A4, the other on one verge, so the motorway would be offset to one side. This would cost 150% of the existing estimate, put the motorway hard up against properties on one side of the road, and required relocation of services, footways and cycle tracks on one side of the A4.
  2. Separate structures for each carriageway, “partly cantilevered”. This would cost 160% of the initial estimate, and result in “ugly structures”, all foundations being located where services ran and extra difficulty at the Lionel Road interchange (J2).
  3. “Three pin arches” supporting carriageways “wide apart” with a central gap. The cost increase this time was to 155%, all the foundations would be among underground services, Lionel Road interchange would be extremely difficult and some piers would be sited partly on private land.

The clinching argument, though, was made after this presentation.

“Mr. Lander reminded the meeting that a decrease to dual 24ft. carriageways was inevitable at the Chiswick Flyover, which had been built, and he wondered what was being gained. It was agreed that despite the agitation for dual 36ft., the model should continue to show dual 24 ft. on a central pier.”

Minutes of a meeting held 21 November 1958 between representatives of Sir Alexander Gibb and Partners and the Ministry of Transport6

The Ministry’s engineers were clearly concerned about the idea of building a dual three-lane motorway to the outskirts of London and then arbitrarily narrowing it to two lanes, but struggling to make headway on the issue since the consulting engineers found all sorts of practical objections that the Ministry could not counter without having the same facts at their disposal. They were not even, it seems, able to counter the point about the existing Chiswick Flyover being only two lanes wide by either suggesting widening it or suggesting dropping one of the three lanes at Lionel Road for the considerable volume of traffic that would be heading for the North and South Circulars.

21 November 1958 is, therefore, the day on which the engineers of Sir Alexander Gibb and Partners determined the fate of London’s motorists for all time, fixing in the design of the M4 an artificial and regrettable bottleneck between junctions 3 and 2.

At the same meeting, the transition between the two- and three-lane formation was discussed, since such a thing had never been designed before.

A “gradual widening” was suggested occupying the whole length of motorway between Jersey and Osterley Bridges, and that it should happen on a curve to “preserve appearance”7. This was evidently not done in the final design; the motorway widens as soon as the viaduct ends, on a straight section where the transition is very obvious.

Design speed

Criticism

Construction

123

Operation of elevated section

Operation of elevated road (cameras, signals etc)

Other notable features

Notable features on opening (lighting, motorwarn)

References

These things in this order:

  1. Bressey, C. and Lutyens, E. (1938). Highway Development Survey 1937: Greater London. London: Ministry of Transport. The route can be seen on SABRE Maps. ↩︎
  2. ABERCROMBIE REFERENCE ↩︎
  3. Referenced at, for example, MT 95/309 document 105. ↩︎
  4. MT 95/309. ↩︎
  5. MT 95/309 document 105. ↩︎
  6. MT 95/309. ↩︎
  7. ibid. ↩︎