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The Chiswick Flyover carries the M4 Motorway over Chiswick Roundabout, and forms the eastern part of the M4 elevated section between Chiswick and Boston Manor. It was originally opened on 30 September 1959 as part of the A4 but was planned as part of the London to South Wales motorway. It was closed for a period while the western end was extended to form the M4 Motorway which opened in 1965. It carries dual two-lane carriageways with no hard-shoulder and is subject to a 40mph speed limit.
The Flyover Details
It connected the Cromwell Road extension with the Great West Road and was almost half a mile long. Beneath it, a 400 foot diameter roundabout replaced the former crossroads with the North and South Circular Roads which had traffic of 40,000 vehicles per day and was a notorious bottleneck.
Work had began at the start of February 1957. It was the first major two-level highway project to be undertaken in the Metropolitan area since the Second World War.
Each of the flyover's 4 spans was 125 feet long with a width between parapets of 59 feet. At the Cromwell Road extension side there was a further bridge over Wellesley Road which was 124 feet between parapets. Cost of the scheme was £820,000.
The Later Westwards Viaduct
The 11 mile section of M4 from J1 to J5, which incorporated the new Viaduct and Chiswick Flyover, was opened on 24 March 1965 by Tom Fraser, Minister of Transport. The dual two lane viaduct of almost 2 miles from Chiswick to Boston Manor was the longest viaduct in Europe. It also had the longest stretch of heated road in Britain and 200 miles of cables were used in the heating panels as well as the many miles of cables for the distribution and control systems, to ensure constant traffic flow in all weathers.
From the point where it left the Great West Road the street section of the viaduct, above the Great West Road, was of steel construction, comprising a main span of 365 feet long, two anchor spans of 211 feet 6 inches, fourteen other spans of between 153 feet and 183 feet, and four approach spans of 60 feet. The total length of this street section was 3,580 feet. It was nearly 20 feet at its highest point, had an overall width of 60 feet, comprising two 24 feet wide carriageways, a 4 foot wide central reservation, and 4 foot wide verges.
For the first time it was necessary to impose a speed limit on a motorway due to the winding section between Chiswick and Boston Manor Road - 40mph for just over a mile west of Chiswick flyover then 50mph for the next 1.5 miles where the road became three lanes.
Television cameras were mounted at several points so that the traffic controller could monitor the traffic flow and determine reasons for hold-ups. Special signs were used to indicate if the viaduct was closed.