JNugent wrote:

I'm sure the Chiswick Flyover was built in the 1960s (along with
what was then the rest of the M4, as far as Maidenhead). It was
originally going to be designated A4(M) [1] - ie, the motorway was
only going to be a bypass of A4 from Chiswick to Maidenhead), but
it was still a motorway.

[1] According to early 1960s street maps I have seen, with the
motorway shown only as "under construction" and designated A4(M).

That's not quite right. The sequence of the various developments of the
A4 and M4 in West London is as follows:

- Before the 1950s, the A4 ran through Kensington (High Street),
Hammersmith (King Street) and Chiswick High Road, then down the Great
West Road.

- Between 1955 and 1957, the new A4 ("Cromwell Road Extension") was
rather brutally pushed through West London by widening existing roads or
creating new ones, linking to the Great West Road at the Chiswick
Roundabout.

- In 1959, three flyovers were constructed on this route: Hammersmith,
Hogarth (for A316 traffic, "temporary" but still there!), and Chiswick.

- In the early 1960s, the Maidenhead and Slough bypasses were built. I
thought they were always M4, but perhaps one or both were A4(M) for a
time. Certainly from about 1963 onwards, the M4 was planned as a
London-South Wales route.

- In 1964, the Langley (j.5) to Chiswick section of the M4 was opened,
including the elevated section. The western end of the Chiswick Flyover
was reconstructed to link to the elevated section of the M4 instead of
coming down on to the Great West Road.

--
Richard J.
(to e-mail me, swap uk and yon in address)