In message , Richard J.
writes

You can't design a road for "no speed limit whatsoever". Britain's
motorways were generally designed for 70 mph,

There was no speed limit whatsoever for the first six years ... it was
not until 1965, after reports of test drivers reaching nearly 170mph on
the M1, that a 70mph limit was introduced on motorways.

but it's obvious that the
first bend on the M4 going west was designed to a lower standard.

If you mean the Chiswick flyover, it wasn't even part of the motorway
when it was built in 1959 by Tory transport minister Ernest Marples'
construction company, Marples Ridgway - it was just a flyover on the A4.

It didn't become the most easterly part of the M4 until six years later
when, by curious turn of fate, the rest of the elevated section was
completed by chief engineer Sandy Darling, father of Alistair Darling,
and became (with Westway) the symbol of the way in which roads had been
put before homes in the 1960s.

--
Paul Terry