John Rowland wrote:

Brimstone wrote:

The Hanger Lane underpass long predates the gyratory which sits on
top of it. The underpass is shown on a very early 60s A-Z map of
London.

Was it lengthened when the gyratory was put in? It seems too long to have
been built just to go under a crossroads.

It is as long as it is due to the need for its depth to be greater than
normal because it passes under "the built environment" (ie, it is more
of a tunnel than a cut and cover underpass) and because of the need to
keep the approach gradients as low as possible.