Yokel wrote on 06 October 2010
20:19:49 ...
On 04/10/2010 14:38, Roland Perry wrote:

There's a bit of the M25 near South Mimms that "used to be" something
else (A6 probably), before the plonked the M25 on top of it. At least
they gave non-motorway drivers a new alignment alongside (albeit a
B-road): http://goo.gl/maps/PFIN

Yes, that alignment between just south of South Mimms and London Colney
became dual carriageway at some time before 1967, and was the A6. I
suspect it wasn't built to motorway standard and had to be
widened/reconstructed to turn it into the M25 (junctions 22 to 23) in 1986.

It used to be the Rickmansworth by-pass.

That was a different road. It was built to motorway standard and opened
in 1975 as A405 (North Orbital Road). It became M25 in 1985, junctions
17 to 19, inculding the long slip road at J.19 to link with A41.

... - I wondered why it had a right-angled bend with an apparently
unnecessary roundabout and slip roads (now part of one of the M25
junctions).

That's J.17, with a link to A412 at Maple Cross.
--
Richard J.
(to email me, swap 'uk' and 'yon' in address)