On 2010\10\06 21:23, Richard J. wrote:
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.

IIRC it was used unchanged in the South Mimms area.