In message , at 23:00:01 on
Wed, 6 Oct 2010, Basil Jet remarked:
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.

My recollection is that the whole stretch of road was rebuilt and
widened.

(By an odd coincidence I also used to regularly drive on the
"Rickmansworth Bypass" section, which wasn't.)
--
Roland Perry