The main road south out of London - or at least, it would have been. It was projected to run south from Streatham towards Crawley, the result of decades of consultation and planning. A proposal for a London to Brighton motorway had existed since the very early part of the century, so the M23 was already a grand old age by the time the Ringways came along. The section outside London was built, but it terminates just inside the M25, or about a mile within Ringway 4. The rest was never started.
R2 Southern Section (Streatham Vale Interchange)
A236 , A237 & A239 (Mitcham Junction)
A232 Croydon Road (and possibly R3 Southern Section)
Possible junction for R3 Southern Section
A2022 Purley & Woodmansterne
A23 London Road (Hooley Interchange)
R4 Southern Section
Continues to Gatwick Airport & Crawley
The final version of the M23 proposal, from 1967 onwards, saw it starting on the Ringway 2 Southern Section at Streatham Vale. It was to run on the east side of the railway between Streatham and Mitcham Junction. On approaching the latter, an interchange was proposed with the A236, involving an incredibly large roundabout straddling the railway line and a short realignment of the A236 onto a high embankment (shown right; click to enlarge).
From there, it would turn south-east and then south to traverse Mitcham Common, reaching its next junction at the south-east corner of the common on the A232. This would probably have been the main junction for access to Croydon. One possible line for Ringway 3 passed through this interchange too.
South of the A232 Croydon Road, there is no obvious line through open land or alongside a railway, so the motorway was to take a new path through suburban housing. It went south to Mellows Park, then along the south side of Sandy Lane to reach the A237. Another junction would be provided with the A2022, though immediately north of here may have been an additional junction had the more southerly route been chosen for Ringway 3's Southern Section.
Curving gently to avoid built-up areas, the M23 would thread its way through Chipstead, rising over the valley there on a tall viaduct. A service area was also to be provided somewhere in this area. A free-flowing junction with the A23 at Hooley (most of which was actually built) is where the proposed line joins the existing M23, from where it ran a little further south-east to connect to Ringway 4 Southern Section (now the M25) at Merstham.
It seems strange that the M23 - planned as the southern equivalent to the M1, and at one time under consideration to take the number "M2" - should stop abruptly on Ringway 2 with no obvious northward connection to central London. In fact, the motorway had been repeatedly pruned back since the first firm proposals were made in the 1950s. The original plan was linked in to the Ringway 1 proposal which included the Balham Loop - a short circuit of motorway enclosing Clapham and Balham that would have connected the M23 and SCRPDR to the South Cross Route. The M23 would have terminated at the northern side of Tooting Bec Common in this proposal.
Various other ideas included the A214 Tooting Bec Road, the A23 Streatham High Road and a vague location somewhere in West Norwood (probably on the SCRPDR itself, which was to pass through there). The ultimate revision was to remove this northerly section altogether, avoiding the difficulty of driving it through densely packed residential districts and scarce parkland, and instead have the motorway stop dead on Ringway 2 at Streatham Vale (shown left; click to enlarge). The M23 was a key component of the Ringway plans, viewed by the Ministry of Transport (who were responsible for the scheme right up to its northern terminus) as being of similar importance to the M1, and yet it was the motorway that would have terminated the furthest from central London.
Traffic heading in to London would either make a short hop on Ringway 2 and join the A23 or A24 inbound, or (if they had any sense) would probably have travelled west and used the motorway link via Wandsworth to reach Ringway 1 at Clapham Junction. The alternative, travelling east and heading in to London on the SCRPDR, would have been even less direct.
The struggle to build the M23 and the reason it never happened (principally that Ringway 2's Southern Section was cancelled) is a long story. However, it is interesting to note that while the Department of Transport did not think it feasible to construct after 1980, it was not formally cancelled until 1995, when the land reserved for the scheme was finally sold off.