A20, M20 or A20(M)
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This route and the A2/M2/A2(M) are the most complete radials in the Southern area: ironic, really, that they should be so close together and so close to the edge of the area, leaving most of South London severely lacking in road capacity.
The uncertainty over which number to use (which is why this page is titled with all three) is an ongoing problem for this route and also the A2 and A40 corridors. Strangely, it's not actually a case of road numbers not being given - instead, there are too many conflicting numbers floating around.
At the London end, GLC documents submitted to the Greater London Development Plan inquiry describe it as the A20(M), but they only cover the section from Ringway 1 at Kidbrooke Interchange through to the B263 Green Lane at New Eltham. The documents showing the alignment of Ringway 2's Southern Section, which were leaked to the Croydon Advertiser in 1969, refer to that same section as the M20. Outside London, maps exist showing the M20's junction numbers much higher than they actually are, which suggests that numbers were reserved for junctions inside what is now the M25, but actual junction numbers on the ground stop at Swanley on what would have been Ringway 3, suggesting the M20 would terminate there.
In all, it's a confusing jumble, so until anything much is known about the route between Mottingham and Swanley, we will continue to hedge our bets with all three numbers.
Ringway 1 & A2, M2 or A2(M) (Kidbrooke Interchange)
A210 Eltham Road
R2 Southern Section (Mottingham Interchange)
A208 Court Road
Local connections to Sidcup & Swanley
R3 Eastern Section (Swanley Interchange)
A20 London Road
R4 Southern Section
Continues to Maidstone & Folkestone
The route
Kidbrooke Interchange would have been a peculiar motorway crossroads: the south-eastern corner of Ringway 1, and the start point of two radial motorways. The A20(M) (or M20, if you prefer) would have been the southern arm of this junction, flowing directly into the East Cross Route. It would have travelled south, through a deliberate gap between Tudway Road and Meadowside, before crossing Sutcliffe Park. A junction would have been provided on the A210 Eltham Road, serving Mottingham and parts of Blackheath.
Alternative designs for Mottingham, with the preferred free-flow on the right. Click to enlarge
Continuing south, the motorway would join the line of the A20 Sidcup Road, curving alongside it to run on its northern side. This would be the site of Mottingham Interchange, the free-flowing connection to Ringway 2. Continuing on the north side of the existing A20 in a deep cutting, the motorway would travel through the suburban area of New Eltham, reaching its next interchange with the A208 Court Road. The A20 would be immediately adjacent to, and easily accessed from, this junction. The cross-section to the right shows this (click to enlarge). An earlier version of this page, claiming that the motorway would be elevated directly above the A20 along this stretch, is now known to be incorrect.
Beyond this junction, no information is available, but it's reasonable to suggest the motorway would have followed the line of the existing A20 around the Sidcup and Swanley Bypasses to reach the current start of the M20 and the junction with Ringway 3. From there it would travel much as it does today past Wrotham to meet the terminus of Ringway 4's Southern Section, and then continue onward to the coast.
Variations
Out in rural Kent, the line of the M20 between Ringway 3 and Ringway 4 (ie, between present-day M25 and M26) is known to have been altered before the M20 as we know it was built. An alternative routing, joining what is now the M26 west of junction 2a, was the original plan, and the M20 west of Maidstone was built with this idea in mind: it continued west to what is now junction 2a on the M26, with the intention that this would be on the mainline of the M20. When the more northerly line for the motorway was chosen, and the M20 joined east of Wrotham, the section of road between junctions 2a and 3 was narrowed from three to two lanes in each direction as a result.
