Easily the most confusing route in the Ringway plans, mainly because of its acute tendency to change its purpose and route every few years. The first section is fairly reliable (though the location of its spur to the Romford area is highly variable), but beyond Noak Hill it begins thrashing about all over the place in a blur of alternative route options. Its three basic destinations were to be Brentwood, Chelmsford or the never-built Maplin Airport near Southend.
The task of nailing the M12 down is made no easier by the terminology used to describe it. While all other parts of Abercrombie's 1940s terminology were studiously replaced by GLC planners - and any official using them quickly reprimanded - the M12 clung to the archaic title 'Radial Route 7' well into the 1970s, despite having long since been allocated the number M12. Further complications arise from the fact that it shares the 'Radial Route 7' title with the M11 from Hackney Wick to Woodford.
The M12 would have begun on the M11 and M15 Ringway 2 interchange at Woodford, where the M11 currently terminates. The junction layout here is designed with vast areas of empty space (now mostly opened up as parkland) to accommodate the M12 and its sliproads, and in several places part of the M11 is carried on bridges or viaducts with nothing underneath them. The mainline of the M12 would have continued as the mainline through the junction and into London as the M11, while the M11 mainline from the north would effectively terminate here.
The motorway would run east through the obvious corridor of open land, south of Hainault then north of Romford. It would turn to head north-east, crossing the A1112 near Dog Kennel Hill, then gradually arc around to the north of Havering-atte-Bower. Somewhere near here, a service area was proposed, in line with policy to provide services about 12 miles from Ringway 1. Havering services would have been equivalent to Scratchwood on the M1, Heston on the M4 and the prepared site on the M11 for Chigwell services.
Beyond here, the motorway would pass north of Home Farm before turning gradually south-eastward, throwing off a short spur at Noak Hill to provide access to and from London for the Harold Hill area. It would briefly run parallel to Ringway 3 (now the M25) for a short distance, with sliproads to an expanded junction where the M25 currently meets the A12. It would then turn east, flowing straight into the Brentwood Bypass.
In the earliest Ringway plans, the M11 was to take a different route down the Lea Valley and it was the M12 that would continue from Woodford to Hackney Wick and Angel. In about 1967 or 1968, the Ministry of Transport won the argument over the M11's line, and it took over the job of reaching Angel, leaving the M12 starting at Woodford instead.
Some early plans had a lengthy spur for traffic to and from London splitting from the mainline M12 north of Havering-atte-Bower, which would have travelled south-east to fly over the Gallows Corner roundabout and flow into the A127, thus allowing the M12 to carry A12 and A127 traffic around Romford and Ilford. The flyover that currently stands at Gallows Corner was intended as a "temporary" fix until this spur was opened. When Ringway 3 was added to the plans, it was decided that A127 traffic could use a section of that to make the connection with the M12, and the Gallows Corner spur was dropped. The Noak Hill Road spur was added in its place, in order to maintain local access for the urban area east of Gallows Corner.
In the early 1970s, the M12 was revised to form the motorway to Maplin Airport - dual four-lane instead of dual-three, and with a parallel four-track high speed railway line. Outside Ringway 3, scores of options were proposed. The basic idea was that the M12 would have continued east to relieve the A127 Southend Arterial Road, terminating at London Maplin Airport - the proposed third London airport which was to be built offshore at Maplin Sands. Documents can be found detailing any number of possibilities, including some spectacular interchange layouts (like the one to the right, a possibility for M12/Ringway 3 at North Ockendon - click it to see a larger version). The route was never finalised before Maplin was dropped.
When Maplin died off (mostly due to its astronomical cost), Stansted was selected as the site of London's new international airport and the focus shifted onto the M11 instead. Beyond Ringway 3, the M12 was rerouted. The strongest evidence so far suggests it reverted to the 1969 layout where it would have run directly into the Brentwood Bypass. By the late 1980s this had evolved into a link all the way to Chelmsford (probably missing out the section between Woodford and the M25), but in the end of course, other than a couple of stubs from the M11 near Woodford, not an inch of the motorway was ever built.