Ringway 2
In the late 1960s the Greater London Council developed the London Ringways Plan to construct a series of circular and radial motorways throughout London to ease traffic congestion in the central area. Under this plan the North Circular Road was designated as the northern section of Ringway 2 and was scheduled to be upgraded to a motorway designated as the M15. Most of the Ringway Plans were cancelled in the early 1970s and the North Circular Road was never upgraded to full motorway standard, although much of it is now grade separated dual carriageway.
The section of the North Circular south of M11 junction 4 (the "South Woodford to Barking Relief Road") was constructed on the alignment planned for the unrealised motorway plans and the section between the M11 and A12 junctions was originally opened as part of the M11 with the interchange at Redbridge designated as M11 junction 3. The intention was to rename this section as M15 once the A406 upgrade to motorway standard had proceeded further but this never happened and the road was eventually downgraded from motorway to A road keeping the existing A406 designation.
Read more about this topic: North Circular Road