East Cross Route (ECR) is a dual-carriageway road constructed in east London as part of the uncompleted Ringway 1 as part of the London Ringways plan drawn up the 1960s to create a series of high speed roads circling and radiating out from central London. The road was constructed between 1967 and 1973 and runs from Hackney Wick in north-east London, through the Blackwall Tunnel, to Kidbrooke in south-east London. The ECR was initially designated as part of the A102, but has, subsequently, been partially renumbered so that sections of it are now the A2 and A12.
More information on the Wikipedia page [1], with some more information here.