the a12 starts just north of the blackwall tunnel at a junction with the a102 and the a13. from here to past ipswich (including the entire section through london) the road is a dual carriageway. north of the junction, the a12 heads northwards as a 2/3 lane dual carriageway mostly at street level. this stretch of road is known as the blackwall tunnel northern approach. this stretch ends at the triple-layer interchange with the a11 at bow road where it becomes the east cross route. this is mainly a 3/4 lane dual carriageway built mainly on flyovers and underpasses and was built in the late 1960s, previously called the a102(m).