Not quite an answer to the question, more of a comment on the way junctions in the UK are numbered, leading to the addition of a suffix 'a' or 'b' etc to the junction number when a new one is inserted. In New Jersey, USA and on some motorways in Spain the junctions are numbered according to the distance from the origin of the motorway; thus, for example junction 256 is 256 kilometers from Madrid. Adding a new junction poses no numbering problems at all and it serves as a quick indication of distance.