I challenged my 9yo with this, not through a line of code or clever tweaking of a website but by graphically tracing the network - it didn't take so long. The brief was not to create a perfect geographical or schematic rendition (especially within large cities) but to do their best in a limited time, so that when reduced in size it would give a reasonable idea of the network of motorways and connected dual carriageways. Isolated sections of dual carriageway are not highlighted.
motorways are in blue