In March 2006, after 15 years of debate, the government authorised the construction of a extension of the M6 from its then northern terminus near Carlisle to the Anglo-Scottish border at Gretna (the so-called "Cumberland Gap"), where it links into the existing A74(M). The road opened on 5 December 2008, the 50th anniversary of the M6 Preston By-pass. The project, which was a mixture of new road and upgrade of the existing A74, crosses the West Coast Main Line and had an estimated costs of £174 million. It completed an uninterrupted motorway from just south of Dunblane (via the M9, the recently opened M80 section near Cumbernauld and the M73) in the north to Exeter (via the M5) and to London (via both the M42/M40 and the M1) in the south.