It has seven tunnels through the El Negrón mountain range separating Asturias from Leon. They were initially built for vehicles travelling in both directions, but have now been adapted for one-way traffic. The latest to be inaugurated was the Negrón Passage, whose second tunnel was opened to traffic in June 1997, running virtually parallel to the original one.
The viaducts are equally impressive pieces of civil engineering. The Carlos Fernández Casado suspension bridge, which spans the Barrios de Luna reservoir, is outstanding both for its architectural beauty and the innovative technical solutions employed in building it.