bicycle paths, biking in the city is convenient and the CTS operates a cheap bike-sharing scheme named Vélhop. The CTS, and its predecessors, also operated a previous generation of tram system between 1878 and 1960, complemented by trolleybus routes between 1939 and 1962. Being on the Ill and close to the Rhine, Strasbourg has always been an important centre of fluvial navigation, as is attested by archeological findings. In 1682 the Canal de la Bruche was added to the river navigations, initially to provide transport for sandstone from quarries in the Vosges for use in the fortification of the city. That canal has since closed, but the subsequent Canal du Rhône au Rhin, Canal de la Marne au Rhin and Grand Canal d'Alsace are still in use, as is the important activity of the Port autonome de Strasbourg. Water tourism inside the city proper attracts hundreds of thousands of tourists yearly. The tram system that now criss-crosses the historic city centre complements walking and biking in it. The centre has been transformed into a pedestrian priority zone that enables and invites walking and biking by making these active modes of transport comfortable, safe and enjoyable. These attributes are accomplished by applying the principle of "filtered permeability" to the existing irregular network of streets. It means that the network adaptations favour active transport and, selectively, "filter out" the car by reducing the number of streets that run through the centre. While certain streets are discontinuous for cars, they connect to a network of pedestrian and bike paths which permeate the entire centre. In addition, these paths go through public squares and open spaces increasing the enjoyment of the trip. This logic of filtering a mode of transport is fully expressed in a comprehensive model for laying out neighbourhoods and districts – the Fused Grid. At present the A35 autoroute, which parallels the Rhine between Karlsruhe and Basel, and the A4 autoroute, which links Paris with Strasbourg, penetrate close to the centre of the city. The Grand contournement ouest (GCO) project, programmed since 1999, planned to construct a highway connection between the junctions of the A4 and the A35 autoroutes in the north and of the A35 and A352 autoroutes in the south. This routes well to the west of the city in order to divest a significant portion of motorized traffic from the unité urbaine. The GCO project was opposed by