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 transportation 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.