Motor traffic in the city grew significantly and in 1972 the trams were replaced by buses. From the 1960s, on the initiative of the young architect Jan Gehl, pedestrian streets and cycle tracks were created in the city centre. Activity in the port of Copenhagen declined with the closure of the Holmen Naval Base. Copenhagen Airport underwent considerable expansion, becoming a hub for the Nordic countries. In the 1990s, large-scale housing developments were realized in the harbour area and in the west of Amager. The national library's Black Diamond building on the waterfront was completed in 1999.