islets west and east of it; Blasieholmen, at the time still an islet, was connected to the mainland by a bridge called Näckenströms bro, and northward to present day Strandvägen by Stora Ladugårdslandsbron, a bridge on poles; and to the west a bridge connected Norrmalm to Kungsholmen over Blekholmen, a now non-existent islet. By the end of the 17th century, population growth resulted in an additional bridge north of Stadsholmen. One of the oldest bridges was located where today Stallbron is found, immediately south of the Riksdag Building. The first stone bridge, Norrbro, was built in front of the Royal Palace under Gustav III. Not until the 20th century, Stockholm was able to surpass the straits and bays surrounding the city. Half of the about 30 bridges in central Stockholm were built 1920–50, most of them during the 1930s. This development was due to increasing traffic loads caused by a fivefold increase of vehicles in the 1920s. At Slussen, passing ships caused stationary rows of trams several hundreds metres long. The situation was solved when a traffic committee in 1930 could present the so-called "clover-leaf solution" of engineer Gösta Lundborg and architect Tage William-Olsson inaugurated in 1935. The modernity of the solution put Stockholm in a state of rapture and impressed even Le Corbusier, who praised the scale of the construction and invited the world to follow the example of Stockholm. Meanwhile, across the Riddarfjärden bay, construction works had started on Västerbron, the large bridge offering a north–south passage west of the historical city centre. Designed by architects David Dahl and Paul Hedqvist and engineered by Ernst Nilsson and Salomon Kasarnowski, Västerbron became the first large bridge designed by this quartet. Tranebergsbron was inaugurated in 1934, with its span of 200 m, for a few years the world's longest span. These large-scale bridges not only tied various parts of the city together, but their mere size changed the cityscape permanently. Considerably smaller but during the decade equally praised was the small Riksbron designed by Ragnar Östberg. National parks and nature reserves The nature reserves in Stockholm involves regulations affecting both landowners and the general public by regulating usage of insecticides and fertilizers, construction of bridges, lighting of fires, camping, anchoring, and keeping unleashed dogs. A plan for the care and development of a nature reserve often means the city as well as private landowners are obliged to guarantee the maintenance