Living in cities is becoming increasingly attractive for many people around the world. According to the United Nations, more than 3.8 billion or 53.6 % of the world's population were living in urban agglomerations in 2014. Especially from an ecological point of view, cities are a central issue for the future. Cities consume enormous amounts of energy, raw materials, and space, additionally producing tons of waste and hazardous materials, while many places suffer from congestion, traffic jams, crime, etc.
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