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The use of tree-lined avenues requires some extra attention. Tree-lined avenues in which the tree crowns merge above the road inhibit the mixture of polluted air with the cleaner air above the city. This causes a negative effect on air quality at street level. An often-used rule of thumb is that tree crowns should not merge overhead when fully grown and that they should not have an overhang covering more than one-third of the street. Trees are generally placed 15 metres apart and preferably not directly lining the road, so that they won’t cover the road as much.