Complex sentence: The structures were popular in the late nineteenth century among smaller cities across the United States and Europe, when standard street-lighting systems--using smaller, shorter, and more numerous lamps--were impractically expensive.
Split into simpler sentences: The structures were popular in the late nineteenth century in cities across the United States and Europe. In some places it was used when standard street-lighting systems--using smaller, shorter, and more numerous lamps--were impractically expensive.