Ice skating, popularized in Central Park, was a key reason for including Prospect Lake in the design of the park's watercourse. In the late nineteenth and early twentieth centuries, red balls raised on the fronts of trolley cars signified that the ice was at least four inches thick. Red flags were also placed at Grand Army Plaza to indicate the ice's sufficient thickness. Later, green flags were used to indicate that the ice was thick enough, and red flags indicated that the ice was too thin. Since then, safety concerns have ended skating on the lake; as a result of climate change, winters have become warmer in the 21st century compared to the 19th century, the ice on the lake has become too thin to accommodate skaters. Ice skating moved to the Kate Wollman Rink in 1960, and again to the Lakeside Center in 2012.