Tokyo, as the center of the Greater Tokyo Area, is Japan's largest domestic and international hub for rail and ground transportation. However, its airspace has been under the US military's exclusive control after World War II. Public transportation within Tokyo is dominated by an extensive network of "clean and efficient" trains and subways run by a variety of operators, with buses, monorails and trams playing a secondary feeder role. There are up to 62 electric train lines and more than 900 train stations in Tokyo. Shibuya Crossing is the "world's busiest pedestrian crossing", with around 3,000 people crossing at a time.