Public bus service in Beijing is the among the most extensive, widely used and affordable form of public transportation in urban and suburban districts of the city. In 2015, the entire network consisted of 876 routes with a fleet of 24,347 buses and trolleybuses carried 3.98 billion passengers annually. Trolleybuses run on over 31 routes including 6, 38, 42, 65, 101-112, 114-118, 124, 128, 301, BRT 1-3. Many of these trolleybus routes are located inside the Third Ring Road but some, such as 301 and BRT 1-3, do extend as far out as the Fifth Ring Road. Since 2013, In an effort to reduce urban air pollution, Beijing has been converting regular bus routes to trolleybus routes by installing overhead power lines on several corridors. Public bus service in the city began in 1921. Today there are