Public transport in Boston is both affordable and convenient, enabling commuters to get pretty much anywhere within the city. The Massachusetts Bay Transportation Authority (MBTA) is responsible for all public transit within the area, and is the fourth largest transit system in the USA. The MBTA operates a grand total of 162 bus routes within the city limits, accounting for a total ridership of 375,000 single trips per day. Walking is also an option, with the city’s abundant scenery and attractive architectural landscape. Known as “America’s Walking City”, Boston was rated the third most walkable US city by Walkscore. Boston’s subway system is comprised of four colour-coded lines, the Red Line, which splits in two directions, the Green Line which splits into branches named B, C, D and E, the Orange Line, which is the oldest rapid transport line in the city, and the Blue Line, which crosses under Boston Harbour.