The Massachusetts Bay Transportation Authority (MBTA) operates 170 bus routes in the Greater Boston area. The MBTA has a policy objective to provide transit service within walking distance (defined as ) for all residents living in areas with population densities greater than within the MBTA's service district. Much of this service is provided by bus. In , the system had a ridership of , or about per weekday as of . Most MBTA bus routes are local service operated in Boston and its inner suburbs and connect to MBTA subway stations. Fifteen high-ridership local routes are designated as key routes, with higher frequency at all operating hours. The MBTA operates a five-route bus rapid transit service branded as the Silver Line, as well as two limited-stop crosstown routes. Three smaller local networks are based in the nearby cities of Lynn, Waltham, and Quincy. Several express routes operate from suburbs to downtown Boston. The MBTA has an active bus fleet around 1,140 buses with diesel-electric hybrid or compressed natural gas propulsion. Several Silver Line routes use dual-mode buses that operate as trolleybuses in the Waterfront Tunnel and as diesel buses on the surface. Replacement of the full fleet with battery electric buses is planned. The entire bus system is accessible; all vehicles are low-floor buses with fold-out ramps. Most routes are operated directly by the MBTA. Five suburban routes are run by private operators under contract to the MBTA, while several small circulator systems are run by other operators with partial MBTA subsidy. MBTA-operated buses operate from nine garages, one of which is under reconstruction and a second planned for replacement. Several sections of dedicated right-of-way for MBTA buses have been opened in the 21st century, including two off-street busways for the Silver Line and a number of dedicated bus lanes. The modern bus system descends from a network of horsecar and electric streetcar lines built in the 1850s to 1910s, which were consolidated under the West End Street Railway and later Boston Elevated Railway (BERy). The BERy introduced buses in 1922 to replace lightly-used streetcar lines and expand into new areas. Over the next four decades under the BERy and Metropolitan Transit Authority (MTA), all but six streetcar routes were converted to bus or trolleybus. (Most trolleybuses were phased out by the 1960s, but four routes lasted until 2022.) The MBTA took over the MTA in 1964, and several private