The Massachusetts Bay Transportation Authority operates a large number of bus lines in the greater Boston area. Some routes are for transport within the city; others bring passengers from surrounding areas to stops on the rail lines (commuter or subway) of the MBTA. The MBTA also operates bus rapid transit service; see the Silver Line article for more information. The MBTA has a policy objective to provide transit service within walking distance, defined as 1/4 mile, for all residents living in areas with population densities greater than 5,000 inhabitants per square mile (1,900 /km2). Much of this service is provided by bus.