The Massachusetts Bay Transportation Authority operates 178 bus routes (list of routes) in the Greater Boston area, many of which are were formerly part of a large streetcar system. Some routes are for local transport within the city; others bring passengers from surrounding areas to stops on the MBTA Commuter Rail or subway lines. 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 service. Much of this service is provided by bus.