Key bus routes of the Massachusetts Bay Transportation Authority (MBTA) system are 15 routes that have high ridership and higher frequency standards than other bus lines, according to the 2004 MBTA Service Policy. Together, they account for roughly 40% of the MBTA's total bus ridership. These key bus routes ensure basic geographic coverage with frequent service in the densest areas of Boston, and connect to other MBTA services to give access to other areas throughout the region.