MBTA key bus routes

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

In recognition of their function as part of the backbone MBTA service, the key bus routes were added to newer basic route maps installed in subway stations and other public locations. These schematic route maps show the rail rapid transit routes, bus rapid transit routes, commuter rail services, and key bus routes. The key routes had been treated as a distinct category for the purpose of service improvement, such as trial runs of late-night service, and due to the high volume of passenger traffic they carry, both individual routes and the category as a whole have been the subjects of urban planning and transportation engineering studies. As part of the implementation of the MBTA's Bus Network Redesign program in 2024, the key bus route terminology is being phased out and replaced by a general high-frequency route network.