66 (MBTA bus)

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.

In recognition of their function as part of the backbone MBTA service, the key bus routes have been 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 have 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.

In November 2006, the MBTA launched a concerted effort to improve service quality on key bus routes. The 2008 Service Plan recommended improvements for various lines, including upgrading the 31 bus to Key Route standards. A second round of upgrades, entitled the Key Routes Improvement Project and costing $10 million in all, was supported by the American Recovery and Reinvestment Act of 2009. The Silver Line services were originally considered part of this program, but were later split off into a separate enhancement project. The initial Key Routes Improvement Project was concluded in 2013, with some planned upgrades not being made. Due to these changes, the reliability of service (as measured by adherence to posted service frequencies) has been found to be greater on the Key Routes than on others, with 75% versus 61% respectively in June 2017.

A 2012 statistical analysis found that proximity to key bus routes was inversely correlated to taxicab trip generation, whereas taxicab trip generation actually increased with proximity to bus routes overall, suggesting that only the routes with highest frequency were able to compete with taxi service among customers with the option to choose.

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