Thanks for posting these!
Sorry if I missed it, but what/when is your data source? You questioned the inclusion of the 71 and 73 as key bus routes, but it’s important to recognize that if your data is from this decade, things are different from when the key bus routes were designated. As a transit data nerd (and former regular 73 rider who went remote during COVID), I’d bet good money that ridership on the 73 is proportionally lower relative to when key bus routes were designated than bus routes that serve a lower income area, like the 28, 15, or 22, for example.
There have also been some minor changes in the bus map and perspective of the map. For example, in the years since, there has been a move towards assessing corridors rather than routes. Back then, the 70/70A were two different bus routes and their individually lower ridership prevented them from being considered key bus routes. Now that the MBTA has woken up to the reality that corridors are a more appropriate thing to assess, as well some minor changes such as the 70 being its own route, the key bus routes would not be designated the same now. In fact, you can see that bear out in the trunk routes of the bus network redesign. I’m happy to see the T71 and T73 remain as trunk routes, but also happy to see that designation extended to corridors like the T70 and the T96.
Thanks for posting. I love reading your stuff as always!