One of the biggest problems of the current key bus+rapid transit network is that the northern half of the Orange Line has NO high frequency bus routes at all. Most of the buses serving Charlestown, Sullivan, Wellington, or Malden all only run hourly service with half hour service rush hours.
In Allston Brighton, the 86 runs 30 - 40 minute headways off peak, and it does not serve Oak Square or Packards Corner (headways on the 64 is even worse). While the 86 does eat some portion of the 57's route that would otherwise need to travel via downtown, that still leaves a decent portion of the 57 where the only viable alternative is to travel via downtown. The Boston Landing station sees 2 hour headways weekends with only hourly service weekdays. The 501 runs on a one way loop and does not run outside rush hour. Once rush hour ends, if one is nowhere near the 86 or the Boston Landing CR, or the CR or 64 timetables are too inconvenient, oftentimes, the only option is the 57 that runs every 10 - 12 minutes rush hour and every 15 - 20 minutes off peak as of winter 2024, and
trips are getting slower.
The Blue, Orange, and Red Lines in the downtown core all run significantly more service at higher frequencies than any of the Green Line branches, running 169, 133, and 132 trips. The Green Line, on the other hand has been
steadily running fewer and fewer trips per day. If fewer Green line trips are running, this means the headways for the D will only worsen. The Green Line has averaged 351 trips/day for the winter 2024 schedule. Assuming they are all evenly distributed for all branches, that would mean 88 trips/day for the D, which is only 2/3rds (66%) the service of other direct heavy rail lines.
The Green Line trunk is also notorious for its unreliability and congestion downtown.
50% of Green Line trips from North Station to Kenmore take longer than 20 minutes or less than 16 minutes. A similar trip on the Orange Line from
Sullivan Square to Back Bay sees 50% of trips within a minute of each other, which is 4x more consistancy and reliability in travel times. Adding a transfer penalty from North Station to Kenmore only compounds the unreliability problems even more than the existing unreliability of a D branch run from North Station to Kenmore.
One of the biggest problems with the MBTA is
a lack of clockface pulse transfers for low frequency bus routes. This means that timetables for buses are all coordinated so that buses arrive at a station, meet a train, and then depart on a 20, 30, or 60 minute clockface schedule. A subway (or bus) to bus transfer on the northside Orange Line is often painful with low frequency buses in isolated hostile industrial areas, as these buses do not wait for connecting Orange Line trains. Oftentimes it is impractical to make a connection to the 86 from a connecting bus if traffic congestion in Charlestown, Everett, or Allston-Brighton, risks missing the 86 and having to wait 30 - 40 minutes for the next bus.
For example, in Charlestown, the Saturday schedules for the 93 is every 26 minutes, the 89 is every 25 minutes, and the 86 is every 28-29 minutes. Yet the schedule is set up so that the 93 arrives at Sullivan a few minutes after the 86 and the 89 depart, meaning there is a 24 minute transfer at Sullivan to connect to a bus to continue westwards. The transfer penalty is too great so the only way to get to Somerville or Allston Brighton is to travel downtown to either the North Station or Haymarket terminals and transfer to the Green Line there.
(GL running fewer and fewer trips, source: TransitMatters data dashboard)
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