Articulated ("Bendy") buses vs. Double-decker (DD) buses
I lived in Singapore for a few years (2012-2016). The vast majority of their extensive bus network features 40' single-decker buses and double-decker buses, with a smaller number of bendies. My stay was also during an era when they were replacing most of the older bendy fleet with newer, more innovative double-deckers
(with a small number of new bendy buses for routes where DDs can't be used). So while I'm not an expert, I feel I may be one of the more knowledgeable people on this board to speak on this topic. (For background knowledge, Singapore's payment system requires boarding at the front door and alighting at the back door(s) where you tap out.)
Capacity: The "standard" 35-ft, dual-axle DDs have lower capacity than bendies. For this reason, all DDs in Singapore are now 40', triple-axle ones, which have similar capacity as 60' bendies.
Dwell times: However, there's still a massive problem with DDs - Boarding and alighting take a lot more time, due to a few factors:
- Two doors on DDs vs. three doors on bendies
- Needing to wait for passengers on the upper deck to go downstairs
- And, one factor you may not even think of: The stairs can only be occupied in one direction. Very often, passengers boarding the bus are stuck just before the stairs, waiting for alighting passengers from the upper deck. In the most extreme cases, this blocks the entire line of boarding passengers.
DDs do worst with two types of ridership patterns:
- When certain stops have high turnover (both boarding and alighting passengers)
- Especially on crosstown routes (Boston's 1, 66, even 28), and routes that through-run two feeder routes at a single transfer station (think Boston's 99, 106 and 108 at Malden Center, or 86 at Harvard)
- When most passengers only take the trip for a few (2-5?) stops, due to resistance to climb stairs for a short trip
- Especially on feeder routes that solve last-mile problems
On the other hand, DDs do much better on routes with the opposite characteristics: When ridership is mostly unidirectional and have long journeys. This is the most applicable to express buses (Boston's 500 series etc), but also to long radial routes (Boston's 32, 34E etc).
Footprint savings: On the other hand, DD's biggest advantage is that they have less footprint than bendies. This is the key reason why Singapore favors DDs, as land is
very scarce on this tiny island. (Another advantage is more comfortable rides for long-distance riders due to more seats.)
Turning radius? I'm not super sure on this topic, but from what I've heard, bendies' minimum turning radius is the same as 40-ft buses (single-deckers and DDs). So they're theoretically not harder to maneuver tight turns, just that you need to be more careful because bendies traverse the junction for a longer period of time.
Singapore's innovations for DDs: To address the aforementioned drawbacks of DD (most notably dwell time) while still reaping the benefits of their footprint, Singapore has recently introduced customized DD units with special designs. As of the time when I left Singapore, the main features were two stairways (a front stairway largely for boarding, and a rear stairway largely for alighting), and three doors (an extra rear door). They might have added even more features since then. However, I don't think these innovations are easy to replicate elsewhere, especially in the US due to Buy America and much lower local demand for DDs.
Transit fans' reactions: During the time I was there, they generally preferred bendies over DDs. Some of it was due to personal preference, or the older bendy fleet being seen as more historic or ironic, or identity issues (at one point, one of the two major bus operators only used DDs, and the other only used bendies). However, some of them have the preference due to DD's drawbacks as mentioned above, and they were especially against putting DDs on the massive number of last-mile feeders. I'm not sure if the perceptions have changed since I left.
How does this apply to Boston?
To be honest, I'm not sure if
that many bus routes here really
need high-capacity buses. For most corridors, frequency boosts and having a more robust bus network (more alternatives) should go a long way. Even for routes that are kind of "maxed out" in frequency due to bunching (1, 66 etc), I think signal and ROW priority should be the next steps in order to boost reliability.
But if we do consider having more high-capacity buses, and wonder what types we need...
TL;DR: There are arguments for both bendies and DDs, or a mix of them.
Here are the bus routes whose riderships are within top 20 systemwide on any of weekdays, Saturdays or Sundays* according to the
2014 Blue Book:
(very outdated, but using it for ease of access)
- Radial, lower turnover*: SL5, 111, 32, 77, 9, SL4, 73, 31, 15, SL2, SL1, 70
- Radial, higher turnover*: 39, 28, 23, 57, 22, 116, 117
- Crosstown: 66, 1, 86
* The weekday rankings uniquely include 9 and SL2, while both Saturday and Sunday uniquely include 70 and 117. Also, the 71 is #21 on all days: it is the Key Bus Route with the lowest ridership on weekends, and only above 117 on weekdays.
** Turnover measurements are subjective, loosely based on the Bus Route Profiles and not thoroughly checked. Some of these are at the boundary of low vs. high.
From the Singapore case study above, it appears that type (1) is better suited for DDs
(aside from vertical clearance constraints) and types (2) and (3) are better suited for bendies. However, DDs for type (2) should also be fine: For most of them, the boardings and alightings are not as perfectly symmetric as (3), and it's possible that they're coming from different directions at different times of the day (so no concurrent boardings and alightings).
Another consideration with high turnover routes (types 2 and 3) is that their total ridership may not translate perfectly to peak load, and the latter is what determines capacity needs, as I explained
here. Especially when considering this comment:
In numbers the 1 and 66 carry 9306 and 10954 daily pax respectively (transitmatters data as of 10/12/23) which equates to 45.6 and 47.2 riders per trip. Of course riders aren’t spread equally across all trips but that’s statistically every trip filling all 39 seats and having a handful of standees.
[...]
The 39 has a daily ridership of 8745 or 37.7 per trip.
Turnover on the 39
(most of which occur where it overlaps the E) is still lower than the 1 and 66. So it's entirely possible that the average load (i.e., number of riders on the bus) on the 1 and 66 is lower than the 39 despite higher ridership, because people who use the 1 and 66 don't traverse the entire route. While that itself doesn't argue against the need for high-capacity buses on the 1 and 66, it may be more crucial for other crosstown routes like 86 with even lower ridership.