This post is a follow-up on my recent post of the Kendall-Back Bay subway, and is particularly driven by subsequent discussions on archBoston.

When I first wrote about the Kendall-Back Bay proposal, I did not intend to treat it as an “Urban Ring” — a popular idea in Boston, and once an official project, that creates circumferential rail connections around the downtown core. Instead, I thought of the hypothetical route primarily as serving the “extended downtown” of Boston.

Nevertheless, is the Kendall-Back Bay subway (still) capable of being an “atypical Urban Ring”? Let’s take a look.

TL;DR

• An Urban Ring should be time-competitive with existing options — but even time savings of “just” 10 minutes are significant. And even without time savings, you can still free up capacity downtown.
• It’s also worth accounting for downtown subway-to-subway transfer as an alternative. Beating circumferential buses in travel time is much less notable if the buses are even slower than a downtown transfer (in which case you aren’t really “creating new connections”).
• Back Bay is much stronger of an Urban Ring destination than you might expect. Its subway ridership is magnitudes higher than Longwood Medical Area (LMA) and likely less peak-oriented. Back Bay also draws notable demand from Cambridge that isn’t adequately fulfilled otherwise.
• Despite this, a traditional “Grand Junction line” between MIT and Fenway/Boston University will likely still be useful, particularly due to local demands.
• A three-way solution (Harvard-LMA, Grand Junction-BU, Kendall-Back Bay) is likely the most ideal. The Grand Junction route need not even be a circumferential line!

Background: The archBoston Discussion

Following my recent post of the Kendall-Back Bay subway, there have been some great follow-up discussions on archBoston. In particular, it inspired user ritchiew to craft their own proposal of an “Inner Ring” and an “Outer Ring”:

Most of this blog post concerns the follow-up conversations about this proposal. In particular:

High-Level Principles for Urban Ring

In this post, I will complement these ideas with additional thoughts and data:

Time Savings: 10 Minutes Are Significant in the Transit World

A successful Urban Ring should obviously save time for origin-destination pairs compare to existing trips. But how much time savings are enough? Are 10 minutes too little?

This part of TheRatmeister’s comment imply that it’s not “mind-boggling”:

> But for an Inner Loop like this, a trip from Back Bay to Kendall/MIT takes around 12-15 minutes currently. Yes the Inner Ring would decrease that to 5 minutes, but that’s already not a ton of time savings for such a costly project. Once you’re up to Lechmere, the travel time savings to Back Bay go down to around 5 minutes. Going the other way, with Washington St Light Rail to Park St, via downtown that trip would be ~13-15 minutes. The Inner Ring would cut that to ~8 minutes. Not bad, but also not the kind of mind-boggling time savings you’d really want from a project in this cost range. This project, apart from the tails, doesn’t really make new trips possible via rapid transit, it makes existing trips a bit faster and adds downtown capacity.

But in the rapid transit world, it’s worth a closer look. 10 minutes are enough for the following trips:

• Aquarium to Suffolk Downs
• North Station to Malden Center (technically 11 minutes, but you get the point)
• Charles/MGH to Porter
• Tufts Medical Center to Jackson Square
• South Station to Savin Hill

In other words, 10 minutes is all it takes to traverse 70% of any HRT “half-line”. It’s clearly significant in the context of rapid transit travel times, especially for those with driving and Uber as alternatives.

My Google Maps queries generally give 15-18 minutes for Back Bay-DTX-Kendall. (I’m not sure where TheRatmeister’s 12-15 minutes are from.) Assuming that a Kendall-BBY subway travels as fast as two stops on southside OL (similar stop spacing), it would be a 3-min ride, or 6 minutes accounting for transfer time. So I feel comfortable with using 10 minutes as the time savings.

This means that to match the travel times from Malden Center to Back Bay today (20 minutes), a Red Line resident needs to live at Central. Having the Kendall-Back Bay subway allows you to live at Davis, maybe even Alewife. That’s a huge difference — it basically opens up residences on the entire northside Red Line as viable options for Back Bay jobs.

(On a side note, I calculated similar time savings for Back Bay-Lechmere. The Orange-Green trip makes 7 intermediate stops that end up taking 20 minutes, so a 4-stop trip on a slightly shorter route can still save around 10 minutes.)

Likewise, all this can also be said for Forest Hills residents working in Kendall.

Beating Other Routes: Not Just Buses, but Also the Downtown Transfer

An important part of calculating time savings is to set a “baseline” for how long a trip currently takes. But choosing the baseline is trickier than you might think.

Whenever there’s an existing circumferential bus route, it’s tempting to treat that as the target to beat. But I argue that there’s an alternative, often-overlooked option: Connecting via downtown.

• This relates to a claim in TheRatmeister’s comment above is that an “Outer” or “Conventional” Ring “mainly [replaces] bus trips or even [creates] new connections”, in a way that an “Inner” or Kendall-Back Bay “Ring” doesn’t. Because of the omission of downtown transfers, I argue that the claim not true and arguably a semantic trick.

A Forest Hills resident commuting to Kendall can make the following choices once their OL train pulls into Ruggles:

Route	Time
Ruggles-DTX-Kendall	23 min
CT2 direct	33 min on the bus, plus transfer time
LMA Ring (best-case, 3.0 miles) (Either Fenway – Park Dr – BU Bridge _ Grand Junction, or LMA Center – Kenmore – new tunnel to MIT athletic fields – Grand Junction)	12 min (possibly longer depending on ring alignment) (3 min transfer; trip time of 9 min estimated using OL’s Mass Ave to Forest Hills, which takes 10 minutes over 3.3 miles and makes the same number of stops)
Ruggles-BBY-Kendall	9 min (3 min Ruggles-BBY; 3 min transfer; 3 min BBY-Kendall)

The first observation is that with the W-SW quadrant in particular, a bus is often even less competitive than a downtown transfer. I’d say that it’s worth adding a downtown transfer into consideration.

You can also see that both the “inner” and the “conventional” rings have substantial time savings from a downtown transfer — primarily because of too many downtown stops. Stops make a much bigger difference in runtime than distance alone. The “best cases” of both rings are already shorter than OL-RL and have fewer stops; the Kendall-BBY subway benefits further by having a more direct route.

Keep in mind that the 12 min via LMA Ring is the best case. With a longer route (e.g. if you want to serve both LMA Center and BU Central), more stops or tighter curves, it can easily go up.

If anything, I’d even argue that Kendall-BBY may create more new connections (not served by buses) than Kendall-LMA. Even in the real-world Bus Network Redesign alone, the 47, 66 and 85 all make the Cambridge-LMA connection, with the former two being 15-min Frequent Bus Routes. But the only realistic Cambridge-Back Bay bus connection is the 1, and it only touches the fringe of Back Bay.

More Than Speed: Freeing Up Capacity Downtown

This is what I think that any Urban Ring should be capable of doing. Sure, there are some crosstown trips where people opt for buses or alternative means of transport over a tedious transfer downtown. But there are probably more trips where people end up using the downtown stations. Moving them onto an Urban Ring reduces crowding at the “downtown diamond”, and frees up capacity on the trains to be used for more irreplaceable purposes.

A downtown transfer is what I always do from Malden to Central/Kendall. Having watched Red Line trains towards Cambridge fill up at Park St practically all times of the day (well outside of commute hours), I have to think that many of them are Cambridge residents heading back home from places like Copley. Heck, even Arlington has significant transfers between Green Line branches (presumably with the E), however ridiculous that trip looks on a map.

The magic is that such a Ring trip doesn’t even need to have huge cost savings. Even comparable travel times to a downtown transfer (accounting for transfer times) can already attract some riders.

An indirect effect is that even if a “small” ring results in seemingly tedious trips, they can already bring some capacity reliefs:

• With a Kendall-Back Bay route, a Davis – Kendall – BBY – LMA trip is somewhat tedious, but some people will still do it.
• With a Kendall-LMA route, a Davis – Kendall – LMA (or BU) – Back Bay trip is so disadvantageous that I doubt it will see much patronage.

Why Back Bay?

The elephant in the room is, everything I mentioned above — time savings, downtown capacity, etc. — apply equally well to Longwood Medical Area (LMA) as they do to Back Bay. Any individual Urban Ring route faces a choice between them.

So why am I advocating for Back Bay, compared to LMA and others?

Establishing one fact first: Kendall, Back Bay and LMA are job clusters of similar sizes, each hosting ~70,000 jobs, give or take. The 4th in place, as ritchiew noted, is significantly smaller: Harvard, at ~25k.

Ideally, we shouldn’t have to choose. Both Back Bay and LMA deserve better crosstown connections given their prominence, which is why building “concentric rings” makes sense.

But Back Bay has some previously understated advantages:

1. Non-Commutes: Back Bay has a much greater variety of trips

The neighborhood’s appeal goes far beyond traditional commuting trips. Shoppers at Newbury St and Prudential, tourists visiting Trinity Church and Boston Public Library, conference attendees… You name it.

In fact, Copley station has one of the most impressive weekend-to-weekday ridership ratios on the entire system. (Curiously, the same can’t be said for OL’s Back Bay station, though.)

2. Existing Ridership: Back Bay’s is much higher than LMA’s

Ridership on a typical weekday in Fall 2024, both directions combined:

Station	Boardings	Alightings
Copley	9973	9936
Hynes Convention Center	7728	7688
Kenmore	5879	6197
Prudential (E)	2546	2400
Symphony (E)	1422	1580
Northeastern University (E)	724	1356
Museum of Fine Arts (E)	535	623
Longwood Medical Area (E)	631	1066
Brigham Circle (E)	733	1015
Fenway (D)	550	726
Longwood (D)	570	863
Back Bay	9912	11074
Ruggles	7979	6708
Kendall/MIT	9660	10803
Central	9346	9413
Harvard	11537	11290

Somewhat surprisingly, the 3 main Green Line stations serving LMA (the namesake stop, Brigham Circle, and Longwood) “only” have combined 1,934 boardings and 2,944 alightings. Their riderships are so underwhelming that Prudential alone matches the total of these 3 stops, and even the often-overlooked Northeastern University has ridership comparable to each of the 3.

Even Copley station alone has 3-5x the ridership of the three LMA stops. Throwing in Back Bay station would double that multiplier.

To be clear, this is NOT saying that LMA is underwhelming or doesn’t deserve better transit. On the contrary, this shows that LMA is severely underserved given its employment density. Faster, more direct trips to LMA should be able to increase its ridership significantly.

But the bottom line is: The Back Bay neighborhood’s ridership today is already an order of magnitude higher than LMA’s, and even Kendall’s. I feel that bringing people to demonstratably popular destinations (the most popular outside of the downtown core) is just as important as growing ridership to underserved destinations.

3. Existing Patterns: Cambridge already has demand for Back Bay — far beyond service levels

In drafting the Bus Network Redesign, MBTA planners used origin-destination travel demand estimates provided by StreetLight Data, which account for trips on all modes (including walking, biking, driving, etc). This is a really cool dataset: unlike traditional employment-based datasets, it also includes more flexible non-commute trips, such as education, recreation, etc.

Here’s the volume of travel between Cambridge and some top destinations:

Region	Volume
Lower Allston (Harvard Allston Campus and Western Ave) & Boston Landing	11640
Copley-Back Bay Neighborhood (Includes north of Boylston St from Mass Ave to Public Garden, plus the “Prudential region” further southwest from Back Bay station to Christian Science Center)	9863
Beacon Hill (incl. MGH)	9801
Kenmore, BU & Fenway	8856
West End & North End (incl. North Station)	5397
“Central-South Allston” (Union Sq Allston, Packard’s Corner, and southwest Allston around Comm Ave)	4961
Financial District	4684
Government Center to Aquarium	3359
Chinatown & Park Plaza	2630
Symphony & NEU	2346
Longwood Medical Area	2113
Logan Airport	2056

The Back Bay region is one of the top destinations for Cambridge residents — despite the lack of a direct subway connection from Cambridge.

Typically, you would expect residents on a rapid transit line to favor destinations on the same line. MGH being the third highest clearly shows this. (Likewise, I did a quick look at neighborhoods on southside OL, and virtually none of them travel to Cambridge.)

But I wouldn’t have expected the Back Bay neighborhood to draw twice as much demand from Cambridge as the Financial District! You get a fast, reliable one-seat ride to two stations that cover most of FiDi, while none of that can be said to Cambridge-Back Bay trips.

Whether this is due to recreational trips, driving commutes, Back Bay residents commuting to Cambridge, or something else… Who knows. But one way or another, the demand is there, and it’s well worth consideration.

4. Geometry: Back Bay avoids “backtracking” better

I had discussed the issue of “backtracking” above. This has been the reason why I typically favor an Urban Ring to go through Kendall rather than Harvard, if a choice has to be made.

The exact same logic applies to Back Bay vs. areas further west:

• With a Kendall-Back Bay ring, LMA riders need a “C-shaped” trip like Alewife – Kendall – Back Bay – [Boston University or LMA]. This is not ideal, but at least tolerable (and still somewhat better than a downtown transfer).
• With a Kendall-LMA ring, Back Bay riders need a “zigzag” trip like Alewife – Kendall – BU or Kenmore – Back Bay This is so roundabout that I don’t see many people using it.

A caveat is that some “outer” half-ring from Harvard will likely exist at the minimum, so not all Red Line transfers will go through Kendall.

Arguments Against Back Bay

While this post primarily supports and justifies a circumferential route through Back Bay, there are also arguments against the idea. Here’s a brief discussion, though much of this will need more analysis in the future.

1. Cost of a New Underwater Tunnel

This is the most obvious drawback of the entire Kendall-Back Bay proposal. I’ve already discussed its cost concerns at length in the previous post, so I will not repeat them here.

To be fair, the “traditional” Urban Ring alignments are not always cheaper. One proposal of a fully grade-separated tunnel, which was once the official Urban Ring Phase 3, involved a direct water crossing between Kenmore and MIT. This will cost just as much as a Kendall-Back Bay crossing.

But any BU/Fenway/LMA ring has a trick that significantly reduces the cost: reusing the BU Bridge, an existing rail bridge along the Grand Junction.

2. Missing Out on Kendall-MIT-BU Trips

This is a much more subtle and less intuitive point. I only realized this when attempting the following (incomplete) analysis of travel demand between various parts of Cambridge and the area of Fenway, Boston University (BU), Kenmore and LMA, compared to demand between Cambridge and Downtown/Back Bay:

Cambridge neighborhoods* to/from…	Fenway, Kenmore and LMA	Downtown (Shawmut Peninsula), Back Bay, and South End**	% of Fenway-LMA riders among both
Lechmere	662	7311	8.3%
Kendall	738	3967	15.7%
MIT	3144	4561	40.8%
Cambridgeport	1042	1185	46.8%
Area Four (Columbia St – Mass Ave South)	910	1708	34.8%
Central & Inman	1611	3432	31.9%
Harvard	1495	4994	23.0%
Porter & north/west	889	2595	25.5%

* I removed “Charles River” from the calculation for a variety of reasons; most notably, it’s actually on the Boston side.
** Unfortunately, this is the most fine-grained resolution in the data source if I want to break up Cambridge into specific neighborhoods.

At a glance, this suggests that demand from MIT and Cambridgeport skew more towards Fenway, BU and LMA than the rest of Cambridge does. The same applies to Central Square and nearby areas, but NOT further towards Harvard and Kendall.

This is not surprising at all: MIT and Cambridgeport are the parts of Cambridge that are the closest to Fenway, right across the river. But the biggest implication is: It justifies a Grand Junction – Fenway/BU route.

Depending on interpretation, you may even draw the following conclusions, although both are disputable:

• Local residents around the Grand Junction right-of-way may prefer a line to BU over a line to Back Bay.
  • (Caveat: This may not apply to residents further north on the Red Line making a transfer.)
• If you want a line to BU, one from Grand Junction and/or Central Square may be more effective than one from Harvard.

Unfortunately, the public data is very coarse, and doesn’t allow us to distinguish Back Bay’s demand from (the rest of) Downtown Boston. Thus, I wouldn’t necessarily use this to justify a choice between Fenway and Back Bay. But this phenomenon is at least intriguing and warrants further analysis.

Conclusion: A Three-Way Solution?

In summary, a direct link from Kendall to Back Bay has great potential that arguably went under the hood. It adds coverage to Back Bay that was ignored entirely in most Urban Ring proposals; it is especially effective at connecting Cambridge to Back Bay, which is currently more popular than LMA and draws demand from Cambridge despite inconvenient transit access. Even southside Orange Line riders see more benefits from this than the more “traditional” rings.

Kendall-Back Bay alone doesn’t fulfill the entire purpose of the Urban Ring in this quadrant, however. At least one other circumferential link, the “Outer Ring”, would be needed to cover other important nodes: LMA, Fenway/BU, Allston, and Harvard.

But a third alignment also has merit: Grand Junction between MIT/Cambridgeport and Fenway/BU. In addition to a conveniently located rail ROW and cost savings, it’s also more effective than the other two in fulfilling more “local” demands across the river.

This makes you wonder… Why can’t we have all three?

To be clear, this is not perfect, and there are probably better alternatives for three different routes.

But there are some unique, intriguing advantages with how the Grand Junction “Lime Line fits into this framework:

• If you can already afford the other two lines, the Grand Junction Line is an extremely cheap addition. You can simply do street-running LRT for the most part.
• There’s great flexibility in how to extend it on both ends: not just circumferential, but possibly even as a “semi-radial” route (what I call a “tangential line” — although this should be used as a last resort). See the dropdown toggle below.
• Thanks to the Kendall-Back Bay Line, the Grand Junction Line is relieved of many Urban Ring duties that it’s not suited for:
  • From Kendall to Orange Line South, and possibly the Huntington Ave branches (D/E)
  • From Kendall to Green Line Extension (GLX) (no longer mandatory to hit East Somerville and an Union Sq branch infill)
  • Red Line transfer, and missing the “center” of Kendall Square (while an RL infill at Tech Square is possible, omitting it is now less bad)
  • How to cross LMA: Tradeoff between a more direct route and better coverage is reduced
• Because of the previous point, the Grand Junction line can specialize on more “local” demands, primarily between East/Central Cambridge and Fenway/BU. This increases tolerance for more grade crossings, more roundabout routes and/or the use of LRT — all of which Grand Junction is a natural fit for.

Regardless, a natural question is: What if we can only choose two out of the three? While most Urban Ring proposals had favored the “Y-shaped” Harvard and Grand Junction lines, this post suggests that the “parallel” Harvard-LMA and Kendall-Back Bay model is a strong contender and worth consideration.