Perhaps I worded it incorrectly. What I meant was more "density needed to be cost-effective", not "density needed to support".
Regardless, what I said before still applies:
Now, this is an old analysis with what I could only describe as an opaque methodology and suspect rigor, but it has some interesting numbers.
https://accessmagazine.org/spring-2012/transit-d-word/
It concludes that a density of 30-40k per square mile (50 people per gross acre) is necessary if cost per mile of heavy rail is $250M:
View attachment 75062
The way I read it, the study's focus is on OpEx penciling out, not the wider public good that transit brings, so that is VERY high estimate for density if cost per mile really was $250M. Unfortunately, cost per mile is in the billions, so even if you consider all of the extraneous benefits heavy rail transit brings, it might not be "worth it" these days if density is too much lower than that.
All that to say, there is obviously a density cutoff where heavy rail transit is no longer a worthwhile investment. Maybe it’s 10k, maybe its 50k, it depends on the cost. But in our lifetimes, Lexington center will never fall into the cost-effective bucket unless it’s a lucky infill on the way to a more important destination.
Opaque methodology, indeed. I don’t disagree with the premise of there being a density cutoff. 30,000 is demonstrably too high of a cutoff though. You’ve also shifted from “has the density to support” to “is cost effective to build” which are quite different concepts.
Your new (and improved) stance that there exists a density cutoff for which even an expensive build HRT becomes justified is one I agree with.
EDITED TO ADD: It's also important to note that population density is a sufficient, but not necessary justification for transit service. Take a look at population densities by census tract per station, posted above. You'll notice South Station has a suburban level of population density. Yet, Downtown Boston is not suburban and South Station is a very highly used station. That's because ridership isn't driven by walk-up ridership from local residents, but rather from the employment density surrounding the station and transfers to/from Commuter Rail and Amtrak, for which South Station is Boston's hub.
It isn't just population density + employment density either. Take a look at Forest Hills. Forest Hills has had higher ridership than any other Orange Line Station south of Back Bay for decades. Even long before recent residential developments surrounding the station. This is due to the fact that it is a major bus transfer hub and that it has a larger catchment area than other Southwest Corridor stations. As the terminus of the Orange Line, people tend to travel further, especially those who live south and southwest of the station, to access the Orange Line.
It isn't just population density + employment density + transfer nodes either. Take a look at Route 128 Station. It has higher ridership than the overwelming majority of Commuter Rail stations in the system, including every station (except North Station) on the Lowell, Haverhill, and Fitchburg Lines, even though many have higher population densities (Porter, Waltham, Brandeis/Roberts, Fitchburg, West Medford, Lowell, Wyoming Hill, Melrose/Cedar Park, Melrose Highlands, Lawrence, Bradford, and Haverhill, to name several obvious ones). This is due to the facts that Route 128 has more frequent service, faster service, better connections to employment hubs, and is sited right next to Route 128 with a major park and ride, serving suburban drivers coming from a very large catchment area.
It isn't just population density + employment density + transfer nodes + frequency, either. Route 128 Station has much higher ridership than any station on the Fairmount Line, even though they all have higher population densities, at least as high of employment densities, at least as good of transfer nodes, and more frequent service with service directly to South Station. That's due to the massive catchment area of Route 128 owing to its location catering to suburban park and ride drivers coming from a wide region.
All of that is to say, population density is a surface level assessment that can be a sufficient, but not necessary justification for transit service. That is nuance that was missing from your determination that transit wasn't justified based on one metric alone, derived through opaque means. Looking at these locations a bit more holistically:
Arlington has a mostly dense-suburban (9,171.6/sq mi) population density, with urban levels of density (>10k) east of Mass Ave and dense-suburban (6k-10k) west of Mass Ave. In my estimation, Arlington could support three or four HRT stations with the ridership to justify service, cost aside, on population density alone. The stop spacing wouldn't be as dense as more urban neighborhoods. ~0.7-1 mile per station over three or four stations would give a comparable catchment per station as the ~0.5 mile per station stop spacing on the Orange Line in Jamaica Plain, for example. Jamaica Plain has a population of 41,000 compared to Arlington's 46,000 population, spread over a somwhat larger (5 sq miles in Arlington compared to 4.4 sq miles in JP), less dense area. Jamaica Plain has certainly shown to be a very viable location for transit, and Arlington wouldn't be far behind.
Lexington is far more debateable on whether it would generate the ridership to justify service, cost aside. With a population of 34,000, there are plenty of residents in Lexington, but the density is low enough that many more would agree it may not justify rapid transit service by itself. It is also lacking employment density or a major transfer node. With a population density of 2,100/sq mi, Lexington is a mid-density suburb without any particularly dense districts. With stations inherently further from the average resident (Lexington is more than three times the area of Arlington), ridership would have to be generated from people traveling further over a larger catchment area. That is going to capture a diminishing proportion of the populace as the distance to the station increases, absent a stellar park and ride opportunity (like Route 128 Station). So, in my estimation, Lexington's most viable rapid transit service models are:
- A single station in Lexington Center.
- This location would be the lowest population density of all HRT/LRT stations on the T, so it would perform poorly in walk-up ridership.
- Employment density is medium, at best.
- Vehicular access is medium, at best.
- Cycling access is great.
- Catchment area would be wide.
- Three mile stop spacing from Arlington Heights would be the second longest in the system.
- Ridership high enough to justify service would depend on people driving, biking, etc from throughout Lexington, Burlington, and Bedford. The Minuteman is well-suited for such a station, but Lexington Center's roads may not be.
- Debatable to justify due to the factors above.
- A single station at Route 128.
- This location would be the lowest population density of all HRT/LRT stations on the T, so it would perform poorly in walk-up ridership.
- Employment density is low.
- Vehicular access is great.
- Cycling access is great.
- Catchment area would be extremely wide.
- Five mile stop spacing from Arlington Heights would be by far the longest in the system.
- Ridership high enough to justify service would depend on people driving from throughout the area (Lexington, Burlington, Bedford, Lincoln, Worburn, Carlisle, etc) for frequent service to Cambridge.
- Debatable to justify due to the factors above.
- Both.
- This would split ridership. Lexington's Route 128 station would serve drivers from Burlington, Bedford, Lincoln, Worburn, Carlisle, etc looking for frequent service to Cambridge. Lexington Center would serve walk and bike-up riders from within Lexington.
- Hard to see there being enough ridership to justify both stations once ridership is split like that.
- Definitely viable (cost aside) if Lexington urbanized their town center added 15,000+ residents within a mile of their town center.