While reading an excellent article by Yonah Freemark, Why should Chicago focus growth near transit?, I thought the Twin Cities should do the same thing. Taking advantage of existing capacity is far more cost effective than building new capacity (and yes, this applies to all modes). But what is the existing capacity of the Green Line? Well, that depends on assumptions and human behavior. In the table below I work through some scenarios based on assumptions.
First, how many hours per day is the Green Line operating? Second, what is the frequency within that time period? Third, how many cars per train are there? Fourth, what is the capacity per car (they are rated at 230, but this includes standees)? Fifth, how long is the line? Sixth, how long (how many stations) is the average trip? Seventh, how many directions are you considering?
This measures capacity in terms of daily boardings. Daily miles traveled is another measure, and is independent of the length of trips.
To calculate this we use the following equation:
Capacity = (Hours of Operation)*(Trains/Hour)*(Cars/Train)*(Capacity/Car)*(Stations – 1) * (Trip Length) * (Directions Operating).
At any rate, the attached table shows some surprisingly high numbers, up to 7 million (under the admittedly silly unconstrained scenario (A) where people only ride the train for 1 stop before alighting, trains run for 24 hours a day, and people are standing at near crush capacity), with more plausible numbers in the 255k territory, assuming everyone gets a seat, but you can run at 5 minute headways (C). Here we are limited by capacity in one section (downtown Minneapolis), which does run at 5 minute headways, but splits the capacity between the Green and Blue lines.
The main point is that there is a lot of capacity on the Green Line yet to go, even if you only run 18 hours a day, and you expect everyone to have a seat, and run at today’s 10 minute headways (which is all today’s fleet can support, to increase headway we either need to increase speed greatly or add vehicles), and assume the average trip is 7 stations (Aaron Isaacs informs me it is 3.5 miles, which at 1/2 mile spacing is about 7 stations) (83,314 – scenario D). At the other end of the spectrum, if everyone expected a seat and was riding from Union Depot to Target Field, the capacity would only be 32,400 with today’s frequencies.
Thus, east–west transportation capacity is not the constraint in development along the Green Line corridor. (One could similarly demonstrate the under-utilization in the north-south direction on buses, and in all directions on roads).
Certainly load balancing is an issue, much of the capacity is “off-peak”, but that is what pricing is for. Higher loads would increase wear and tear on the cars, and add costs, but hopefully the added revenues would more than compensate.
Compare with current ridership of about 37,835/day (Sept. 2014).
Given there is also a lot of developable land in this corridor, why are new corridors being subsidized for development? [I do actually know the answer to this, it was a rhetorical question].
Capacity of the Green Line
|Fully Unconstrained||Fully Constrained||Needlessly Constrained|
|Trains per Hour||12||12||6||6||6|
|Runs per Day||288||288||144||108||108|
|Cars per Train||3||3||3||3||3|
|Capacity per Car||230||115||115||50||50|
|Capacity per Train||690||345||345||150||150|
|Stations - 1||18||18||18||18||18|
|Average Trip Length (in Stations)||1||7||7||7||18|
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