Sound Transit has stated in a few presentations that, despite the cuts, under Somer’s proposal Sound Transit will get to “92% of the estimated boardings from ST3” (see also here). This seemed strikingly high given what was being indefinitely delayed, so I dove into the ridership numbers they have released (see sources at the bottom of the post).
To estimate system boardings, it is first important to note the 2nd Downtown Seattle Tunnel (DSTT2) duplicates existing service between SODO and Westlake. While that stretch has a high number of boardings, the vast majority of it will be one of two types:
- Riders who are currently served by the current system but now use the rerouted 1-Line. Many of these will be transfers.
- Riders who’s trip starts or ends at one of the other stations added to the line outside the downtown core, such that they will be counted in those stations’ boardings.
Either way, the numbers for DSTT2 do not represent uncounted additional riders. Even though it is an essential part of the overall system, its duplicative nature means it is not generating new trip pairs.
With that in mind lets consider the extensions which do produce new trip pairs (note I have included Redmond, as it is part of ST3):
| Segment | Status | Daily Boardings |
|---|---|---|
| Denny – Seattle Center | Funded | 37,100 |
| West Alderwood – Everett | Funded | 27,100 |
| Smith Cove – Ballard | Indefinitely Delayed | 19,900 |
| S Federal Way – Tacoma D. | Funded | 16,400 |
| Delridge – Alaska Junction | Funded | 13,500 |
| Kirkland – Issaquah | Funded | 7,500 |
| Graham Street | Indefinitely Delayed | 4,100 |
| Boeing Access Road | Indefinitely Delayed | 2,100 |
| SE Redmond – DT Redmond | Funded (Built) | 5,900 |
| T TCC T Line Extension | Funded | 3,400 |
Using these numbers, the percentage of daily boardings funded in the plan is 81% !
This is far below the 92% figure given, and I’m being quite conservative. I have not tried to include the lost boardings from other canceled projects (Sounder South Extension, Sounder frequency increase, multiple park and rides).
Nor have I attempted to account for double counting where a trip pair begins and ends at a one of the new ST3 stations, which will almost certainly cause the percentage to decrease further. This is because the only type of double-counted trips who’s correction would increase the funded percentage are those which start and end at canceled stations, and the number journeys between pairs in that set would be relatively quite small.
The take away is that 81% is something of an upper bound based on these numbers.
So, how did they get that number? Let’s add back in the boardings in the duplicative DSTT2 that I left out:
| Segment | Daily Boardings |
|---|---|
| CID-Westlake (new DSTT2) | 123,600 |
| SODO (new) | 14,600 |
With these included the percentage of ST3 boardings that are funded increases to 90.5%. Still bellow the 92% given, but close enough that it is likely explained by more recent projections that they are using.
What this suggests is that they are not attempting to present the impact of the cuts on the number of daily trips added by ST3, but have instead given a number which includes both trips served by the current system and double counts many of the new trips. I’m not positive this is what they did, but nonetheless something is definitely fishy.
TLDR: Sound Transit appears to be using a flawed methodology which significantly underrepresents the impact of the ST3 cuts on additional ridership.
Sources:
Ballad Link Extension, West Seattle Link Extension, Everett Link Extension, Tacoma Dome Link Extension, Kirkland-Issaquah, Redmond, TCC T Line, Graham Street & Boeing Access Road. Note that for Kirkland-Issaquah and TCC I was only able to find ridership figures, so I halved those to convert to boardings (either way they are small enough that they don’t impact the conclusions).
submitted by /u/recurrenTopology
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