Frequency Where It Matters: Right-Sizing ST3

Tiffany Von Arnim (Flickr)

When Sound Transit decided to split the spine for the ST3 package - sending Everett trains to West Seattle and Tacoma trains to Ballard - it did so for a number of reasons. The unprecedented length of the spine corridor meant it was always infeasible to run trains end to end; and capacity concerns in the current downtown tunnel made adding a third line undesirable, leading Sound Transit to propose a fantastic second subway through downtown.

Now that ST3 has passed, it's time to start making a good thing better. In particular, three operational disappointments lie ahead, all of which are fixable:

First, the still-quite-lengthy lines are planned to offer uniform frequency, meaning trains will cruise through Fife just as often as they do through SeaTac or South Lake Union. Even ardent defenders of the 'light rail spine' would have to admit both the asymmetry of demand between the city and outlying areas, and the need for additional urban frequency.

Second, the length of the lines and the nature of the radial commute mean that during peak hours passenger turnover will be relatively limited, leading to crushloads as early at Northgate or the UDistrict for the Red and Blue lines, leaving thousands of downstream passengers at UW or Capitol Hill with a consistently poor experience.

Third, this poor experience will occur while the brand new subway squanders its potential capacity, as the Ballard-Tacoma line cannot exceed 5 minute headways due to at-grade running on MLK Boulevard. With the same 400"^2 platform length constraints as the other tunnel, this means our brand new subway would be just half as utilized as the current tunnel.

What can be done? Expanding on an idea by Page 2 writer Devon Jenkins, I'd offer two possible solutions: turnback trains and point-to-point service. With regard to turnback trains, Sound Transit is already mulling the possibility of UW to Stadium trains as supplemental service to mitigate the likely 2018 closure of the tunnel to buses, so additional turnback options could potentially be considered under ST3, such as Ballard to SeaTac.

But it's point-to-point service that offers the most dynamic (and operationally complex) option. Cities such as Washington DC, (WMATA), San Francisco (BART) and Denver (RTD), for all their faults, tend to offer greater frequency on core corridors by consolidating in the core and branching on the periphery. The limitations of this approach are clear: without grade separation between lines, the needed crossovers create fragile chokepoints that limit the realistic frequency of each line. Most of San Francisco and Denver's individual lines run every 15-20 minutes because of the limitations of the Transbay Tube or Denver's Central Corridor.

RTD System Map

But in thinking about such a multi-line concept in Seattle, these limitations don't necessarily apply as severely, especially with a second tunnel. A multi-line concept could go a long way toward right-sizing ST3 while retaining frequent service for everyone, matching supply and passenger demand while still providing the full suburban service we voted for.

Consider the hypothetical below. In addition to the planned Red, Green, Blue, and Purple lines, this concept would add 3 more: Orange (Ballard-Redmond), Silver (Ballard-West Seattle), and Black (Lynnwood-SeaTac).

Map by the Author

Consider the benefits:

• Right-sizes frequency: central corridors would be served as often as every 4-6 minutes, while outlying stations would see service every 12 minutes.
• Respects the frequency limits on MLK.
• Fixes the crushloads: with turn back trains from both Lynnwood and Northgate, passengers weary of crushloads could know to wait for a Black or Blue line train.
• Evens out demand in the tunnels: If each line ran every 12 minutes, each tunnel downtown would carry 3 lines at up to 4 minute headways.
• Zero new track-miles: The only additional capital costs of this proposal are switches or possibly flyovers immediately south of International District Station, so that trains from either tunnel could head towards Redmond, Tacoma, or West Seattle.
• Ease of maintenance: Any train could be maintained at any Operations and Maintenance Facility.
• One-seat rides to SeaTac from both Ballard and Lynnwood.
• One-seat rides from the Eastside to South Lake Union, Queen Anne, and Ballard, where bus service can never hope to be competitive. (and cough, Expedia).
• Creates the traditional West Seattle to Ballard line many had in mind since the monorail's failure.

Weaknesses include the possibility of uneven headways depending on chosen turnback stations, massively increased operational complexity, continued wild cards such as MLK and the Ballard Bridge, and planned development near the Int'l District tunnel portals. But we should be thinking creatively about how to fix the known problems that lie ahead, and this admittedly radical concept is intended to get the conversation started.