On March 16th, the two downward escalators between the mezzanine and sub-mezzanine at the University of Washington Link Station failed. With only elevators available to move people into the station for four hours, a line snaked around the station. On April 4th, a presentation to Sound Transit’s Operations Committee reviewed the event and discussed changes that could be made to prevent and mitigate future outages. While the presentation recognized the poor customer experience during the event, it described the impact in a strictly qualitative fashion.
This is a quantitative assessment of the escalator outage event. Any mitigation or preventative measure is going to have a cost to put in place. Without the ability to assign a cost to the event, it would be difficult to understand which actions should be on the table.
This assessment uses a measurement called Spontaneous Accessibility, published in this year’s Transportation Research Record. At a high level, this measurement describes how well an individual can make an unanticipated, unplanned transit trip throughout a given area. From a technical standpoint, it divides an area into a high-resolution grid of sectors. For each sector it computes, for every minute of a time window, the number of other sectors that can be reached within 30 minutes using transit and walking. This yields a heat map of how easily reachable each sector is, as well as a Network Accessibility Ratio that measures the proportion of time-origin-destination combinations that can be reached within 30 minutes against the total number of combinations. By making modifications to the modeled transit network and calculating the change in the Network Accessibility Ratio, it is possible to make a quantitative assessment.
To model the outage, Spontaneous Accessibility was computed under two circumstances. The first considers the scheduled transit service within the city of Seattle on March 16th, between 3:30 PM and 7:30 PM. The second uses the same temporal and spatial parameters, but eliminates the University of Washington stop from southbound Link trains. This is not a perfect model of the outage, as customers could still reach trains at UW Station after a substantial wait. Nevertheless, to a person arriving at the station and viewing a long line, the station may be considered effectively unreachable. This analysis yielded a change in Spontaneous Accessibility of -0.339%. A maximum reduction of 12.3% was observed in the vicinity of the Capitol Hill Link station, with other measurable reductions clustered largely around Link stations and a portion of the Aurora Avenue corridor. This Spontaneous Accessibility map shows the distribution of impact.
To extract a meaningful cost from the change in Spontaneous Accessibility, it is necessary to bring in some additional data. Over the four-hour period of the outage, there were 1988.28 vehicle-hours of in-service trips serving the city of Seattle. Using King County Metro’s $140.86 cost per vehicle-hour figure as an estimate for operating costs across all transit providers, the cost of in-service trips for the four hour window was approximately $280,069. In the same period, the Network Accessibility Ratio for the unimpaired network would have been 0.10378. Thus $280,069 was necessary to sustain a 0.10378 Network Accessibility Ratio in Seattle over those hours. When impaired, Seattle had a 0.10342 Network Accessibility Ratio. Sustaining this for four hours should cost approximately $279,097. Thus, from a Spontaneous Accessibility standpoint, the cost of the outage was $972.
It is important to be cognizant of aspects of the measurement that may distort this value. Because Spontaneous Accessibility is an isochrone-based measurement, it is not sensitive to the outage’s impact on trips that initially would have taken longer than 30 minutes. Spontaneous Accessibility measurement also acts as though riders have perfect knowledge of the transit network. Actual riders may not be aware of the other options that they have for completing their journeys, and thus may queue at the station instead of finding alternatives. Even if riders were to have this knowledge, Spontaneous Accessibility does not incorporate vehicle capacity, and thus would not account for alternate routes becoming congested as riders switched to them. Particularly thorny in this case is that Sound Transit would largely be relying on King County Metro to absorb that missing capacity.
Any attempt at assigning a single cost to an event is going to have limitations, and this is a basic one using publicly available data, open source software, and back-of-the-envelope math. At this point, however, it describes the impact more quantitatively than Sound Transit has put forth at this time. Sound Transit plans to evaluate certain mitigations by the next Operations Committee meeting on April 20th. These will surely have a price, and it will certainly be interesting to see how they compare to this assessment of the UW escalator event’s cost.