A few months ago there was some anecdotal evidence that Central Link wasn’t very reliable with respect to its schedule and scheduled headways for various reasons. My subjective impression is that this has improved somewhat, but there’s no way you’d be able to tell from Sound Transit’s publicly available data. More after the jump.
Reliability for Sound Transit buses and trains is reported in their Quarterly Ridership Reports. Reliability metrics, fittingly, are different for buses, Sounder, and Link. The 3rd Quarter ridership report, which is the first one to cover Central Link and covers the time of the reliability panic, reports on-time performance as 99.5%, better than Sounder (97%) and ST Express (94%). Great!
However, the fine print references the definition of “on-time” in the 2009 budget, Appendix C.
Standard is greater than or equal to 98.5%. A train is late if it a) departs a terminal station more than one minute late or, b) arrives at a terminal station three or more minutes late and is unable to make it’s departure time.
This definition, which was adopted before Central Link opened, seems convoluted and confusing, and more importantly measures the wrong thing. First of all, a common-sense reading of this language indicates that trains that meet condition (b) are a subset of those that meet condition (a), rendering (b) pointless.
More importantly, late departure is of little consequence to the rider, except to the extent it creates late arrival. It’s late arrival that causes me to take an earlier train than I might otherwise choose, to make doubly sure I make my appointment or catch my connecting bus.
For instance, Sounder’s definition is pretty good:
Standard is 95%, defined as having the average of all trains in a month arriving at terminus within seven minutes of schedule at least 19 out of 20 trips.
I have no developed opinion as to whether the proper metric for Central Link should be measured at each stop or only at the terminus, or if the right number is 7 minutes, 1 minute, or 30 minutes. Indeed, another sensible approach is to use headway-based reliability metrics. The main issue, however, is that using these figures we have no way of knowing how bad the problem was at first, or if it’s getting better.
Sound Transit did not take the opportunity to explain the thinking behind the metrics. Unfortunately, the 2010 proposed budget has the same metric definitions in it.