On September 30, 2014, the TTC’s Bloor-Danforth subway suffered a shutdown from just before 8:00 am until about 3:00 pm on the segment between Ossington and Keele Stations. The problem, as reported elsewhere, was that Metrolinx construction at Bloor Station on the Georgetown corridor had punctured the subway tunnel. While the weather was dry, this was not much of a problem because, fortunately, the intruding beam did not foul the path of trains. However, rain washed mud into the tunnel to the point where the line was no longer operable.

In the wake of the shutdown, there were many complaints about chaotic arrangements for alternate service, although any time a line carrying over 20k passengers per hour closes, that’s going to be a huge challenge. The point of this article is not to talk about that incident, but to something that showed up the next day.

According to the TTC’s internal measure of service quality, the BD line managed a 92% rating for “punctual service”. This is lower than the target of 97%, but that it is anywhere near this high shows just how meaningless the measurement really is.

The basic problem lies in what is being measured and reported. Actual headways at various points on the line and various times of day are compared to a target of the scheduled headway plus 3 minutes. This may look simple and meaningful, but the scheme is laden with misleading results:

• On the subway during peak periods, service is “punctual” even if it is operating only every 5’20”, or less than half the scheduled level. Off-peak service, depending on the time and day, could have trains almost 8 minutes apart without hurting the score.
• There is no measurement of the actual number of trips operated versus the scheduled level (in effect, capacity provided versus capacity advertised). Complete absence of service has little effect because there is only one “gap” (albeit a very large one) after which normal service resumes.
• There is no weighting based on the number of riders affected, period of service or location. A “punctual” trip at 1 am with a nearly empty train at Wilson Station counts the same as a train at Bloor-Yonge in the middle of the rush hour. There are more off-peak trips than peak trips, and so their “punctuality” dominates the score.

An added wrinkle is that the TTC only includes in its measurements periods of operation when the headway is unchanged. With the service being so often off-schedule, it would be difficult to say just what the value of “scheduled headway plus 3” actually is at specific points along the route during transitional periods.

All the same, we have a measurement that has been used for years in Toronto and it gives a superficially wonderful score. Sadly, the formula is such that falling below 90% would require a catastrophic event, and some silt in the tunnel does not qualify.

Continue reading →