Monday, May 7, 2012

Clock-Face Scheduling and the 16

There's a concept in transit timetables called clock-face scheduling. Basically, the idea is that you have a route run on a consistent headway every X minutes, and that you set X to a value such that the pattern repeats every hour. In Riverside, the #1 is scheduled so that departures from the Downtown Terminal are clock-face scheduled. Before 7:30, westbound departures are at :10 and :40 past the hour- every 30 minutes. Between 8:00 and 18:00, departures are every 20 minutes, at :00, :20 and :40 past the hour. At 18:00 they return to every 30 minutes, at :00 and :30 past the hour. This makes trip planning easy-- if you know the route, you know that it will pass your stop at a certain time past the hour, every hour. You simply remember that time, and you know when the next bus is coming without having to consult a schedule or web site.

Right now, the #16 is also clock-face scheduled, with eastbound departures every 30(ish) minutes from Downtown at roughly :10 and :40 past the hour-- although the schedule does lose time over the course of the day, eventually becoming at :01 and :31 and then fluctuating wildly past 17:31. I know that the westbound bus passes my stop at roughly :20 and :50 past the hour, and the eastbound at roughly :00 and :30. On weekdays, I never need to consult a schedule to figure out when to catch the bus-- I just need to memorize a few numbers, and I can plan my trip accordingly.

Which is why something that I'd normally be excited about-- increased frequency on the route that serves my apartment-- has given me some reason for pause, because the frequency is being improved from 30 to 25 minutes.

The disadvantage of clock-face scheduling is that you can't simply pick an arbitrary frequency and make it work. Only certain times produce repeating patterns every hour. 5, 7.5, 10, 15, 20, 30 and 60 minute headways work. 40 and 45 minute headways produce patterns that repeat every two hours, which is okay but not ideal. (This is the situation on the 15, for example.) Any other combination, and you're basically either checking the bus book or guessing. A 25-minute headway produces a pattern that repeats every 5 hours, which is basically useless for memorization. Furthermore, you don't gain all that much-- only two hours out of those five see three buses an hour (more than the 2/hour from a 30-minute headway). The average wait goes from 15 minutes to 12.5.

Thus I am torn. I'm always happy to see a frequency improvement, especially on the route that I use most. However, this improvement comes at a significant cost to system legibility. It means I now need to check the bus book/BusWatch site every time I want to use the system. And it isn't all that big of an improvement- the average 16-hour service day will only see 6 more buses, or one every 3 hours. Obviously, if they were to improve the frequency to every 20 minutes, it'd be fantastic, but I'm not sure if the increase in service is worth the significant drop in usability here.

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