On the tipping point of buses

No, we’re not going to go around tipping over buses. Though it would make a great urban legend to counter that rural legend of cow tipping.

There are plenty of bus routes in Kitchener-Waterloo — and in most cities that are not New York, I would imagine — that run every half-hour or less frequently. On the other hand, the “mainline” 7 buses run every 7-8 minutes along their common spine. I claim that the difference between a 30 minute and a 8 minute headway is not just in quantity, but also in quality. This might be obvious.

The difference is in how one may use such routes. For 30-minute buses, on average you’ll wait 15 minutes; but sometimes you have bad luck and the bus is running late, so a worst case is something like 40 minutes. These are 40 minutes that you could have used to walk to your destination. Which is pretty bad, so you get yourself a bus schedule and plan your time around the bus stop time. If you don’t get there sufficiently ahead of time, you still run the risk of the bus arriving early and screwing you over. The obvious way out is to avoid all this hassle with some other means of getting from point A to point B, such as driving.

Why is an 8-minute headway different? The answer should be clear: if you need to take the bus, you go to the bus stop and wait. On average, that’s a 4 minute wait. Worst case, 8 minutes, which is manageable. You don’t need the bus schedule! If you know where the bus goes, then you can just go to a stop and expect the bus to ferry you to your destination. It’s easy, requires no access to schedules or the Internet, and especially requires no advance planning. (With a short headway, the schedule might actually give you a false confidence due to inherent variability in the route.)

The bus headway tipping point is probably somewhere in between 5 and 30 minutes, and is some kind of average of individual patience curves. Around that tipping point, more buses will lead to disproportionately more riders and a more user-friendly bus route. The greater point is that there is a serious qualitative difference between low- and high-frequency service, and this deserves more prominence in discussion of transit.


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2 responses to “On the tipping point of buses”

  1. smably says :

    I’d be interested to see ridership figures from before and after a change in headway (e.g., when the iXpress went to 15-minute frequency during the day).

    It occurs to me that this is an argument in favour of smaller vehicles. I wonder whether transit planners take this into account when introducing articulated or double-decker buses. I also wonder whether running smaller, more frequent buses might pay for itself in increased ridership on routes with frequency close to the tipping point.

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  1. In transit, less is more « Psystenance - May 2, 2009

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