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Most people familiar with public transport know about clockface timetables. We would all prefer a "turn up and go" service, but if your train, bus or ferry service is not high frequency (10 minute headways or better), second best is one where departures are at the same minute intervals every hour.
Let's say your local ferry leaves in the direction of the city terminal at 10 and 40 minutes past the hour, every hour. You might like it to run more often, but at least the schedule is memorable. You don't have to consult a timetable because it always leaves at the same minute intervals.
Clockface timetables have other benefits which may not be obvious to the passenger. If the ferry operates at regular 30 minute intervals, then it is easier to schedule connecting buses. It's also easier to make ferry to ferry connections at an interchange like Circular Quay.
So are all clockface timetables equal?
If two ferry lines have 30 minute headways, all connections at an interchange terminal can be timed conveniently. In the example shown in Figure 1 below, passengers have an eight minute wait for any transfer from Line A to Line B or from B to A. Just like a memorable timetable, it is reassuring to the passenger to know they can make a good connection between Lines A and B regardless of which ferry they catch.
But a timetable with 20 minute headways can also be considered "clockface" because departures are still scheduled at the same minute intervals every hour. The problem is that if other lines in the network run every 30 minutes, then good connections are only possible once an hour. In other words, the connections are actually made worse.
In practice, fluctuations in demand dictate that frequency may need to be greater in peak periods, or some lines must operate more often all day. So what is the best way to accommodate this without mucking up connections? The answer is to halve the headways. If 30 minute headways are not frequent enough, change them to 15 minutes.
This approach has at least two important benefits. The first is it doesn't alter the underlying simple, easy to remember pattern. To use our original example of the ferry stop with departures at 10 and 40 minutes past the hour, the departure times in the peak are now at 10, 25, 40 and 55 minutes past the hour. The extra services merely supplement the off peak pattern.
The second benefit is demonstrated in Figure 2 below. Passengers transferring from the lower frequency Line A retain a good connection with Line B for all services.
There is of course a problem with transfers in the other direction - from the higher frequency Line B to the low frequency Line A. This can be ameliorated by timetable apps which highlight the services on Line B which have good connections to Line A.
So what this means is that all clockface headways are not equal, especially if a network design objective is to improve line integration and bus connections. With plans afoot to increase off peak frequencies on the Parramatta River, a 15 minute headway is a much better option than 20 minutes. And if we are concerned about efficient use of taxpayer funds, would it be better to leave the River at 30 minute headways off peak on week-days?
This article first appeared on sydneyferry.blogspot.com
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