Looking at bus frequencies

One interesting element of the new Outer Link bus service is that it operates at the same frequency all the time: one bus every 15 minutes. This is pretty unusual for a bus service, as typically you see a huge increase in service frequency at peak times and then a big drop-off during off-peak times. Obviously demand is ‘peaked’, and we’re already starting to see Outer Link buses being filled up (only yesterday one drove past me in Herne Bay because it was too full to pick up passengers), but it seems the point of keeping frequency the same whenever the service runs is to ensure we get good off-peak frequencies – enabling people to think about using PT for more than just the trip to work.

Personally I think the Outer Link will need to add additional peak time capacity in the not too distant future, which will mean a shift away from its “same frequency all the time”. But you can go too far the other way, with peak frequencies being good but off-peak frequencies being fairly rubbish (most of Auckland’s bus routes are like this). While demand is obviously the highest at peak times, providing peak time services are really expensive (because you need to own the bus and employ the driver for just a few trips a day), so you really want to be sure that the balance between peak and off-peak services is nicely fine-tuned.

Some experiences of Campaign for Better Transport member Andrew along Sandringham Road in recent weeks and months suggest that this is a route which has the opposite problem to the Outer Link: it has too many services at peak times and not enough during off-peak times. Here’s what Andrew sent me recently:

Peak loadings are very uneven. The early AM expresses I used usually have most seats taken, sometimes some standees. The later AM expresses are pretty well used.

The PM expresses are the emptiest, although I suspect it’s because they often depart the CBD just after, not before, all stopper services. Those who just get on the first available bus that comes along would therefore end up on the all-stopper, even if the express were the better bus for them.

Interpeak and evening services are often full standing, turning passengers away at some points as described in those tweets. Both the 233/243 and the 249 have this problem.

To explore this issue, I’ve compared service frequency patterns of Sandringham Road services with those along New North, Dominion and Mt Eden Roads. All four routes are generally similar, following key north-south arterials across the central part of the isthmus: (All the routes tend to split away south of Mt Albert Road and some have variations in their routes to what’s shown but this is a general guide).

If we look at the timetables for each of the four routes and measure the time each service either arrives (for inbound) or departs (for outbound) the city centre, we get an idea for the peak/off-peak split. The first graph below shows the varying frequencies for inbound services: Obviously for inbound services the big spike in frequencies is during the morning peak. In the 8:00-9:00am hour most of the routes have around a bus every three minutes, although Dominion Road has a significantly higher frequency than this – presumably to cope with the higher demand. Interestingly, Dominion Road maintains a far higher level of inter-peak service than any other route, but drops off in the evening peak whereas for all the other routes they get a bit of an increase (presumably because they’re sending so many buses into town to shift people out of town).

Looking at the PM peak, things are a bit different – with a more spread out peak in services from around 3:00-6:00pm: The precipitous drop-off in service frequency after 6pm on all four routes is quite surprising – and perhaps suggests that you might be most likely to find your bus completely full if you try to catch it between 6pm and 8pm. Not exactly ideal if we want to encourage more of a ‘spreading of the peak’ to enable the more efficient use of our bus resource.

Putting the inbound and outbound together and you get the full picture of how frequencies fluctuate throughout the day:  Some quite interesting observations can be made of the graph above. Firstly, it’s rather surprising that we have almost as many inter-peak buses on Dominion Road as we have evening peak buses. And with all routes we see that huge fall in service levels after 6pm.

If we look at the numbers of buses specifically, take the morning peak as buses arriving in town between 7am and 10am and the evening peak (as determined by bus numbers) as those leaving town between 3pm and 6pm, we can compare how “peaked” each of these routes are. There’s an interesting spread: While there’s not too much difference between New North Road and Sandringham Road services, we do find that Sandringham Road buses are the most “peaked” in terms of the service provision. This appears largely due to the poor inter-peak frequencies that both New North and Sandringham Road have – which more than make up for Dominion Road’s huge inbound peak numbers.

There doesn’t seem to me to be any particularly logical reason why Sandringham and New North should have much more “peaked” service provision than the other two routes although I know Dominion has particularly high demand so it perhaps justifies a higher inter-peak level of provision than the others. Perhaps it’s most surprising that New North Road has such a highly ‘peaked’ level of provision, as it duplicates the inner part of the Western Line and seems to be mostly used by university students (whose trips are likely to be spread throughout the day more).

It seems to me that if we were a bit smarter about service provision for both Sandringham and New North Road services we could perhaps knock a few peak time services away and use the significant money we save to spend on boosting inter-peak frequencies. For all routes it seems that a big boost in evening frequencies (6-9pm) is likely to be justified. With a bit of clever thinking, many of these changes could be made at little or no cost.

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