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Bus Clumping
11 August 2007


Public transport. It's a highly useful service, often a lot cheaper than using a car, you don't have to worry about parking, and in most capital cities you can get nearly anywhere using it. Sometimes it's even faster than driving, depending on your destination. But one thing continually holds back public transport from being widely accepted, especially among those who rely on it to get to work: Reliability.

Public transport is notorious for being late, or having services that just don't come. I don't think this is just a Brisbane thing, either. The issue isn't quite as prevalent with trains, but if you don't happen to live near a train line, you're getting about by bus. This brings me to the point of this post. If you ARE relying on buses for transport, there's something you should understand. Consider this a community service announcement.

Buses are not just late because the bus driver is lazy or irresponsible. There are complex arrays of dynamic factors that influence the arrival time of your bus. The most prevalent of these is a phenomenon I like to call 'bus clumping'.

Consider the things that will make a bus late; the first is traffic. On a busy day the traffic will be slow, or even the bus might just get unlucky and catch all of the red lights. For a car driver, this might only make five or ten minutes difference on average. But it is important to consider that buses are beholden to another environmental influence - their passengers. Obviously, a bus picking up an unusually heavy load of passengers will take longer, as there will be more stops made to load passengers on and off, and said loading takes longer. It also significantly increases your chances of picking up an old lady who'll rifle for ten minutes for the correct change or newbies learning to fumble through the bus system, having to ask directions before being able to decide what ticket they want.

So the bus is late. Big deal right? But you see, this one late bus has a carry-on effect - this is when the phenomena of bus clumping begins to occur.

I couldn't quite get the graphing application to work, so just minus 2 on the dark purple bar.  It's just a visual aid anyway.
Fig.1 - The ideal bus cycle.

Upon careful observation, you�ll notice that people arrive at their bus stops roughly as per the diagram - the majority five minutes before the bus arrives, with a number of stragglers who miss and have to catch the next bus. It's important to note that a late bus will pick up these stragglers that it would normally miss, again making it later and later by small increments. Eventually, the bus is so late that it's starting to pick up the early passengers waiting for the next bus. The longer the route the bus has to take is, the later it is likely to be towards the end of the line.

Bus clumping then occurs when the next bus on the same line sets out on time, only it has fewer passengers to pick up - since the stragglers it would normally pick up caught their intended bus, and a portion of its regular passengers have been picked up by the late-running bus ahead as well. So the bus become earlier and earlier, usually until it catches up with the late running bus. This can occur for numerous buses at once, so almost always during morning rush hour will you notice that four buses which normally come spread out over a thirty minute period will all arrive at your stop at the same time. This of course depends where along the line you catch your bus.

I apologise for that hard-to-read light green text.  I'm sure Hypnopixel's design overlord will be scandalized.
Fig.2 - See how bus clumping completely screws up that graph?

Think it's over yet? That bus that was early because its predecessor is late is also causing schedule problems. Since it's running progressively earlier, it will be missing more and more of its passengers who don't arrive at the bus stop on time. What do you think those passengers are going to do?

That's right. They're going to catch the next bus, which will become progressively later the further down the line.

That, ladies and gentlemen, is the highly irritating phenomena of bus clumping.

Then, just to make the bus even later, you'll get these righteous indignant types - usually 30-40 year old busy bodies who'll make a great big fuss about complaining how the bus is late, berating the bus driver as though it is their personal responsibility that they're going to miss their appointment. First clue - if the appointment is so damn important, DON'T TAKE PUBLIC TRANSPORT. Catch a cab, or at least make sure you've given yourself enough leeway so that if your bus doesn't come, you'll still make it on the next one. More importantly, and this brings me to the entire point of this rant - COMPLAINING ABOUT IT WILL NOT TURN BACK THE CLOCK. Bus clumping is near unavoidable, so your complaints then and there will change nothing. Sit down, shut up, and please do not make the rest of the also-late people on the bus even later while you embark on your tirade. If it really bugs you so much, call people who can do something about it - the people who organise the bus timetables. If they get enough complaints, maybe they'll assign extra buses to the route, and that's about all you can do about it.

If you do rant and rave and hold up a whole busload full of people, I think it�s only right I warn you that your local transport authority has started hiring bus ninjas to dispose of such nuisances. Don�t feel safe to let loose just because you can�t see them. They�re ninjas � just because you can't see them, doesn't mean that they aren't there.





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