Why do late buses cluster in threes?

There’s a saying about buses: You wait ages for one, and then two come along at once. The same is often said about opportunities and romantic partners. But in the case of buses, you don’t have time to weigh your options. When you’ve been waiting at the stop for what feels like a lifetime and then two or three buses barrel toward you like a public-transit parade, you’re likely to wonder two things: Why on Earth are they all clustered together like that? And, which one should I get on? Let’s find out.

Believe it or not, there is a lot of research into the phenomenon of bus clustering, or “bunching,” and experts have figured out precisely why it happens in all sorts of situations. Here’s how it works: First, a bus (Bus 1) gets delayed for whatever reason — there’s a lot of traffic, a passenger needs help loading their bike onto the bus, an entire Girl Scout troop is paying their fare in coins, whatever. When the bus finally gets to its next stop, passengers that might have missed that bus ordinarily now get to catch it, so the bus takes on more passengers than usual.

With those late passengers gone, the next bus (Bus 2) has fewer passengers to pick up, so it moves on sooner and narrows the gap between it and Bus 1. This is a self-perpetuating cycle since Bus 1 keeps arriving to its stops late and picking up more passengers than usual, which makes it even later and narrows the gap between it and Bus 2 even more.
Of course, if you’re in charge of the bus drivers, you might step in and tell Bus 2 to overtake slowpoke Bus 1. But then Bus 2 will hit the overcrowded bus stops while all but eliminating any gap between it and Bus 1. That added slowdown could create a chain reaction that brings Bus 3 ever closer — until all three arrive at your stop, right when you think you’ll never see a bus again.

Since this all hinges on the amount of time between buses, it’s most likely to happen on routes where buses arrive with high frequency.

So three buses have shown up at once. Which do you choose? In an article on The Conversation, two transport experts from the University of Huddersfield explained how you should decide. Your best bet is to opt for Bus 2 since, as we saw, it’s probably less crowded and is likely to overtake Bus 1 soon. But you need to be careful: That overtaking may have already happened, in which case you’d really be getting on slow Bus 1. To avoid this, check to see if there are plenty of seats on your bus of choice. If there are, it’s likely the true Bus 2, and you’re in the clear.

What about Bus 3? That’s out. “This should be avoided wherever possible, because two things, which could cause more problems, are likely to happen,” they explain. One problem is that the operator could instruct it to get ahead of the mayhem by overtaking the other two buses and skipping the stop — and that stop could be yours. It might also be instructed to terminate before it reaches the end of the line, meaning you might have to get off and wait for another bus to take you the rest of the way. It might not work for opportunities or romantic partners, but when you have to choose between multiple buses, always go for the second one to come along.

Check out my related post: What is the history of the public bus service in Singapore?

Interesting reads:







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