Theory: “Bus bunching” or why several buses sometimes arrive at a bus stop at once
Sometimes waiting for the bus can take longer than expected, and then once the bus finally arrives with some delay, it’s not rare for another bus to come right after or just moments later.
This is in most cases by no means a coincidence, and a variety of names exist for this phenomenon: Aside from “bus bunching“, it can also be referred to as “platooning”, “clumping” or “piggybacking” or “equal headway instability” in some scientific literature. The buses involved are either called a bus bunch or a banana bus.
It is a well-researched phenomenon that has a specific cause and for which a number of solutions have been proposed: One bus has a slight delay, leading to more people waiting at a bus stop. This adds to the delay, and so the next bus stop has even more people waiting than in the case of punctual buses. On the other side, the bus following the delayed bus has fewer passengers to pick up, and so will take less time for its trip, often catching up to the slower bus and being held up by it.
The delay is often not just a few minutes, but can accumulate significantly. It also affect trains and other modes of public transport, and as a result during rush hour in Mexico City trains often take three times as long to do a single trip alongside their line.
A computer model created by Carlos Gershenson and Luis Pineda at the National Autonomous University of Mexico shows that adding more trains or buses to the line tends to just compound the issue and that allowing faster trains to overtake slower ones will only change the order of trains, and which one is holding up the other ones.
The best way to prevent banana buses (or trains) from forming is to require minimum and maximum times spent at any given stop, even if there are no passengers at it or not all have managed to disembark. Those times would be adjusted for rush hours, and passengers waiting at stops told to wait for a bus or train that is less crowded.
Read more:
New Scientist: Why three buses come at once, and how to avoid it
International Journal of Advanced Research in Computer and Communication Engineering: Bus Bunching Avoidance System
Why Does Public Transport Not Arrive on Time? The Pervasiveness of Equal Headway Instability
Image credit:
Felix O, CC BY-SA 2.0, via Wikimedia Commons
