Bus bunching

In public transport, bus bunching, clumping, convoying, or platooning refers to a group of two or more transit vehicles (such as buses or trains), which were scheduled to be evenly spaced running along the same route, instead running in the same location at the same time. This occurs when at least one of the vehicles is unable to keep to its schedule and therefore ends up in the same location as one or more other vehicles of the same route at the same time.

The end result can be unreliable service and longer effective wait times for some passengers on routes that had nominally shorter scheduled intervals. Another unfortunate result can be overcrowded vehicles followed closely by near-empty ones.

Theory

A bus that is only slightly late will, in addition to its normal load, pick up passengers who would have taken the next bus had the first bus not been late. These extra passengers delay the first bus even further. In contrast the bus behind the late bus has a lighter passenger load than it otherwise would have, and may therefore run ahead of schedule. The classical theory causal model for irregular intervals is based on the observation that a late bus tends to get later and later as it completes its run, while the bus following it tends to get earlier and earlier. Eventually these buses form a pair, one right after another, and the service deteriorates as the headway degrades from its nominal value. The buses that are stuck together are called a bus bunch or banana bus; this may also involve more than two buses. This effect is often theorized to be the primary cause of reliability problems on bus and metro systems.