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 result is unreliable service and longer and more inconsistent effective wait times than scheduled. Another unfortunate result can be overcrowded vehicles followed closely by near-empty ones.

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.

The classical bus bunching theory is an example of chaos theory. The orderly procession of buses is presumed inherently unstable, and buses are presumed to tend towards bunches if left unchecked. According to proponents of this theory, it is impossible to predict from the outset which buses will be bunched and which buses will proceed on schedule to the destination, because bunching is presumed to be caused by random conditions such as traffic congestion, stoplights, and the number of passengers at a stop.

Simulation studies have successfully demonstrated the extent of possible factors influencing bus bunching, and they may also be used to understand the impact of actions taken to overcome negative effects of bunching.

Clumping can be caused by random heavy usage of any particular vehicle, resulting in it falling behind schedule. The leading vehicle eventually lapses towards the time slot of a later scheduled vehicle. Sometimes, the later scheduled vehicle gets ahead of its own timetable, and the two vehicles meet in a location in between their scheduled times. In some cases, a later scheduled vehicle may actually get ahead of an earlier scheduled one.

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