In public transport, bus bunching, clumping, convoying, piggybacking or platooning refers to a group of two or more transit vehicles (such as buses or trains), running along the same route, which were scheduled to be evenly spaced, but instead run in the same place 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 which are supposed to be behind it.
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 running slightly late will, in addition to its normal load, pick up passengers who would have taken the next bus if the first bus had 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 theorised to be the primary cause of reliability problems on bus and metro systems.
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 between their scheduled times. Sometimes one scheduled vehicle may pass another. These effects tend to accumulate, unless the later vehicle deliberately idles on stops or slows down, or the lead vehicle skips some stops where it has no drop-offs, leaving the pick-ups for the following bus.
When buses clump, one can pass another on the street or road. When trains clump, this is often not possible due to the limitations of a single track in a given direction.
Another cause is that some drivers are faster than others. This results in catching up on long or high-frequency routes.
When a transit vehicle becomes overcrowded as well as delayed, the transit operator may choose to designate the vehicle as a temporary "express" trip. This means that the expedited vehicle will bypass many of its usual stops, operating on a limited stop basis, or heading non-stop directly to the route terminus. This usually entails some additional delay, needed to notify passengers already on board, and to let some of them to disembark to avoid being taken past their intended destinations.
These non-express passengers, as well as others waiting to board, may be told that another less-crowded vehicle is "right behind them". This may or may not be true; in extreme cases, the following vehicle(s) may also be overcrowded or even more severely delayed.
Other corrective measures include adding more vehicle trips or higher-capacity equipment. Aside from possible extra costs, the chief difficulty is in responding quickly enough to delays and overcrowding before they spontaneously dissipate. It is preferable to closely monitor transit headways and take immediate corrective measures, rather than allowing delays to "snowball", causing a near-complete breakdown of service.
Clumping can be prevented or reduced as follows:
A different approach is to abandon the idea of a schedule and keep buses equally spaced by strategically delaying them at designated stops. This is used to control the buses on the campus of Northern Arizona University, where it  outperforms the previously scheduled system.
More efficient on-board fare collection, or prepaid fare collection, can reduce dwell time at stops and greatly speed up service. Multiple-door, level-boarding setups can also greatly reduce stop durations, as demonstrated by the best Bus Rapid Transit (BRT) services.
Heavy ridership caused by special events (such as sporting events, large concerts, inaugurations, or holiday celebrations) is even more likely to cause problems with bunching and delays. Extra service may be added, and special rules (such as waiving of fare collection) may be used to improve service.
Merely adding more vehicles to the schedule without making other changes has been proven not to be a reliable solution to the problem of bunching.