Scheduling Self-Coordinating Buses
By: Don Eisenstein & John Bartholdi (from IFORS News June 2012)
Buses tend to bunch. This is because inevitable disturbances will cause one or more buses to fall behind the others, and any large gap tends to grow because the trailing bus must serve more passengers and so will be further delayed. Large gaps get larger, and small gaps get smaller, with the result that riders experience long waits, after which several buses may arrive together.
Schedules have only limited ability to resist bunching. If a disturbance is sufficiently large, such as a bus breaking down, severe bunching will occur as a schedule is unable to recover any regularity of service.