In public transport, bus bunching 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, according to Wikipedia. Dave Sprogis, Volunteer Software Developer, and Data Analyst in Watertown, MA noticed a constant complaint from residents about bus service provided by the MBTA. On a route that advertises a bus “every 10 minutes or less” during rush hour, waits were frequently more than 30 minutes, sometimes an hour!
As you probably assume, buses do not start out bunched — they start out evenly spaced according to the schedule that deploys them. However, as the buses run the route, some run a little faster while others run a little slower. There is variability in traffic lights, pedestrian crossings, numbers of passengers loading and unloading, or even differences in driver pace.