ANYONE WHO RIDES a high frequency bus route regularly has probably seen a “bus bunch,” which is two or more buses on the same route driving together in a group. Bus bunching may appear to be a random fluke, but in reality it is a visible symptom of a bus system that is unreliable, makes poor use of its limited resources, and fails to provide high quality transit service that people need and deserve.

When a bunch forms, what was supposed to have been two or more trips with regular intervals between buses becomes what is essentially a single, supersized trip with large, irregular intervals between the bunch and vehicles coming before and after it. Artificially reducing the number of trips on a given bus route in this manner forces passengers at stops before and after the bunch to wait longer for the next bus. In addition, within each bunch the delayed vehicle in front is typically overcrowded, while the bus that caught up to the delayed vehicle is largely empty. As a result, when bus bunching occurs, the T functionally runs fewer trips with more vehicles, some of which remain largely empty. This forces passengers to endure long and unpredictable waits for delayed and crowded buses, which is no way to run a public transit system.

What causes bunching on high frequency routes? There are two main underlying issues: delays and poor management. Every bunch starts with a bus trip that is delayed, which typically occurs due to traffic or passengers taking a longer-than-expected time to get on and off the bus. Whenever a bus is delayed, it picks up two sets of passengers: those who arrived on time for the bus and had to wait, and riders who would otherwise have missed this bus and gotten on the next one. Because the delayed bus is carrying more passengers than it normally would, the ride slows down further since more people are getting on and off the bus.

If the bus behind the affected vehicle isn’t similarly delayed, it will speed up as it makes fewer stops for fewer passengers, since riders who otherwise would have boarded this bus got on the previous, delayed vehicle instead. Eventually, as can be seen in this simple interactive simulation created by Lewis Lehe, an assistant professor of transportation engineering at the University of Illinois Urbana-Champaign, this faster vehicle will catch up with the slower bus in front of it, leading to a bus bunch.

Poor management makes this sequence of events more likely. Currently, buses are sent out on trips along a route – dispatched, in transit terms – based on fixed schedules. If buses are late in heading out on a given trip, they’re dispatched as soon as possible, without considering when other buses are being dispatched. Then, to best adhere to the fixed schedule, operators drive the route as fast as possible. Under this system, bus operations do not try to account for regular disruptions, such as traffic or above-normal numbers of passengers. This leads some buses to be delayed, and others to speed up, setting up the chain of events outlined above that leads to bunching.

To beat bus bunching on key bus routes, two things need to happen: the T needs to reform how buses are managed, and state agencies and municipal governments need to make a coordinated effort to address the root causes of bus delays.

To start, the T should abandon fixed schedules on high frequency routes: these schedules are almost never adhered to, and riders typically don’t plan their trips around them. Instead, the emphasis should be on keeping consistent intervals between buses along a route – a strategy known as headway maintenance. This strategy, which has been implemented on key bus routes in Austin, Honolulu, and other major metropolitan areas, calls for dispatching buses based on the intervals between vehicles – known as headways – and having drivers and dispatchers work to maintain these regular intervals between buses as they proceed along a route.

Headway maintenance can eliminate the most severe instances of bus bunching, which in turn would lead to shorter and more consistent intervals between buses arriving at stops since effectively more trips would run on a route on a given day, reducing average passenger wait times and improving service frequency and reliability. Moreover, this approach would utilize the T’s limited number of buses and operators more efficiently, as there would no longer be empty buses driving immediately behind crowded vehicles.

While adopting headway maintenance is a good first step, there also needs to be a concerted effort to address traffic delays and excessive variability in the amount of time buses need to stop to let passengers get on and off (known as dwell times). The T can minimize dwell times by prioritizing the implementation of AFC 2.0, the long-awaited new payment system that will speed up fare payments when boarding the bus, and allowing all-door boarding, which has been successfully used on the Silver Line at Logan Airport to reduce dwell times and improve service reliability.

Reducing traffic delays, however, is largely in the hands of municipal governments and traffic enforcement. Only cities and towns can install dedicated bus lanes – such as the center-running lanes on Columbus Avenue that have improved service on Routes 22, 29, and 44 – and improve traffic signals so that buses are not stuck at red lights (known as transit signal priority). Cities and towns are also best positioned to keep existing bus lanes and stops clear of vehicles through rigorous parking enforcement and creating alternate areas for delivery vehicles (a common source of obstructed bus lanes and stops) to park.

These changes will not be easy – they will require major operational shifts on the part of drivers and dispatchers, significant investment from local governments and the state Legislature, and long-term political commitment from agencies and municipalities across the Commonwealth. But by taking the steps necessary to beat bus bunching, the T would improve service for some of its most dedicated riders, attract new ridership, and lay the groundwork for future efforts to provide the region with the bus network that it needs to build a stronger, more equitable economy.

Christopher Marks is a member of the Next Generation Bus Program at TransitMatters.