Bus Bunching

I find myself talking about transit a lot with those in my social networks, and passions seem to be greatest when talking about bus reliability. A typical conversation with someone in my social network goes like this:

“What do you do for work?” “I’m a transit planner. I adjust bus routes and schedules.” What’s wrong with the 66 bus?

Boston’s rapid transit network is a hub-and-spoke system consisting solely of rail lines heading in and out of the downtown area. Route 66 is one of only a few circumferential bus routes, and there is nothing inherently “rapid” about it except that it beats taking the subway if you’re traveling between points far from downtown.  The MBTA considers it a “key bus route,” meaning that the scheduled frequency is never more the 15-20 minutes at all times. However, the key bus routes are widely known for their persistent bus bunching and resulting long gaps in service.

Consider this common scenario: you’re standing outside in the rain, having waited over 20 minutes for a bus that’s supposed to operate every 9 minutes. When the bus finally arrives it’s jam-packed and there are two empty buses directly behind it.

This is bus bunching, a persistent yet difficult problem to manage. Bunching happens when one bus falls behind schedule and the following bus catches up to it. As the gap in front of the first bus widens, it picks up passengers who would have been on the next bus, and the next bus moves along faster.

Any source of delay can contribute to bunching if the service is frequent enough. Among these factors are traffic congestion, traffic signals, heavy or irregular passenger loads, fare payment issues and wheelchair boardings.  It happens less often on subway lines that are not affected by traffic. Vehicles operating every 5 minutes or less are virtually guaranteed to bunch without corrective action.

Several strategies may be employed to minimize bunching:

  1. Make sure the schedule is accurate and includes enough recovery time at the end for buses to load passengers and start the next trip on time. A good standard is 95% of trips leaving the starting point on time.
  2. Monitor departure times to make sure operators are ready to go but not leaving early.
  3. Never leave time points early. Most agencies do this on low-frequency routes where passengers arrive for scheduled times, but it’s also important to prevent bunching.
  4. Unusual delays such as road closures or use of the wheelchair lift should be reported to the dispatcher immediately so that all buses on the route can be held.
  5. Change public schedules for high-frequency routes to show only the frequency of service.  This gives you flexibility to make real-time adjustments.
  6. Use available means of publicity to encourage helpful passenger behavior: having the fare ready, exiting through the rear door, etc. Post the fares next to the door.
  7. Assign spare operators to “run as directed” work pieces at strategic locations.
  8. Prepare emergency schedules that dispatchers can implement if you have a reduced fleet, need shuttle buses for a rail outage, etc.  Dropping a single trip will cause bunching, and if this happens on a regular basis it’s time to hire more operators or cut service.
  9. Run special schedules on severe weather days. Less capacity will be needed and running times may be longer.
  10. Anything that reduces travel times and variability will reduce bunching. Examples are off-board fare payment, stop consolidation/relocation, curb extensions, queue jump lanes and signal priority.
  11. Technology now exists for a piece of software to use location data to adjust headways in real-time. This is especially useful for dealing with special events and irregular running times. All the operator has to do is follow the screen.

The key is to intervene early; it only gets harder if you wait until two buses come together. The following interventions can be helpful:

  1. If bunching happens near the end of the route and there is enough recovery time, intervening makes no sense since almost everyone is already on the bus.
  2. Holding the second bus a few minutes usually solves the problem. Be careful to avoid compromising all of that trip’s recovery time.
  3. Covering the fare box will get passengers on more quickly and reduce dwell time.
  4. A bus can catch up by running express or “drop-off only” for a segment of the route.
  5. Turning one bus around before the end of the route (short-turning) allows it get back on time and prevents bunching from worsening or cascading to other routes on the corridor.
  6. Direct spare operators or those pulling back to the garage to cover very late trips.

It is undesirable to make passengers get off and wait for the next bus, but we should not be afraid to do that if the benefit to others already waiting ahead is greater than to those inconvenienced. If you start receiving complaints, a publicity campaign to promote more reliable service could be helpful.

What other tips do you have?