Why Buses Bunch—and Why the Second Bus Cannot Always Just Pass

Few transit experiences are more frustrating than waiting a long time for a bus and then seeing two buses arrive together.

Passengers naturally wonder why the buses are traveling as a pair instead of maintaining their scheduled spacing.

This problem is known as bus bunching.

It is common on frequent routes, and once it begins, it can become worse without some form of intervention.

How Bunching Starts

Imagine a route where buses are scheduled every 15 minutes.

The first bus encounters a delay. It may be caused by traffic, a long red light, a mobility-device boarding, a blocked stop, or an unusually large number of passengers.

The bus is now five minutes late.

Because it is late, more passengers have accumulated at the next stops. More passengers take longer to board, which makes the bus even later.

Meanwhile, the following bus is experiencing the opposite condition. Because the first bus is carrying many of the waiting passengers, fewer people remain for the second bus.

The second bus spends less time at stops and begins to catch up.

The late bus gets slower. The following bus gets faster.

Eventually, the two buses arrive together.

The Snowball Effect

Bus bunching is self-reinforcing.

The lead bus may become increasingly crowded because it reaches each stop first. A crowded bus takes longer to load and unload.

The following bus may remain lightly loaded and continue gaining ground.

Once the spacing becomes uneven, the normal schedule alone may not correct it. Dispatch or field supervision may need to intervene.

This is why bunching can continue even after the original traffic delay has ended.

Why Does the Second Bus Not Just Pass?

Passing can sometimes help, but it is not always simple or appropriate.

There may be legal, safety, customer-service, and operational reasons not to pass.

The Lead Bus May Still Have Passengers Waiting

If the second bus passes the first, passengers at the next stop may be expecting the lead bus to serve them.

The passing bus may have a different destination, route variation, or stopping pattern. Even when both buses are on the same route, the operator must know whether passing will help restore spacing or merely create more confusion.

The Street May Not Allow Safe Passing

The buses may be operating on a two-lane road, in heavy traffic, near an intersection, or alongside parked vehicles.

A transit bus cannot always maneuver around another bus safely.

Even on a multilane road, the passing bus may immediately become trapped at a signal or need to return to the curb at the next stop.

The Buses May Not Be Operating the Same Trip Pattern

Two buses displaying the same route number may not have identical assignments.

One may:

• Terminate earlier
• Continue farther
• Serve a deviation
• Operate an express segment
• Be scheduled to connect with another route
• Be returning to the yard after a particular trip

Allowing one bus to pass without considering the complete assignment can create a new problem later.

Passing Does Not Restore Proper Spacing by Itself

If the second bus passes, the two vehicles may simply exchange positions while remaining close together.

The original lead bus is still crowded and delayed. The passing bus may now begin collecting passengers ahead of it.

Without additional action, the pair may continue operating only a few minutes apart.

The goal is not merely to change which bus is first. The goal is to restore useful spacing between them.

How Transit Agencies Respond

There are several ways to address bunching, depending on the route and circumstances.

Holding

The following bus may be held at a timepoint or transit center to recreate space behind the lead bus.

Passengers on the held bus may understandably be frustrated, especially if they feel their bus is being delayed unnecessarily.

However, the hold may improve service for passengers along the rest of the route.

Holding works best when used carefully. Excessive holding can create another gap behind the delayed bus.

Stop Skipping

Dispatch may direct one bus to skip selected stops, usually when another bus is available to serve those passengers.

This can help the delayed vehicle regain time, but it must be used cautiously. Passengers need clear information, and accessibility obligations must be respected.

Short Turning

A bus may be turned around before reaching the normal end of the route so it can return in the opposite direction and fill a large service gap.

Short turns can improve overall route spacing but inconvenience passengers who need to travel beyond the turnback point.

This strategy requires careful communication and a safe place for the bus to reverse direction.

Expressing One Bus

One of the bunched buses may be directed to operate with limited stops for part of the route.

This can redistribute passengers and separate the buses, but it depends on road conditions, route design, and the availability of another bus to serve skipped stops.

Adjusting the Schedule

If bunching occurs regularly at the same time and location, the underlying timetable may need to be changed.

Possible adjustments include:

• Adding running time
• Changing recovery
• Revising departure times
• Modifying timepoints
• Adding service
• Changing stop spacing
• Adjusting traffic-signal priority
• Revising route structure

Dispatch can manage an unusual event. It should not have to compensate every day for an unrealistic schedule.

Why Frequent Routes Are Vulnerable

Bunching is most noticeable on routes with frequent service.

On an hourly route, each bus is usually operating as a separate event. On a route running every ten or fifteen minutes, buses continuously interact with passenger demand and traffic conditions.

Even a small delay can represent a large percentage of the scheduled spacing.

A five-minute delay on a 60-minute service is inconvenient. A five-minute delay on a 10-minute service eliminates half the intended separation between buses.

This is one reason high-frequency service requires active management.

Passenger Information Matters

Passengers often judge bunching by what they can see at one moment.

They may not know that:

• One bus was delayed by an incident
• The buses have different terminal points
• Dispatch is attempting to restore spacing
• One vehicle is operating a replacement trip
• A road closure has changed the service pattern

Real-time information can help explain what is happening, but it does not solve the underlying operating problem.

The best outcome is still reliable, evenly spaced service.

Prevention Is Better Than Correction

Bunching cannot be eliminated entirely. Streets are unpredictable, and passenger demand varies.

It can be reduced through:

• Realistic running times
• Adequate recovery
• Active supervision
• Reliable vehicles
• Appropriate frequency
• Well-designed stops
• Transit-priority measures
• Clear operator instructions
• Timely service data

Good schedule design creates a stronger foundation. Good operations management helps protect that schedule when unexpected conditions occur.