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Buses are scheduled evenly down a route. Without any interruptions, that space (called ‘headway’) remains even.
But if the front bus is delayed, that space shrinks. Traffic congestion, accidents and special events are the leading causes.
That extra time allows riders who would have caught the second bus to now be picked up by the first.
Meanwhile, fewer riders are left waiting for the second bus. Not as many people get on or off the second bus, so it runs faster and faster.
Each stop takes longer as more people get on and off and the bus has to wait. Stops that normally attract few riders are now more likely to have riders waiting.
Every stop brings the second bus closer and closer, eventually catching up to the front bus.
This makes passengers angry. Buses run late and are more crowded. It can be confusing when two buses show up at the same time.
The good: The CTA can schedule more buses on that route to handle the extra riders and prevent bunches.
The bad: This costs the agency money, which isn’t always available.
The good: CTA can build in extra wiggle room between buses and keep them evenly spaced.
The bad: Buses have to wait longer and slow down, which can frustrate riders.
The good: To speed up, the lead bus can run express to a transfer point farther down the route.
The bad: That inconveniences riders who miss their stop and those who have to watch the bus pass them.
The good: The trailing bus can jump ahead, filling the gap.
The bad: Riders see a later bus run ahead. The trailing bus may still have passengers needing to get off, keeping it from running any faster.
The good: The front bus can “short turn” by dropping off passengers, going out-of-service and turning the other way.
The bad: Passengers are inconvenienced. The CTA has to make sure the trailing buses have enough room for additional riders.
What did we learn? Without planning and intervention, buses naturally bunch. While the CTA has a number of options to combat bunching, each has the potential to frustrate riders.

BUS BUNCHING

Illustrations by Simran Khosla

Reporting by Chris Hagan and Jennifer Brandel

Edited by Shawn Allee