The Chicago Transit Authority says it wants to tackle a major rider complaint: bus bunching. But why does bus-bunching happen?

Joseph Schofer teaches transportation at Northwestern University. He says while busses shimmy down the street, random delays—like double-parked cars—can make buses creep closer. Usually, buses make every stop, so they never have a chance to separate. Schofer says, then, more and more riders gum up the system.

SCHOFER: And so that delayed bus is further delayed because waiting times get longer than were originally planned. The delay is exascerbated and the bunch gets bigger.

Schofer says there are remedies, but the best is expensive.

SCHOFER: What you might think about doing is going to a bus rapid transit solution where you put buses in exclusive lanes that didn’t have to deal with interference from side traffic.

It turns out the CTA will test a rapid transit system this year, but it won’t say which neighborhoods will see the rollout.

I’m Shawn Allee, Chicago Public Radio.