Watch equally-spaced buses on a circular route bunch together due to a positive feedback loop: a delayed bus picks up more passengers, gets more delayed, while the bus behind finds fewer passengers and catches up.
A classic example of positive feedback instability. When one bus falls behind schedule, more passengers accumulate at stops, increasing dwell time. The following bus encounters fewer passengers and speeds ahead — the gap grows at one end and shrinks at the other.
Observe how evenly-spaced buses gradually cluster together. Toggle the holding control to see schedule recovery — when buses wait at timing points, bunching is suppressed. The space-time diagram reveals bunching patterns.
Bus bunching affects virtually every high-frequency transit system worldwide. The holding strategy (forcing early buses to wait) is one of the simplest and most effective remedies, reducing passenger wait time by up to 30%.