Breaking a Harmful Feedback Loop: Mitigating Bus Queuing and Headway Irregularity on Busy Corridors

We unveil that a previously-unreported and undesirable feedback loop can be created when bus queues frequently form at congested curbside stops along a corridor. Buses caught in this loop exhibit growing variation in headways as they travel from stop to stop. Bus and patron delays resulting from these queues accumulate in like fashion and can grow large on long, busy corridors. We show that this damaging feedback loop can be abated by applying various bus holding strategies at a corridor's entrance. Specifically, holding buses not only helps reduce headway variations---a well understood benefit---but can surprisingly also mitigate bus and patron delays. We further introduce a modest variant to the simplest of these strategies, which releases buses at headways that are slightly shorter than the scheduled values. It turns out that this variant strategy can effectively compensate for bus delays caused by holding by reducing bus delays at queued stops. Benefits can outweigh costs in corridors that contain a sufficient number of serial bus stops. The simple variant is shown to perform about as well as, or better than, other bus-holding strategies proposed in the literature in terms of saving delays, and is more effective than other strategies in regularizing bus headways. We also show that grouping buses from across multiple lines and holding them by group can be effective when patrons have the flexibility to choose buses from across all lines in a group. Findings come by formulating select models of bus-corridor dynamics and using these to simulate part of the Bus Rapid Transit corridor in Guangzhou, China.