Keeping bus service reliable and evenly spaced is important to riders in normal times. With the imperative to minimize crowding during the COVID-19 emergency, preventing bunching and gapping is even more urgent now. Agencies like the SFMTA have responded by adopting new dispatching methods that prioritize even headways on frequent routes, as opposed to getting each bus to adhere to its individual schedule. This post presents recent research from Sam Schwartz Engineering about headway-based dispatching that was produced before the pandemic but remains timely and relevant to the current moment.
As more transit agencies aim to operate routes where buses arrive every 15 minutes or better, it’s increasingly important to maintain even, reliable headways. The primary benefit of frequent transit — that riders know the next bus is never that far away — is negated when buses bunch together, opening up long gaps in service.
City DOTs are important partners in achieving consistent headways, because bus lanes and other transit priority measures are necessary to reduce the variability caused by traffic congestion. To complement and build off transit-priority infrastructure, transit agencies have their own suite of tools to improve bus reliability.
Recognizing that frequent service requires different methods of maintaining reliability, U.S. transit agencies are shifting from schedule-based dispatching to dynamic headway management that prioritizes even spacing between buses and the prevention of bunching and gapping.
This post provides an overview of why agencies are making this transition, and what they’ve learned so far. It summarizes the findings of a white paper from Sam Schwartz Engineering, which interviewed eight U.S. transit agencies about their current practices for reliably delivering frequent service.
You can read up on these developments in more detail in the full white paper.
For routes that arrive at least every 15 minutes, keeping headways consistent matters more than sticking to the schedule. “On-time performance” assesses reliability according to the amount of time each bus deviates from published schedules. But on routes that run frequently, it’s possible for buses to be counted as “on-time” even with gaps in service that create significantly longer waits than advertised. For this reason, a growing number of agencies are assessing the performance of frequent routes according to a different metric: regularity.
“Regularity” conveys how well buses on the same route maintain consistent headways. On high-frequency routes, where riders expect to “show up and go” without worrying about the schedule, measuring the time between buses is more aligned with the rider experience. Four of the transit agencies interviewed for the white paper now assess the reliability of frequent bus routes differently than non-frequent bus routes.
Precise vehicle location data makes a difference. Automated vehicle location (AVL) technology is improving, and the new generation of AVL can provide more accurate and detailed information about bus location in real-time. With better AVL technology at their disposal, more agencies are updating their reliability strategies for frequent transit routes.
In Baltimore, for instance, the Maryland Transit Administration’s previous vehicle location system updated the position of buses only every 1 to 3 minutes. For its current AVL system, the agency installed transponders on its vehicles to collect a continuous feed of real-time location data. It is now using the data to modify run times, making schedules more realistic and increasing operators’ “chance of success.”
Real-time data helps agencies better manage headways. Agencies deploy a number of strategies to maintain headways mid-route, such as holding some buses on standby so they can be inserted into gaps in service, or detouring to bypass traffic congestion. All eight agencies that were interviewed use real-time data to help staff decide how to execute these mid-route reliability strategies.
Real-time data also helps bus operators maintain consistent headways on the fly. Observing dispatching techniques in Atlanta and San Antonio that provided bus operators with real-time data about their position relative to other buses on a route, researcher Simon Berrebi found that bus operators maintained more consistent headways when given this information. Operators should have the leeway to act on this information, he concluded, and use their own judgment to respond to conditions on the fly — slowing down, say, when the bus ahead of them is too close. Some agencies, including New York City Transit, are beginning to equip bus operators with real-time displays that show the exact position of their vehicle in relation to the buses ahead and behind them.
Starting routes off right. If a bus starts off behind schedule, it’s very difficult to catch up. So in addition to mid-route adjustments, better dispatching at the terminal is a must to keep headways consistent. At its garages, for instance, WMATA ensures on-time departures with a “division yard management system,” which tells bus operators which vehicle to use and its precise location on the lot.
One underutilized technique at terminals is drop-back (or fall-back) dispatching. With drop-back dispatching, once bus operators arrive at the terminal, they are “decoupled” from their vehicle, which becomes immediately available for another operator. This preserves time off between runs for operators (“recovery time”) while increasing the utilization of vehicles. On frequent routes, this may enable the number of buses required at peak hours to be reduced.
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