If you’ve ridden Metro for any length of time you’ve probably experienced a “schedule adjustment,” where the train holds for a minute or two at a station. Why does Metro do that?


Graphic by the author.



The basic answer is that your train has gotten too close to the train ahead of it or the following train has gotten too far behind. Schedule adjustments are a way that Metro keeps headways (the time between trains) consistent. And that’s important because not having an even headway can lead to “bunching.” Also, uneven headways can lead to customers getting stuck waiting for the delayed train.

Bunching

Bunching is when a vehicle doesn’t come for a long time and then several show up at once. The basic cause of bunching is that one bus or train gets slowed down for some reason, and that initial delay means that every stop down the line has more customers waiting to board than usual. That leads to longer dwell times at each stop.

Buses are particularly susceptible to bunching because all boarding happens through the front door, people have to pay when they board, aisles are narrow, and they can get stuck in traffic. Buses that get delayed fall behind, lengthening dwell times for riders waiting for the bus while also shortening the headway until the next bus, which now has fewer passengers to pick up.

The uneven passenger loads that come from bunching are hard on transit. One way that Metro curbs bunching on the rail system is by holding trains for schedule adjustments when they’re getting too close to the preceding train.

In the center of the graphic above, you can see that the train running early is just one minute behind the preceding train. But the train behind is lagging by a minute because of the additional loading. So instead of a three minute gap between trains, it’s doubled here to six minutes.

If Metro were to hold the early train by one minute, it would then be two minutes behind the preceding train and five minutes ahead of the following train, which is closer to the scheduled headway.

Shortening delays

Metro also uses schedule adjustments to help when there’s a delay behind the one it’s holding.

For example, let’s say you’re on a Glenmont-bound Red Line train approaching Fort Totten. The operator announces that due to a disabled train at Judiciary Square, you’ll be holding three minutes at Fort Totten. You’re probably wondering how a delay behind you can mean your train needs to wait.

Doing this allows Metro to mitigate the delay for people who’ve yet to board your train. Yes, everyone on your train will be delayed three minutes. But by holding the train, Metro allows the people who arrive at Fort Totten (and any downstream station) during the three minute hold to board. Without the schedule adjustment, those people would be stuck waiting for the originally delayed train to arrive, which could be quite a while.

Schedule adjustments also keep there from being too many people who need to board the first train to come through after the wait. Because it’s been a while since the last train, the first train following the gap is often too crowded to board, which means it dwells at each station longer than usual, creating more delays. The downstream schedule adjustment clears some of those passengers off the platform ahead of the gap.

In the graphic above, you can see what it might look like without a schedule adjustment, where the last train before the gap is still three minutes behind the preceding train. But there’s an eleven minute gap behind it.

With a schedule adjustment of, say, three minutes, the spacing between those trains would go from three and 11 to six and eight, which is much closer to the desired interval.

Metro can also “express” the lagging train to further reduce the gap, but that’s a topic for another day.

Schedule adjustments aren’t always pleasant, especially if you’re already on the train. But they do help keep passengers who’ve yet to arrive on the platform from facing a long wait. In more serious delays, schedule adjustments can make a lot of sense. They’re one tool that Metro uses to try and keep trains evenly spaced.

Tagged: metro, transit, wmata

Matt Johnson has lived in the Washington area since 2007. He has a Master’s in Planning from the University of Maryland and a BS in Public Policy from Georgia Tech. He lives in Capitol Hill. He’s a member of the American Institute of Certified Planners, and is an employee of the Montgomery County Department of Transportation. His views are his own and do not represent those of his employer.