Why Buses Run Late - The Mechanics of Delay, Beyond Traffic Jams

Running late is the fate of any vehicle that shares the road

"The timetable says one every 5 minutes, so why is nothing coming?" Anyone who waits for buses has had that thought. The short answer is that buses run late not because of a driver's skill or a lapse of attention, but because of what a bus fundamentally is: a vehicle that travels ordinary public roads, sharing them with other traffic and with pedestrians.

Trains run on dedicated tracks and are almost never blocked by other traffic. The roads a bus uses, by contrast, carry cars, trucks and bicycles, and they come with traffic lights, pedestrian crossings and vehicles parked at the curb. A timetable is a set of expected times, drawn up on the assumption that all of those conditions turn out average. When conditions depart from average, the arrival shifts earlier or later by the same margin. This article walks through the main things that hold a bus up, one at a time, and then explains the mechanism by which a small delay grows like a snowball. Once you understand the reasons, the way you deal with buses changes as well.

Reason 1 - Road congestion: the largest factor, and the least predictable

Top of the list for bus delays is, unsurprisingly, road congestion. Traffic concentrating in the morning and evening commuter peaks, more people driving on rainy days, traffic backing up from beyond an intersection, lanes lost to road work, closures after an accident. Every one of these is an outside circumstance that the bus itself can do nothing about.

What makes congestion awkward is that both its location and its severity differ from day to day. On the same route at the same scheduled time, there are days when the bus glides along and days when clearing a single intersection means waiting through the lights several times over. Rain and snow are especially unkind: more cars take to the road, the flow slows down, and boarding and alighting take longer too, so every ingredient for a delay lines up at once. Going into a day of bad weather thinking of it as "a day when buses are more likely to be late" will spare you some irritation.

Reason 2 - The time boarding takes is different at every stop, on every trip

One factor that counts for more than you might expect is how much the dwell time at each stop varies. Unlike a train, a bus is boarded at a single door, with passengers lining up and moving through in turn. The more people getting on, the longer the bus stands still, and if someone struggles with paying the fare, everyone behind them waits.

Here are some situations, all of which stretch out a stop.

• Someone boarding or alighting with bulky luggage or a stroller
• A wheelchair user boarding while the driver sets out the ramp
• A run of passengers paying in cash who are hunting for coins, or who need change from the farebox
• A busy stop in front of a station where a lot of people get off and a lot get on, so the turnover takes time

Each of these lasts only tens of seconds, but a route has dozens of stops. If every stop runs a little long, by the time the bus reaches the terminus the difference amounts to several minutes. Put the other way around, having your IC card or coins ready in your hand before you board is not just smoother for you: it is a modest act of cooperation that helps keep that bus running to time.

Reason 3 - The luck of the traffic lights, and how the road is built

Traffic lights, too, are a factor you cannot ignore once they accumulate. Every time a bus calls at a stop it drops out of the flow of traffic, then merges back in as it pulls away. If that takes it out of step with the run of green lights, it is more likely to be caught at red at the next intersection. A single wait at a signal may be only 1 to 2 minutes, but on a central Tokyo route with many intersections those waits add up.

The way the road is built has an effect as well. Intersections where the bus is stuck behind a vehicle waiting to turn right, narrow streets where it has to swing wide around parked cars, entrances onto arterial roads where merging takes time. None of this shows up on the timetable, yet on every trip it produces variations measured in tens of seconds. On top of that, the stop itself can get crowded. Where several routes serve the same stop, a following bus cannot pull up at the boarding point until the bus already standing there has departed.

Delay breeds delay: how bus bunching comes about

Taken one by one, the factors above are small fluctuations. Buses, however, have an amplifying mechanism whereby a small delay grows all by itself.

When one trip falls slightly behind, the wait before it reaches each stop grows longer, so more passengers than usual build up at the stop. The more people boarding, the longer the dwell time, and the wider the delay becomes. For the trip running immediately behind it, the reverse happens. Because the bus ahead has "picked the passengers up first", fewer people are waiting, dwell times are shorter, and it steadily gains on the trip in front. The result is a late bus and an early bus running coupled together like the cars of a train. This is what is popularly known as bus bunching.

Once buses bunch, you get the situation that is most baffling of all for those waiting: "two came one after the other, and before that nothing came for 15 minutes". The service interval may be perfectly even on the timetable, but out on the road this amplifying dynamic is always at work. The more frequent the service on a route, the more readily the phenomenon occurs.

Operators are not simply sitting on their hands, either. They adjust departure times at stops along the way to even out the spacing, and when the timetable is drawn up they allow generously for running times in the periods most likely to be congested, building in one measure after another to absorb the fluctuations. Even so, road conditions change from day to day, so the practical reality is that delays cannot be reduced to zero.

How to live with delays: understand the mechanics and plan ahead

As we have seen, bus delays arise from the overlap of structural factors, namely congestion, boarding time and traffic lights, and are amplified by the bunching mechanism. In other words, a late bus is not a piece of chance bad luck but a fluctuation that was bound to happen. Understand it that way, and the measures available to you come into view.

• On days when you have to be somewhere at a set time, aim for the trip one departure earlier on the timetable
• In rain or snow, and during the busy morning and evening peaks, plan on the assumption that buses will run late
• Before you set off for the stop, check the arrival information that tells you where the bus is right now

That third point in particular makes a real difference: simply knowing "how many more minutes I need to wait" transforms how long the wait feels. If you are curious about how a bus's position is tracked and turned into that information, do read how bus location systems work alongside this article. And for how to look up departure times in the first place, see how to read a bus timetable. Understand the mechanics, and wait for your bus the smart way.

We also have a service data page, where you can see at a glance, color-coded by day of the week, which hours have the most buses running and which hours are most prone to delays. Use it as a guide when deciding what time to head out.