When I started working at the TTC, I didn’t know that I would also become the person to rant to, whenever something goes wrong.

At first, it was quite annoying to have to endlessly hear about problem after problem, but through the course of these several months, I developed a taste to listen to people’s concerns—it’s often tastes bitter, but I’ve gotten used to bitter. Especially from my transit-hating suburbanite friends and relatives.

So without further ado, here’s a popular question that often infuriates passengers.

Figure 1: I’m a big nerd who likes diagrams. Here’s a situational diagram of some buses on a route with a traffic problem.

Buses don’t arrive on time, mainly because of congestion. As you all know, most bus and streetcar (“surface”) routes share the road with other traffic, and for that reason, if there is a traffic situation on the road, buses and streetcars are impacted.

When there is a recurring problem with congestion on surface routes, schedules are changed to reflect this—usually in the form of additional recovery time. Recovery time provides an established “buffer” time at the end of the route, so as to ensure better reliability in service. But this cannot always be accommodated, as the conditions on the road can often vary from day-to-day. Dedicated right-of-ways for transit, which is anticipated on many arterials according to Toronto’s new Official Plan, would help to reduce problems related to service delays.

Now it comes to the question of why buses always come in clumps when there is a delay. Maybe transport engineers have a more persuasive answer to this question. But after some research, here is my take on it:

One of the factors, aside from congestion, that influence the time it takes for buses to finish a route is the number of passengers boarding and alighting the bus; the higher the passenger activity, the longer it is for the bus to continue along the route.

When Bus A, for example, is delayed, obviously there would be more people accumulated at stops along the route. Figure 2 shows what happens as Bus A tries to catch up to the delay in service on the route— but usually with no success, since there are more riders that Bus A has to pick up.

Figure 2: During service delays, there is a higher than expected volume of riders along the route—Bus A has the luxury to have to pick up all passengers in the service gap.

The delay for the bus then starts a ripple effect. See what happens in Figure 3 when Bus A reaches the next stop.

Figure 3: (Corresponds with Figure 2) Once Bus A reaches the next stop, there are more riders waiting at the stops where there was a gap in service— adding to the delay for Bus A. Green pedestrians are the additional riders. Meanwhile, Buses B and C, have comparatively less passenger activity than Bus A, enabling them to travel more quickly.

And so the ripple effect begins. Bus A becomes increasingly more delayed stop after stop, which causes an even greater amount of passengers being accumulated.

This doesn’t seem to answer the question. Let’s say that Bus B, was evenly spaced between Bus A and Bus C. Bus A, trying to make up for lost time, has trouble doing so, because of the accumulated passengers at each stop. This causes the gap between Buses A and B to narrow (as demonstrated in Figure 3). Bus B will then arrive at stops sooner than Bus A, and would likely collect fewer passengers as a result. Thus, especially on routes travelling long distances, Bus B would likely catch up to Bus A; Bus A would likely be overcrowded with passengers, while Bus B sits half-empty.

This then leads to another question:

Buses operate short turn service, simply to mitigate this bunching effect.

Figure 4: Overall picture of what happens in a short turn.

Short turns only occur when there is unexpected congestion and buses need to fill a gap in service caused by the delay. With reference to our original example, a short turn bus (“Bus Z”) would operate only a limited portion of the route, then squeeze into service just ahead of Bus A, so as to mend the gap that was created by the unexpected congestion. Bus Z would then help to relieve the accumulated passengers at stops caused by the delay, which would then help to ensure that Bus A does not become even more delayed—due to the increased boardings.

If you have any questions— about this topic or not, feel free to ask. I’ll answer them with the best of my ability. If I don’t know the answer, I’ll try to find out.