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Why Do Buses Bunch?

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Re: Why Do Buses Bunch?

#51

Post by nicoburns »

tim333 wrote: post #10
caymanjim wrote: post #7

This is very common in Manhattan. Whenever I saw a full bus, I'd skip it and wait for the next one, because it was almost certainly going to come along within 5 minutes and be pleasantly uncrowded. On a busy route, it's the sensible choice. On longer-distance commuter routes, where the scheduled times are 30-60 minutes apart, I wouldn't usually take a chance on waiting for the next one, but I even did that on occasio…

It's mostly been fixed in London. They used to be famous for no busses for 20 mins then 3 at once but they are now controlled by a central computer system which knows all the positions via gps and cell data.

It still happens on the tube (one very full train, followed by a couple of emptier ones). Of course it makes a lot less difference when the trains are every minute...

Re: Why Do Buses Bunch?

#52

Post by hamandcheese »

sethammons wrote: post #37

I've always wondered why we don't just account for full bus/train loading deloading plus a buffer. Each stop has the mass transit stop for multiple minutes. Bingo, automatic buffer. If the bus is running late, it stops briefly.

The trade off for this, though, is increased travel times.

Re: Why Do Buses Bunch?

#53

Post by bobthepanda »

kilbuz wrote: post #40
bobthepanda wrote: post #5

It is also worth noting that in real life passengers are not uniformly distributed through time. When school lets out, or a train arrives at the station full of transfer passengers, etc. you can get a huge burst of traffic at one stop that screws up everything. This can have knockback effects, because if buses show up late to their terminal they have now also started their return trip late. EDIT: edited a word

Not to be a pedant (too late), but you probably meant 'uniformly distributed' in this case.

That is indeed what I meant. Thanks for the correction.

Re: Why Do Buses Bunch?

#54

Post by brian_cloutier »

I enjoyed the callout to this when I was reading Google's Percolator paper a few years ago:

> The random-scanning approach requires one additional tweak: when it was first deployed we noticed that scanning threads would tend to clump together in a few regions of the table, effectively reducing the parallelism of the scan. This phenomenon is commonly seen in public transportation systems where it is known as “platooning” or “bus clumping” and occurs when a bus is slowed down (perhaps by traffic or slow loading). Since the number of passengers at each stop grows with time, loading delays become even worse, further slowing the bus. Simultaneously, any bus behind the slow bus speeds up as it needs to load fewer passengers at each stop. The result is a clump of buses arriving simultaneously at a stop [19]. Our scanning threads behaved analogously: a thread that was running observers slowed down while threads “behind” it quickly skipped past the now-clean rows to clump with the lead thread and failed to pass the lead thread because the clump of threads overloaded tablet servers. To solve this problem, we modified our system in a way that public transportation systems cannot: when a scanning thread discovers that it is scanning the same row as another thread, it chooses a new random location in the table to scan. To further the transportation analogy, the buses (scanner threads) in our city avoid clumping by teleporting themselves to a random stop (location in the table) if they get too close to the bus in front of them.

From here: https://ai.google/research/pubs/pub36726

Re: Why Do Buses Bunch?

#55

Post by cimmanom »

pushpop wrote: post #44
cimmanom wrote: post #36

Earlier quoted context omitted.

Are they allowed to also skip stops where people want to get off?

Clearly not. You’re not going to hold people captive on the bus just to make up a few minutes. However that doesn’t mean the bus drive has to accept new passengers when someone does get off (it’s not common that happens but I have seen that happen before and it’s highly infuriating for those waiting for the bus. Which is likely why it’s not common)

My experience on crowded buses is that typically every stop gets rung for anyway, so skipping isn’t an option.

Of course, if it were possible to open only the back doors and let people off but not on, that might help, at the expense of leaving the people waiting to get on enraged. But then you also have the problem that on a crowded bus it can be really tough to squeeze past the rest of the passengers to get off, which also slows everything down.

The whole thing is miserable, and mostly these routes just need more buses (plus the articulated ones that hold 60% more people).

Re: Why Do Buses Bunch?

#56

Post by phinnaeus »

brian_cloutier wrote: post #54

I enjoyed the callout to this when I was reading Google's Percolator paper a few years ago: > The random-scanning approach requires one additional tweak: when it was first deployed we noticed that scanning threads would tend to clump together in a few regions of the table, effectively reducing the parallelism of the scan. This phenomenon is commonly seen in public transportation systems where it is known as “platooni…

TL;DR - Teleportation will solve public transit problems.

Re: Why Do Buses Bunch?

#57

Post by phinnaeus »

r0ffles wrote: post #33
jboles wrote: post #27

Earlier quoted context omitted.

It's always been real-time location data. At one point it was pretty low-tech (odometer hooked up to a radio transmitter) but it's now been GPS-based for ~5 years.

Are we talking about OneBusAway?

These days OneBusAway is just one frontend for the data, the Transit app has the same information.

Re: Why Do Buses Bunch?

#58

Post by gizmonty »

pushpop wrote: post #44
cimmanom wrote: post #36

Earlier quoted context omitted.

Are they allowed to also skip stops where people want to get off?

Clearly not. You’re not going to hold people captive on the bus just to make up a few minutes. However that doesn’t mean the bus drive has to accept new passengers when someone does get off (it’s not common that happens but I have seen that happen before and it’s highly infuriating for those waiting for the bus. Which is likely why it’s not common)

Actually, the train system in Melbourne, Australia was doing exactly this to passengers in the very recent past. The private operators realised that there were penalties for getting to the end of the line late, but not for missing stops, so they just told the passengers who wanted to get off to suck it!

Re: Why Do Buses Bunch?

#59

Post by bbarn »

grawprog wrote: post #43
adrianmonk wrote: post #26

Would it help to have the trailing bus pass the leading bus when they bunch up? My reasoning: (1) As the article said, full buses move slower because more passengers means more boarding/unboarding time at stops. (2) The lead bus is the fuller, slower one. Slower bus in front means buses get closer together. (3) If one bus passes the other, then you have the faster bus in front and the slower one behind. They should t…

When busses get bunched here, the lead bus just tends to skip stops and leave them for the trailing bus. This doesn't really seem to help much. They usually both stay pretty close together and, as a passenger, you end up standing there confused as to whether the bus just left you or not if you can't see the one behind.

The lead bus is much less equipped to skip a stop if it has more passengers, because there's a greater chance of a passenger needing to stop to disembark. In Chicago, you will see a trailing bus skip but it can do so because the driver hasn't gotten and cord-pulls to signal someone getting off.

Re: Why Do Buses Bunch?

#60

Post by watersb »

vecter wrote: post #50
kilbuz wrote: post #40

Earlier quoted context omitted.

Not to be a pedant (too late), but you probably meant 'uniformly distributed' in this case.

Not pedantic at all, this is a very crucial distinction.

"Uniformly distributed" : the number of passengers is the same for every time of day

"Normally distributed": number of passengers versus time of day roughly looks like a bell-shaped curve, a Gaussian.

I hesitated writing this, pushing the pedantry up a notch, but then considered spacecraft crashing into Mars due to failure in technical communication, and recalled colleagues with college degrees in computer "science" who rejected the paradigm of Evolution of species as "only a theory", so here I am writing it down to remind myself of the difference between normal and uniform distributions.

Also, I worked briefly with some Ph.D. Queuing Theory people that one time.

Maybe there should be an academic discipline of "Commuter Science".

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