[NEW PAGE] Routing performance tradeoffs

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[NEW PAGE] Routing performance tradeoffs

Postby DwarfLord » Sun Jul 08, 2018 4:12 pm

The last few years have seen increasing awareness among experienced editors of the tradeoffs involved in adding junction nodes. However, our documentation has not kept up with this awareness and we are now struggling with how to codify these tradeoffs for each specific situation where we'd like to add junction nodes. Instead, I'd like to suggest writing them up in one central place for reference.

Here is some text that could either be put in a new article, or added to an existing article. Once it is in place, it could be referred to elsewhere, for example by discussions of adding junction nodes for speed limits, true elevation, railroads, etc.

Big question is, where would this go? I considered the Junction Style Guide but I'm not sure it fits there. Maybe it needs its own article?

Tradeoffs of creating new junction nodes wrote:Waze's routing performance depends on its accuracy measuring the time it takes each individual Wazer to complete a maneuver. By design, Waze can only measure this time over the space of the single segment leading up to the maneuver. The longer the segment, the higher the confidence in the timing measurement. This means that adding junction nodes, and thus shortening existing segments, can affect Waze's timing.

In most circumstances this effect is negligible and it is OK to add junction nodes when justified by best practice. In specific cases where the wait for one upcoming maneuver may be different than the wait for another, however, adding a junction node can degrade routing performance.

For example, consider a highway approaching a stoplight with a short left turn light and common backups for straight ahead, but typically free-flowing right turns. Before the intersection, the highway flares out to provide turn pockets for left turners to wait and for right turners to bypass the straight-ahead backups. Consequently, the delays to pass through the intersection differ profoundly depending on whether one is turning left, turning right, or continuing straight. Waze will measure those times over the space of the final segment approaching the junction. Provided this final segment is long enough to include the turn pockets and any additional distance over which wait times could be different, Waze will accurately capture the different delays.

Now consider that, for reasons of best practice that would normally be fine, one adds a new junction node on the approach in the midst of the turn pockets. This will force Waze to average together the delays associated with turning and going straight for all drivers approaching the new junction node. Only the delays after the new node will continue to be measured separately. The loss in timing quality will degrade routing performance.

Because of this effect, one must always evaluate potential timing impacts and consider alternatives before adding a junction node. In some cases it may still be necessary to add a junction node despite the drawbacks. In others, an alternative may be found, or it may be that the benefit of adding the junction node is too small to justify the potential routing degradation.


[EDITS: Updating thread title to reflect the way the thread conversation is going.]
Last edited by DwarfLord on Fri Jul 20, 2018 10:31 pm, edited 2 times in total.
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Re: Tradeoffs of adding junction nodes

Postby kumiankka » Sun Jul 08, 2018 8:28 pm

Own article maybe but it should be referenced from multiple pages, like from pages where retention principle is mentioned, junction style guide, etc.

Regarding the text itself, I think you should add that the negative effects of an extra junction node can sometimes be mitigated by adding a junction box on top of the troublesome intersection. It works because, when considering delays and average speeds, JB works like a single segment even when there are multiple nodes and segments inside the JB.

For example, Here in Helsinki we have this JB covering a left turn where, on the rush hour, bumper to bumper traffic starts couple of segments before the actual turn but on the left lane only:
https://www.waze.com/editor?env=row&lon ... ,143356599
Just from routing perspective that JB is unnecessary but it's needed to fix the exact problem you described.
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Re: Tradeoffs of adding junction nodes

Postby DwarfLord » Mon Jul 09, 2018 12:49 am

Thanks for the suggestions!

I must apologize, I thought I had submitted this in the USA-only forum. My last understanding is that the global wiki is effectively no longer maintained. I will seek an administrator to move it.

Nothing about that negates your thoughts, which are very helpful!
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Re: Tradeoffs of adding junction nodes

Postby ehepner1977 » Mon Jul 09, 2018 2:53 pm

It is in the US Wiki forum already. That said, kumiankka, you are always welcome to look over our suggestions. Who knows, you might find some stuff worth borrowing for your local guidance.
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Re: Tradeoffs of adding junction nodes

Postby DwarfLord » Mon Jul 09, 2018 3:59 pm

ehepner1977 wrote:It is in the US Wiki forum already.
Actually a very kind and responsive administrator (thanks vince1612!) moved it this morning, it did start out in the global forum.
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Re: Tradeoffs of adding junction nodes

Postby dfortney » Thu Jul 19, 2018 3:19 am

I agree we need a wiki page on this, and having a separate page referenced by multiple pages is the most efficient approach. I recommend we add parking lot roads and driveways to the list of editor actions that cause the shortening of segments and deserve consideration of routing tradeoffs. One situation deserves special mention: the U.S. has many major highways through towns and cities that devolve into a prolonged series of strip malls. If every parking lot entrance got its own road, the major road gets chopped into bite-sized pieces.

DwarfLord eloquently lays out how junctions along turn pockets can inhibit Waze from collecting accurate differential turn/transit times. In addition, there’s another way in which short segments can foul-up route timings — short segments provide fewer opportunities for GPS data collection and timings, and any inaccuracies have a larger percentage impact vis-a-vis longer segments. I don’t know the measurement accuracy of GPS on consumer mobile devices, and what impact poor cell signals or multi-tasking apps have on data accuracy, but if those kinds of factors result in several seconds of variation, and if that kind of inaccuracy is repeated multiple items along a potential route, resulting routing decisions could be woefully suboptimal.
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Re: Tradeoffs of adding junction nodes

Postby DwarfLord » Thu Jul 19, 2018 5:03 pm

There does seem to be a leaning towards a separate article, and I lean that way too. What I can't sort out is how to title it! I've been stuck.

Today I thought, perhaps we should set our sights a bit broader and leave room for the article to expand. Instead of making it specifically about Junction Nodes, why not title the article...

Performance tradeoffs in mapping

...starting out with just two sections, Principles and Junction nodes & segment lengths.

Later, we could add sections on other performance tradeoffs, like (1) road typing and routing and whether or not to use the "one type higher/lower" feature or (2) whether to set dead-end private roads to PR type. I'm sure there are zillions more examples.

Right now I am NOT advocating for any other content to the article, I'm just suggesting that calling the article "Perfomance tradeoffs in mapping" would provide a home for other concerns that are broader than the "when to use and when not to use" binary guidance generally followed by the rest of the wiki.

How does that sound? If it sounds OK I will draft a beginning "Performance tradeoffs in mapping" article for review.
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Re: Tradeoffs of adding junction nodes

Postby Kartografer » Thu Jul 19, 2018 5:37 pm

I like this idea, but wouldn't the single-word title Tradeoffs work? After all, the whole wiki is about mapping
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Re: Tradeoffs of adding junction nodes

Postby DwarfLord » Thu Jul 19, 2018 6:57 pm

I like the word "performance" in the title because it immediately clarifies the kind of tradeoff (as opposed to things like whether to make a Point-type versus an Area-type Map Comment, etc).

I'm certainly not married to "in mapping", in fact I'd be happy if we could lose it. But isn't the wiki "bigger" than just mapping? The Main Page has a lot of stuff that doesn't involve mapping guidance.

It won't take a lot of persuading to get me to drop the "in mapping" part of the proposed title...

[EDIT: I've changed the thread name to reflect the way the discussion is going.]
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Re: [NEW PAGE] Performance tradeoffs

Postby abc1357 » Thu Jul 19, 2018 9:56 pm

I'm not sure I like Performance tradeoffs. When I first saw the title before opening up the topic, I thought it may be related to performance of WME in Chrome vs. Firefox, or the performance of the various scripts on different computer systems.

Since this is about the tradeoffs of junctioning and segment lengths, why not title this article as what it is Segmenting and junctioning considerations ?
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