Missing some border intersections

[michaelkirk]

As I understand it "border intersections" (those which are marked with a big arrow in the screenshot) are intended to show entry from the perimeter roads into the interior roads.

Is that correct?

It appears that we're missing some (see the pink circles I've used to annotate the screenshot).

I can take a look into this further, but wanted to make sure I'm understanding the expected behavior.

Also, I want to make sure I'm using our vocabulary right, please confirm or deny:

A neighborhood is defined by a single boundary. It is a 2-d polygon which can be drawn by the user (or auto-generated).

"Border" and "Perimeter" are synonymous with eachother, but slightly different from a boundary - they refer to the set of roads closest to the boundary that "outline" the interior roads.

Does that coincide with your usage of the terms?

[TFCx]

A neighborhood is defined by a single boundary. It is a 2-d polygon which can be drawn by the user (or auto-generated).

Does that imply that "the purpose of the LTN tool is to divert the trafic flow from inside the neighborhood to the perimeter road" ? @dabreegster

[dabreegster]

It appears that we're missing some (see the pink circles I've used to annotate the screenshot).

Yes, they indeed look missing and this would be a bug.

"Border" and "Perimeter" are synonymous with eachother, but slightly different from a boundary - they refer to the set of roads closest to the boundary that "outline" the interior roads.

Almost. "Perimeter roads" are the Road segments "close to" (the new Frechet parallel definition) the boundary. "Border intersections" are the Intersection points on that boundary. They're shown by the arrows. The shortcut algorithm looks at every pair of border intersections and finds the shortest path restricted to the "interior roads".

Does that imply that "the purpose of the LTN tool is to divert the trafic flow from inside the neighborhood to the perimeter road" ?

Yes. To be even more specific, if a driving trip begins or ends inside a neighbourhood, that's fine. Unless a neighbourhood contains a POI like a shopping centre that would expects lots of traffic, then if you can make sure the traffic in a neighbourhood is just people entering or leaving, then generally you've got an "LTN". The through-traffic that passes through the neighbourhood is "shortcutting" (or the less polite term "rat-running"). They might be doing that because it's the quickest path regardless of current traffic conditions; there's just a convenient shortcut, or way to avoid a slow traffic signal, or something else. Or it happens because the roads outside of any neighbourhood (usually "main roads" by whatever classification) have too much traffic, and sat-nav / local knowledge finds a shortcut through the neighbourhood. The LTN tool's main purpose is to help you stop shortcuts within a neighbourhood, and instead -- in the short-term -- push that traffic onto the perimeter roads, which are supposed to be designed to handle lots of traffic, unlike local roads in a neighbourhood.

(There's much more about traffic dissipation and spillover effects elsewhere that I will need to write up for docs for this project, but... later)

[michaelkirk]

Almost. "Perimeter roads" are the Road segments "close to" (the new Frechet parallel definition) the boundary. "Border intersections" are the Intersection points on that boundary. They're shown by the arrows. The shortcut algorithm looks at every pair of border intersections and finds the shortest path restricted to the "interior roads".

Ah ok, so we don't talk about "Border" only "Border Intersections" - that makes sense.

To nail this down a little bit more, a "Border Intersection" is only one which connects the Perimeter to the Interior - an intersection from Perimeter to Exterior is irrelevant to the LTN, and we wouldn't call it a "Border Intersection. (right?)

[dabreegster]

a "Border Intersection" is only one which connects the Perimeter to the Interior - an intersection from Perimeter to Exterior is irrelevant to the LTN, and we wouldn't call it a "Border Intersection. (right?)

I want to say yes, most of the time. I can imagine an edge case:

Pretend the black boundary is drawn freehand in the sections where red interior roads meet the green road (which we don't have a word for, completely exterior to the neighbourhood). There could be a long bridge over water, and the boundary crosses an intersection on the bridge. In that case, the bridge is an entry/exit and there should be a border intersection there. There are no perimeter roads on that side.

But I'm having trouble finding a real example of this and constructing it. So... if you're looking to calculate border intersections using the perimeter and interior roads, I'd say that's likely to get us 95% of the way to what we want.