[Talk-us] Complex intersection mapping

Minh Nguyen minh at nguyen.cincinnati.oh.us
Tue Oct 15 09:51:32 UTC 2013


On 2013-10-14 10:42 AM, Martijn van Exel wrote:
> So what are we talking about? Intersections like this one, where one
> or more dual carriageways come together at an at-grade intersection:
>
> https://www.evernote.com/shard/s9/sh/6438c196-bb92-4f66-81dc-9b75186286ba/0e8f07ff527c6a85c0dec426b9b79f1e
>
> One of my colleagues at Telenav has remapped this intersection as follows:
>
> https://www.evernote.com/shard/s9/sh/3491f1fe-6afa-4571-bc43-7cb31c9c2625/9dd47d1445fdcf03d3f0bbd93b8e0f92
>
> The main difference, and the source of some feedback we have received
> over the past few days, is that the dual carriageway roads are
> straightened out, creating multiple intersection nodes (4 in this
> case) instead of the original single intersection node that connects
> all the incoming and outgoing ways. That technique turns out to yield
> more reliable and correct routing and guidance ('keep left, turn
> right') through these intersections in our testing. But of course,
> that cannot dictate how we map as a community, so let's discuss.

I'm one of the troublemakers who complained about your colleague's 
edits. However, the example you give bears little resemblance to the 
intersections I disagree on. Your "before" screenshot depicts individual 
lanes (ew) that converge into a single-point intersection, even when the 
main road is divided on both sides of the intersection (ew). My quibble 
relates to divided roads that become undivided at an intersection.

Screenshots tell it best, but unfortunately we don't seem to have a tool 
to visualize historical revisions of ways. So I recreated their changes 
from memory in iD (because that's how I roll).



** Example A **

Ryans Way and Sycamore Grove Ln. meet Fields Ertel Rd. at the same 
intersection. Fields Ertel is undivided. Ryans Way is briefly divided at 
the subdivision entrance, a very common configuration in newer 
subdivisions, but Sycamore Grove is not.

I mapped the intersection as a single point:

<http://nguyen.cincinnati.oh.us/minh/osm/talk-us/braided_intersections/ryans_before.png>

Your colleague redrew it as a two-point intersection, dividing the very 
tip of Sycamore Grove (to the south):

<http://nguyen.cincinnati.oh.us/minh/osm/talk-us/braided_intersections/ryans_after.png>

I prefer the former approach, because the latter shows a false traffic 
island on the south side of the intersection. Imagine a pedantic 
navigation tool that tells a driver coming from Sycamore Grove to 
"keep/bear right and immediately turn left".

** Example B **

A divided Main St. intersects a divided Remick Blvd. Like everyone else 
here -- and unlike the "before" example Martijn provided -- I prefer a 
four-point intersection. But just to the east, Remick and a service road 
both become undivided at the same intersection. I mapped it as a single 
point:

<http://nguyen.cincinnati.oh.us/minh/osm/talk-us/braided_intersections/remick_before.png>

Your colleague redrew it as a four-point intersection, this time with 
two triangles:

<http://nguyen.cincinnati.oh.us/minh/osm/talk-us/braided_intersections/remick_after.png>

** Example C **

Originally, State Route 4 became undivided at an intersection with 
Walden Ponds Cir. (divided) and Fairham Rd. (undivided). SR 4, with a 
speed limit of at least 45 mph, was redrawn to shuffle about 25 feet to 
the left right after the intersection. But on major roads like SR 4, the 
landscaped median ends several hundred feet before the intersection to 
make room for a long left-turn lane. So I prefer to join the 
carriageways atop the left-turn lane, at a much gentler angle, without 
cutting into the median.

Since I first mapped the area, the median on SR 4 was extended well past 
this intersection, so no "braiding" was necessary:

<http://osm.org/browse/way/240893333>



In all three examples, my original rendition was called "braiding", but 
the ways were never intertwined as in the much-ridiculed TIGER data.

I don't know what specific issues you found with the way I'd been 
mapping. But I think routers should handle both styles gracefully, 
because mappers will intuitively gravitate towards one or the other, 
depending on what factors they consider. As intersections go, these 
examples are rather straightforward. On the other hand, I've mapped 
plenty of intersections where the traffic engineers clearly got carried 
away. If someone corrects me on one of those, I'm all ears! :-)

<http://osm.org/browse/relation/1843583>
<http://osm.org/browse/relation/1284976>
<http://osm.org/go/ZR~9kObUM>

-- 
minh at nguyen.cincinnati.oh.us




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