Blocking u-turns

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Blocking u-turns

Postby AggieJM » Wed Dec 28, 2011 4:55 pm

What is the easiest way to prohibit a "u-turn" at an intersection on a divided road?

This is the location in question:

https://www.waze.com/cartouche_old/?zoo ... FFFFFFTTTT

In this example, I could just block the turns on the small connecting road section. But if there were parking lot roads (or actual side streets) at that intersection, that would potentially block the allowed turns.
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Re: Blocking u-turns

Postby bgodette » Wed Dec 28, 2011 11:47 pm

AggieJM wrote:What is the easiest way to prohibit a "u-turn" at an intersection on a divided road?

This is the location in question:

https://www.waze.com/cartouche_old/?zoo ... FFFFFFTTTT

In this example, I could just block the turns on the small connecting road section. But if there were parking lot roads (or actual side streets) at that intersection, that would potentially block the allowed turns.

There's a few solutions:

The easiest is to create a Mapcat Bowtie, collapse everything to a single junction, disable all turns on the junction, then manually enable each allowed turn and fix up the geometry to look reasonably ok.

A second solution is to use dedicated turn lanes in combination with appropriate turn restrictions to prevent the u-turn, this works well with large medians and wide streets with actual dedicated turn lanes. For a full on dual-carriage to dual-carriage intersection you end up with a Crossed Box. This actually looks reasonably good on the client as there's no pinch as with the Bowtie and the intersection is just one blob.

A third solution, probably most error prone and confusing, is to used stacked two-way roads with appropriate turn restrictions. You have to temporarily modify their geometry enough to set turn restrictions correctly, but it does work.
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Re: Blocking u-turns

Postby bgodette » Thu Dec 29, 2011 9:13 pm

PhantomBlack wrote:
bgodette wrote:A third solution, probably most error prone and confusing, is to used stacked two-way roads with appropriate turn restrictions. You have to temporarily modify their geometry enough to set turn restrictions correctly, but it does work.


I changed my intersection to this option I like the look over the bow tie. I tried to do like this at first but had problems getting the roads to stack. After reading your post I tried again and figured it out not to hard if you draw the second street out to give you a triangle while you adjust turn restrictions then up it's level and drop it over the original street.

Just had a look at what you did, and that's exactly what I was describing. I try to avoid doing that, but it does look clean and routes correctly.

There are a few side-effects to this method:
Automatic and manual reports will be ambiguous, and the warnings in the client may or may not appear depending on what route is being used.
Starting a route on such a setup will also be random as to which street the client snaps to.
There's two roads with independent historical data so there's the potential for odd routing results.
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Re: Blocking u-turns

Postby bgodette » Fri Dec 30, 2011 3:39 pm

fvwazing wrote:Someone should make an effort in categorizing different types of crossings... however, for the T-crossing with three divided roads (call it a 2-2-2-crossing?), there is an elegant solution. Meet:

"The triangle of no U-Turn"

Not only does it use 1 segment less than the original, it also works with only 3 turnrestrictions. All routinginstructions as expected and no hidden segments.

Yes this method works really well for T junctions of two dual-carriageways and there's no concern about the angles. Of course if we were ever given a new polygonal junction object that worked like the current junction node (eg it connects A->B...Z) all these map tricks would be a thing of the past.
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Re: Blocking u-turns

Postby CBenson » Mon Jan 30, 2012 4:05 pm

My understanding is that if the angle between the entering street and the exiting street is less than 135 degrees (that is deflected more than 45 degrees) you will get the turn instruction. Yours looks to me to be right around this angle. Bowties do need to start fairly far from the instersection in order to avoid turn instructions.
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Re: Blocking u-turns

Postby fvwazing » Fri Dec 30, 2011 11:51 am

Someone should make an effort in categorizing different types of crossings... however, for the T-crossing with three divided roads (call it a 2-2-2-crossing?), there is an elegant solution. Meet:

"The triangle of no U-Turn"

Not only does it use 1 segment less than the original, it also works with only 3 turnrestrictions. All routinginstructions as expected and no hidden segments.
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Re: Blocking u-turns

Postby gettingthere » Thu Dec 29, 2011 9:19 pm

My opinion PhantomBlack, I don't like these stacked roads. Too difficult for future editors to understand what is going on here and too hard to work with the turn restrictions without moving the roads around.

'Mapcat Bowtie' approach is easier for other editors to understand when they come across it and routes well.
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Re: Blocking u-turns

Postby gettingthere » Thu Dec 29, 2011 9:55 pm

PhantomBlack wrote:I agree it can confuse future editors although unlike the bow tie it is well hidden and may not draw the attention.


We can assume eventually someone will be there editing. Especially if any user initiated Update Requests get submitted or a map problem pops up. This will confuse someone thoroughly until they figure that two roads are stacked up with different turns permitted/denied from each.
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Re: Blocking u-turns

Postby gettingthere » Mon Jan 30, 2012 4:04 pm

PhantomBlack wrote:I will probably just replace the bow ties with dual cross streets and hope for the best any thoughts?


I would suggest altering the bow tie geometry. You won't be able to restrict the U-Turn with the dual cross streets.

I have altered the bowtie at the intersection so that it should not give extra turn instructions. You had extra geometry points that were likely causing the issue.
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Re: Blocking u-turns

Postby gettingthere » Mon Jan 30, 2012 4:12 pm

The Waze provided route on the Update Request (with the prompted turn turn at the intersection vs. no turn instruction) is exactly how the bowtie was set up prior to me editing it several minutes ago. To CBenson's point, I just backed up the first geometry node a little bit further from the intersection.

I'll leave the Update Request to the OP to click on Solved.
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