via-way no_straight_on breaks no_u_turn

[willwhite]

ron_cowan_parkway.osm.zip

This OSRM file has a no_straight_on relation:

This relation appears to prevent the the following no_u_turn relation from working:

Testing this locally, the no_straight_on relation works:

But the no_u_turn relation does not:

[MoKob]

The problem here is that the two relations together form an implicit multi-via-way restriction.

The not working restriction for no_uturn is conceptually the same as saying from A via B and D to F.

The no_straight_on is telling OSRM that coming from A and via B that C isn't allowed, but D sure is.
Due to the created path from the no_straight_on, we can bypass the restrictions from the no_u_turn.
To add the no_uturn restriction into the mix, we would now need to detect that A via B to D is allowed and disallow B to D and only allow B to E, making it implicitly to a multi via-way restriction (via B and D).

I would consider this bad modelling, though. Instead of two interacting via-way restrictions, a single via-way restriction can model the required restrictions.
The, in my view, better modelling would be a single via-way restriction of the form:

from B, via D, to E: only_left_on.

[MoKob]

Via-way restrictions are also tracked over #4439 as a feature-request for multiple via points.

[MoKob]

I have to correct myself. The proposed modelling wouldn't work and the original modelling is correct. The support is missing due to the implicit multi-way restriction needed. This requires an implementation of #4439 to be fixed fully.

Since no road is coming in from below, a local work-around could be to add a node-restriction d->e as only_straight_on. The additional node-restriction should work and would model correctly what we see at this intersection.

[Andygol]

According to this recommendation https://desktop.arcgis.com/en/arcmap/latest/extensions/network-analyst/turns-in-the-network-dataset.htm#GUID-5BC5E313-1444-4471-954F-9D1068FAD01A

The first or last edge (any exterior edge) in the turn cannot be an interior edge of another turn.

we have incorrectly modeled no-U-turn restrictions, where the same way plays different roles for the TRs:

• to-way for one no-U-turn and
• from-way for another no-U-turn,
• via-way for no_straight_on and no_u_turn TRs

All interior ways should have only role via but in the example we have a mix of roles for one interior edge.

for right modeling, there is required that no-U-turn TRs will have 3 via-members

[github-actions]

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