How to mark a trunk way under construction changing to a motorway

Traffic is still going on the old trunk road, but is changed to a motorway during a 3 year period.
How should the trunk be mapped?

So it is not going to be a motorway after construction completes?

Sorry if my question is incomprehensible (English is not my native language).

The road is now a trunk, with physical separated traffic in each direction (2 osm ways). This trunk is now changed to motorway standard. But during the construction/changed, traffic is still allowed on the trunk, but with lower speed limits.

I would just map these lower speed limits for now and when the road is changed change the classification from trunk to motorway.

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highway=trunk construction=minor (3 years is long enough to map it)

optionally map highway=construction construction=motorway as a separate line if it is changing geometry

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Do you mean: highway=trunk construction=motorway?

Your comment says “construction=minor”).

no

For minor road-works (where the road in question remains open), use construction=minor (and don’t use highway=construction, but leave it at its default value).

https://wiki.openstreetmap.org/wiki/Key:construction#Highways

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Ok, thanks for the answer.

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FWIW construction=[anything] will break routing with OSRM’s default profiles (including the ones available on osm.org). Whether or not it should is a different question, but it will.

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So what do you suggest instead?

It is not something we usually tag on the whole length of the trunk road, rather you could map the local effects (e.g. sections with fewer lanes, speed limits), you also could map construction areas with landuse=construction (but I don’t think it is done a lot outside of cities, as it requires paying attention to the progress and update frequently), and finally you will promote the trunk to motorway when it’s opened.

Do you know where osm.org routing config and default OSRM config can be reported as buggy?

So during a 3 year period, the road will still defined as “trunk” in osm, even if there are lot of changes of available lanes, speed limits, access etc. Changes which is impossible to follow for me and I also assume not a good idea to document, because these are all temporal until the motorway is finished.

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In that period, will this still be the main connecting road between whatever destinations it already connected? Or will a large part of the existing traffic be redirected to follow a different route?

The old trunk road will be used, but gradually traffic will be directed to new lanes for the motorway. The motorway will be built almost on same place as the trunk is today.

so the motorway runs parallel, rather than the trunk structure is improved? In this case you can add the motorway as under construction and switch those parts that are ready when traffic uses them. For a highway=motorway the requirements are motorway signs/designation, until then, you can just raise the lane count of the trunk at these sections where it is already augmented, provided it is the same carriageway.

The new motorway is built by reusing some lanes of the trunk, and some new parts/lanes are added.

But after reading all comments on this matter, I don’t see any good solution, so I have reverted the trunk to the old state. Without any construction tag.