[Talk-GB] Where does a motorway start?

[Paul Norman]

On 2022-08-03 1:18 a.m., Andy Townsend wrote:
> Hello,
>
> The DWG has recently had an email from someone asking about some 
> recent motorway edits.  These have been in various places, so I'll 
> pick an example local to me, where the A64 joins the A1(M): 
> https://nrenner.github.io/achavi/?changeset=123673170 .
>
> The edit moved the start of the motorway from where the roads diverge:
>
> https://www.mapillary.com/app/?focus=photo&pKey=2478290038984023&lat=53.856169399972&lng=-1.3365381&z=17&x=0.48881009070232434&y=0.6403049169362514&zoom=3 
>
>
> to just after the “chopsticks” sign visible in front of the lamppost 
> at the right of the carriageway in edit mode at 
> https://www.openstreetmap.org/#map=18/53.85550/-1.33895 :
>
> https://www.mapillary.com/app/?focus=photo&pKey=493444768591097&lat=53.855654360067&lng=-1.3384409007607&z=17 
>
>
> I suspect that this is more a question of style than anything else - 
> no router is going to send non-motorway traffic down a oneway section 
> of trunk link which only exits to a motorway link. Personally, I'd say 
> that the motorway starts where the roads diverge, but I didn't feel 
> strongly enough to comment about it when I saw another of these 
> changes a few days ago.  I said to the DWG's correspondent that I'd 
> ask here, so here this message is.

I would say this is fairly clear. There is a link road between a trunk 
and a motorway, and it should be tagged as a motorway_link. In some 
cases it can be difficult to say what a link is, but it's clear in this 
case.

For routing, as you've said, it doesn't really matter much. Because 
there's no other roads connected where the two link sections join, they 
can be combined into one (along with the ones closer to the motorway) 
and the only difference is going to be a tiny cost difference. If you 
were trying to route to or from a point on the section in question with 
a mode of transport that is allowed on trunk_link but not motorway_link, 
it could matter, but in practice the access restrictions are the same 
for the entire link.