[Talk-GB] Where does a motorway start?

[Andy Allan]

On Wed, 3 Aug 2022 at 09:19, Andy Townsend <ajt1047 at gmail.com> wrote:

> 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 that, in terms of OpenStreetMap modelling, the motorway
link starts at the point that the roads diverge.

I can sort of see why you could say that it starts at the chopsticks
sign, but honestly it's not a robust logic since you can only place
the pair of chopsticks signs in certain locations. It's not physically
possible to place them where the gantry is, for example, for safety
reasons you can't put them in the painted gore, nor can you place them
between the lanes where the solid white line divider starts. So the
precise location of the chopstick signs is not super relevant.

In terms of "where can non-motorway traffic go to" then I think the
point of no return is where the solid white line divider starts, so
that's where I would start the motorway link, at the latest. Given the
further fuzziness on where the join is modelled in OSM, then I lean
towards the whole thing being motorway_link.

https://www.openstreetmap.org/user/daniel-j-h/diary/43148 has some
good diagrams for what the different parts of a junction are called.

Thanks,
Andy