If we want to have some sense of what routes are "related" to each other when not in the same connected route, I think we need to augment the data with information about what we mean by that.
So what
do we mean by that?
Obviously there's a few reasons why routes that are part of the same single route, but cannot be connected as one route due to discontinuity caused by:
- taking a ferry
- crossing over a border where its a differently numbered route
- sections are not yet constructed
- cases like the E68 at the Romanian/Hungarian border - same route, lack of international communication as to where it should run (or rather, Romania's dislike of signing E roads on A roads)
And then we have stuff like I-35 with W/E routings and same-numbered (no directional suffix) spurs/loops (eg the E55 is signed both ways around Berlin). The route isn't quite the same single route, but they clearly are one route. Likewise bannered routes - it's very clear that they are part of the same route as the vanilla highway.
Then there's more debatable stuff...
Various places give routes a number, but use different shields/prefixes to reflect to standard/status - eg Australia M/A/N/B/C1, Poland A2/S2/DK2, Ireland M3/N3, or Israel F4/H4. I'd say these are related routes.
There's the question of whether parent-child relationships would be related enough - Interstates, sure, but US routes? There's also, say, parent/child stuff in France - N1xxx is typically future Nxxx (with zero-padding), but sometimes it's former Nxxx, and D Roads sometimes are similar. There's a bit of a can of worms here.
Further to that N1xxx - the former Routes Nationales in France are obviously meant to be a continuous route but different departments' different methods of numbering means they change numbers a lot, but they are all numbered related to the original route D1xxx, D6xx, D9xx, etc.
And, perhaps at the extreme end, are collections of routes that don't share numbers or names - eg the routes in Yellowstone NP, or the Mourne Coastal Route in N Ireland + the '
scenic loops' off it.