Reduce edge count by merging ways with same tags

[easbar]

Sometimes roads are split into multiple OSM ways for no real reason and with our current approach this means our routing graph will contain more edges. A possible optimization would be merging such neighboring ways if the corresponding tags are identical (or at least the subset of relevant tags is identical).

It is quite easy to count the OSM nodes that only connect two ways (rather than being 'real' junctions), see here: 10d1a19

For Germany I got:

end: 3640291, junction: 12725162, connection: 2901432, intermediate: 52455693, all: 71722578

which means there are 2.9mio 'connection nodes', which only connect two OSM ways. If we merged all these nodes we would save 2.9mio edges, so 11% of the total 25.9mio edges we create for this map (no ignored highways). But of course in reality we won't be able to merge them all, because not all of them are connecting ways with identical tags and sometimes they might use the same tags but we still don't really want to merge them, for example at country borders.

If we could merge every second connection node we would be able to save around 5% of edges (but not sure if this is realistic). We would also lose the ability to assign a single OSM way ID to each edge. Overall I don't think this optimization will be really worth the effort.

[otbutz]

for no real reason

The OSM API has a limit of 2000 nodes per way. The number of ways with +1.9k nodes isn't that low:

https://tools.geofabrik.de/osmi/?view=geometry&lon=21.28869&lat=43.03894&zoom=4&baselayer=OSM%20Carto%20(OSMF)&overlays=long_ways

In addition, most mappers avoid large ways because editors like iD do not handle them well.

That said, both of these problems don't affect GraphHopper. We could even solve the missing antimeridian support that way. see #2745

[easbar]

Ok thanks the node limit is of course a valid reason to split the way.