Good question. Why are we putting roads into relations? I'm just doing
it because everyone else is.
I'm not sure myself. Possibilities are:
1. Hint to routers for good path to follow over long distances without
searching all roads. In practice, the latest highly optimized routing
algorithms probably don't need this; they do better by looking at road type,
maxspeed, etc.
2. High level view of roads on map at low zoom number levels. In
practice, some guys implementing low number zoom views won't touch route
relations because they're for routing.
3. Hint to GPS turn by turn instructions to know that multiple "ways" make
up the same route, despite having different names / refs, thus avoiding
needless chat like "Continue straight across the Hoffman Bridge, then
continue straight on route 45..." when it's all part route 45.
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