I see, it can be tricky - looks like you may use couple solutions:
- for the round part - I would rather make the road simple - even one track divided by half (as I speak of materials) and put over it curbs as objects - less polygons for sure, no merging. It should be easy, since the road seems to be flat. You could draw couple textures painted differently (white lines, tyre marks etc).
- for junctions - you can create some other textures with junction picture and attach them in the places. Are the junctions going to be like 90 degrees?
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after couple deeper thoughts, I would definately create some objects for curbs and pavements - straight part, quarters of a cirlce inside/ouside and build from them the whole thing. That could take too much time to merge and to model everything from road profiles and also to fit textures could be hard task.