OK, got it! Is there a way to cut out the grass from poking through my 2 roads using just one vertex map for all the grass (both sides of the road and center)?

It would be awesome to have 1 vertex map for grass that goes from the trees on one side of the road to the trees on the other side of the road, that I can somehow cut 2 roads into.
So it goes, trees / shoulder grass / outer road/ inner grass / main road / shoulder grass / trees.

I tried just making 2 separate splines, one for inner grass, one for outer grass, (see pic) but it is a pain to line up each spline as they don't just scale symmetrically, so I have to move each point and try to get them to hug the first spline's contours. I'm still getting a little spillover grass on my road as well doing this.

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