The way I see it, you have to use mixed materials for each part. For the sidewalk, use a mix of grass and your concrete texture, and drive it with a function that will let the grass appear on the out side of the pavement, like a b&w bitmap, and use the smooth blending strip. Same for the road material, mix it with the sidewalk material, and drive the proportions by the same bitmap, and then use the smooth blending strip to your liking. Might work!