They eat your frame rate. I have them, but mostly I use streets with sidewalks joined to streets without at the intersections. The unmarked roads make good intersections.
I lay the main street to the start of the intersection, lay a small strip of unmarked, then continue with the regular spline.
Then I go back and do it again from the other direction and then set all four splinepoints to the same height and line them up to join any sidewalks at the corners.