This is not very realistic indeed, and very small scale. I don't quite get your question, but I think it's best to paint a mask in some software and import it. Easiest to get exactly what you need. Procedurally; did't I provide you with a 'closed circuit' file quite some time ago (2013)? But that is harder to alter, unless you can use the idea Jon (Hetzen) came up with for my Limes road straightening, with the build vector... that's somewhere here to be found.
