10 yard line

I have been trying to simulate in post the yellow 10 yard line you see superimposed in football games. A simple project I have had is a marching band going down a parade route and the camera pans them.

I tried making a rotospline which made a black line across the road, and then applied a tracker to it so it would stay in place while the band marched over it, similar to what you would see in a football game 10 yard line.

It worked, kind of. But the uniforms of the marching band cut through the line, making it apparent that it was not part of the road.

Am I going about this the right way? Maybe I need to paint the line by hand so it tracks the road. But I am still unclear if I can even do this. I was wondering too if I can select certain colors to not apply themselves when they cross the plane of the line so the line will allow shadows and such to fall on it, making it appear as though it is actually on the ground.

Ideas?