Tracks Effect with Alpha

Hello friends,
Im wondering if someone know how to make the effect as a lawn mower going and leaving the marks of cut grass or a car leaving marks in the sand. I think it will be use alphas to do that, but i dont know how.
I appreciate the help.

this is really nifty:
http://blenderartists.org/forum/showthread.php?t=159680&highlight=4weel+drive

Thanks Walshlg, i will look the .blend file and the manual to discovery how everthing work :wink: