Here is a link where you can see some pictures, and read about how they were done. I believe she used stencils from BAMS, aka Bad Ass Mini Stencils, to do them.
http://www.sweetsugarbelle.com...stencils-guest-post/
Hillary, aka The Cookie Countess did a guest post for Callye, aka SweetSugarBelle covering this exact topic, and even made number shaped cookies with tire tracks.