I would like to add the visual of tire tracks in some gravel, so I would think in areas where tires consistently roll, the gravel will be slightly indented and maybe smaller (if the tires push away the larger pieces on top). You can see this effect really well in Peter Guthrie's Quest house (linked below).http://www.peterguthrie.net/#/quest/
My first thought would be to add two long vertical planes that would represent the paths of the tires and somehow have the Forest Pack decrease the z height, scale, or both of the gravel within 6" or so of these planes. That is just my first thought, but perhaps there is a better, more proven method.
I appreciate any input!