Well, every time i drive through the reservations in my area, the sides of the highway are completely covered in empty beer cans, broken bottles, and trash.
I'm sure that there is alot of trash on the roads, just because its on an indian res doesnt mean that it was put thier by the native americans. Those roads are public roads and since people drive on them and the police have little to no rights on those roads there is nothing anyone can do about it. The native americans have enough trouble with society and yet society still throws trash on them (more ways than one).