Why does gravel, broken glass and all crud end up at the sides of roads? Because every time it?s driven over it moves randomly towards the centre or towards the verge.
This happens every time it?s hit. Ultimately through, even if the road has no camber, random processes will move it to a point where it no longer gets driven over, namely the verge/gutter.
So, all litter and crud ultimately ends up where it no longer gets driven over irrespective of which side of a car it?s dropped out of.
Tim