Loads of litter
The answer would seem to include stringent enforcement of litter laws, and better education in schools – with, perhaps, schools helping to clean the mess their pupils create. (You can tell if a school is nearby by the increased amount of litter).
However, I simply don’t believe that non-urban road litter is simply caused by irresponsible road users chucking rubbish out their windows. On several occasions, I have seen lorries taking rubbish to landfill, leaving in their wake a plume of litter.