My husband and I have just spent 3 person-hours picking up litter along a one mile plus stretch of quasi-rural road, both sides. We have been doing this twice a year for a long, long time, but things have gotten worse since the county repaved the road.
As for the 2 larger roads that it connects -- ugh!
Is there evidence that "Adopt a Highway" programs reduce litter?
I can see it going either way: (1) the sign that the next mile is adopted by Group X makes some people think and not throw out that empty plastic water bottle; or (2) they think "someone's going to pick this up so why not toss it?" And then there is (3) litterers don't think at all.
This may be off topic, but there is lot of beautiful outdoors marred by roadside litter.