When I was attending UBI (United Bicycle Institute) in Ashland, Oregon in 1984, I borrowed UBI's best bike (they loaned you a bike if you didn't trust the airlines to take care of it) and struck out for California. The roads in Oregon were clean and I saw very little litter of any description.
When I crossed into California - it changed dramatically. There was discarded everything along the roads. TV sets, dryers, cases of beer (empty) cans...just yuck! I wheeled the Univega I'd absconded with and headed back to Ashland. I wished I'd taken a camera.
The prevalence in one place, but not another, can be attributed to the social mindset of the people who live there. Apparently people in Oregon actually care for their environment and don't want to be the first one to screw it up. California seems to tell it's citizens that they live in a black-topped garbage can. Where I live now I see the same thing. In Vermont there is very little roadside trash. We don't allow billboards advertising ANYTHING by the side of our roads and highways. But go south into Massachusetts - open-air trash can. They had a saying in Oregon:
Don't Californicate Oregon.