I had the same experience in Spain... we were doing some climbing around Costa Blanca and parked/walked along a few random stretches of highway and they are all covered in trash thrown from cars. That was really surprising, I had a stereotype of EU being more civic-minded/civilized than the USA, and there isn't much litter on US highways that I've seen in similar remote-ish areas.

I just don't even understand what process operates in a head of someone tossing an empty bottle out of a car... sometimes, I feel like I have trouble recognizing them as a fully-formed human, more like someone who needs house-training, like a dog.