There are some stretches in the UK where rubbish collects by the roadside in large amounts, tangled in hedges etc, presumably because winds concentrate it there.
Even in my area, the most visible litter is along waterways. Regardless of where the trash originated, it will pretty much always find its way to waterways. Which might be part of why it's rare for you to see along the highways unless you have a very active group working off their community service hours.