I disagree. These eyesores are all over the place. The right of way is maintained. That means the town/county/state/whatever cuts back grass(to prevent an eyesore and keep down down mosquitos and pollen). Postings of any kind interfere with that.
During elections, the easements bristle with wire/ paper election poster. These are removed by the highway depart right after elections. Freedom of speech acknowledged and is temporary.
If the highway easement is for all to exercise free speech then the roads will look like a third world slum as not only memorials but people selling everything imaginable posting there junk.
I firmly beleive in free speech. Doesn’t mean you can litter or put up signs anywhere you want.
There’s also the relative importance of matters.
Typically one of these will appear due to some kind of careless driving.
“Be careful with your car” is a plausible public service message.