When I was growing up in the 50's, this state was very conscious of littering..."Every Litter Bit Hurts" was the motto, and we heard it on the radio and TV...and "Don't Mess With Texas". Most of us older folks did not, and still do not, throw anything from our cars or dump trash on the road. Consequently, our roadsides were very clean. Plastic bags were not in wide use like they are today, so most stuff deteriorated after about a year or so.
Today's generation seems to care less; trash is somebody else's problem. I see a lot of beer bottles and beer cans, diapers, plastic bags and other such crap today...and even the volunteer cleanup crews seem to have disappeared.