1980s kid & have lived in Texas my whole life. The slogan had a instant impact on behavior. Litter was a pretty bad problem. I remember it was common behavior to just toss stuff out the window while driving. Drink can/bottle, wrappers, cigarette butts, pretty much anything; some people would even dump trash on the side of the highway. It got a lot better and fairly quick so I believe the 70%+ stat mentioned.
While I'm on this train of thought. Most/many trucks in Texas had a gun rack with a rifle or two hanging across the rear window. Nobody wore seat belts until kids my age were taught to go home and nag their parents through PSA/Sex Ed/DARE style school lessons. It ended up being pretty effective.
