World population has more than doubled since 1960, so there could be both a decline in the rate at which each person litters and an increase in the net amount of trash we see at the same time.
Also, smoking is down, and most of the people I see who are smokers have a much more relaxed attitude toward dropping unwanted things on the ground (everything, not just butts).
That said, I live in a planned, environmentally conscious community in one of the wealthiest and best-educated counties in America, with a community association aggressive about policing up trash. Absolutely optimal conditions for the minimum amount of litter.
…and there is always trash visible in every square yard of woods, bushes, and treelines, and a fair amount rolling/blowing around openly in streets and gutters.