During the summer of 1967, when I was in college, a group of students from another university came to town on a regional accent and word use/definition survey. Their key was in lugging the big tape recorders of the time to record the accents. Many local folks worked at the college were asked to participate and the interviews were done in the library where I worked. There was also recordings done at the town library. The organizer said they estimated that this was one of the last years where they could get good regional accents and terms because of the on going influence of television.
In retrospect, they probably forgot to factor in the influence of aviation. As our forefathers migrated by wagon train, beginning about the mid 60's, families began to relocate to other parts of the US, bringing with them their local accents.
Looking at the study on this thread, one can imagine that within another 25 years, the next one will reflect the differences in pronunciation of Spanish ;-)