Suppose some apocalyptic event occurred such that the population

[raccoon]

Of the US was greatly reduced and society/technology was like in England in 1200’s or so and people didn’t travel much.

How long would it take before people in NYC and SC, for example, wouldn’t be able to understand each other’s speech?

[virgogal]

1. LOL...they often don't understand each other now.

[raccoon]

2. I knew somebody would say something like that!

But I understand trump. I mean, I understand the WORDS he says. What he means, if anything, is another story.

[Historic NY]

3. I think it already did..

[link:https://www.businessinsider.com/22-maps-that-show-the-deepest-linguistic-conflicts-in-america-2013-6?op=1#americans-cant-even-agree-how-to-pronounce-crayon-4|]

[raccoon]

6. Thanks for the link. I guess I'm the only person who calls

The interstate “the interstate”. Or it’s number.

[Historic NY]

8. I blame the invention of the conestoga wagon & iron horse...

and then it just went wild.

[The Velveteen Ocelot]

4. I already can't understand some southern accents.

Used to work with a guy from GA and I was always asking him to repeat what he said - half the time I couldn't understand him. But if there was no communication at all between populations I'm guessing it would take at least 100 years of isolation before different versions of English significantly diverged.

Not to get too nerdy about this, linguists think it took about 200 years after Germanic tribes migrated to Britain for Old English to become isolated from other Germanic languages; but even by the time the Vikings started invading Britain around 700 Old English and Old Norse were probably still somewhat mutually intelligible.

[trackfan]

5. My wife and I grew up about 10 miles from each other in Los Angeles,

where, according to the map, cray-awn is the the local pronunciation. I say cra-ahn; and she, inexplicably, says cran.

[democratisphere]

7. I don't understand a word out of Leningrad Lindsey Graham's mouth.

So, INSTANTLY.

[OkSustainAg]

9. Remember the "Hillbilly" dialect

Is left over 17th century Scottish English. E.g. "Victuals" pronouced vittles.
I also don't see us going back before Victorian or old west tech no matter what catastrophe. Most of my friends and I are tinkers trying to come up with simpler recycled farm tools and equipment.