I grew up 20 miles west of DC and now live 20 miles east of DC. The difference in accents and dialect was quite surprising, so it’s interesting to see the clear delineation on the map. My 10 year old daughter’s friends point out she doesn’t have a “Maryland accent” which I find hilarious. Funnily enough it’s sensitized me to other mid Atlantic accents and now I can immediately pick people out from New Jersey (even DC transplants with quite subtle suburban Jersey accents).