Compare this image of New York City street signs to one of Seattle street signs.
I have to say that I agree; it is much easier for me to read signs with upper and lower case letters than just upper case letters. I think the credit goes not just to the combination of the two cases, but maybe even more so to the ascenders and descenders of the lower case letters. If I am looking for a street that ends in the letter "y" it is much easier to identify the street from a farther distance when I can spot the descender of the lower case "y" than when the letters are in all caps and the "Y" is the same height as all of the other letters.
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