The text that did appear on these early signs [1910 and a bit later] was largely hand-painted and all in uppercase, simply because no one could effectively draw lowercase letter forms by hand.
Explaining the task of drawing letters, Meeker said: “All capital letters are either straight lines or curved lines. The worst-case scenario is pretty much ‘B.’ ” But lowercase letters, maddening knots full of arcs and curves, present a more serious challenge to the Sunday-afternoon road-sign painter.
This sounds hopelessly weird. The curves on small letters are tighter than on large letters. A few small letters like the double-looped g and the a with a top hook are complex, but they aren't that much more complex than large letters. I find it hard to believe that a B is harder to write than an S, though both can be tricky. So are R, K, and X.
Back in the 80's, those big paper signs in supermarket windows were hand-lettered by brush, and extremely well, imho. Maybe knowledge of how to write small letters had advanced dramatically in 70 years, but it seems unlikely. Maybe the early sign makers happened to include a person or two with odd quirks about small letters. Meeker certainly sounds like he's peculiar on the subject.
I haven't researched the period. I work a good bit smaller than they did. So I might be wrong, but that quote runs opposite to my knowledge and experience.