I just came upon this, which seems like sage advice for would be designers:
… Two aspects of a type are fundamental to its effectiveness. Because the common meaning of “legible” is “readable” there are those – even some professionally involved in typography – who think that the term “legibility” is all that is needed in any discussion on the effectiveness of types. But legibility and readability are separate, though connected aspects of type. Properly understood the two terms can help to describe the character and function of type more precisely than legibility alone.
In typography we need to draw the definition of legibility to mean the quality of being decipherable and recognisable – so that we can say, for example, that the lowercase h in a particular old style italic is not legible in small sizes because its in-turned leg makes it look like the letter b; or a figure 3 in a classified advertisement is too similar to the 8.
In display sizes, legibility ceases to be a serious matter; a character that causes uncertainty at 8 point size is plain enough at 24 point.
Walter Tracy, Letters of Credit