Took me 5 years to do this font

The designer Phil Baines used a strategy of subtracting elements from each character to design a difficult-but-possible-to-read font in the 90s called You Can Read Me. It was actually inspired by an experiment by Brian Coe which was reproduced in a book from 1968 called The Visible Word by Herbert Spencer. You get the idea--the development of typefaces is quite a slow moving activity which owes a lot to the subjective judgements of idiosyncratic personalities, usually male. Over the last century there has been a minority of practitioners who strive for scientific rigor, though.

"[The Visible Word] is a major contribution to legibility studies and presents a summary of over one hundred years’ worth of investigations by one of the UK’s most influential typographers. Spencers extraordinarily detailed 24 page bibliography is testimony to his investigation. The visible word is part of a programme of research into the readability of print in information publishing. In this book legibility is explored with equal thoroughness and objectivity. Resulting in the fact that people read most easily the kind of lettering they are used to. Although this may seem obvious in todays comprehensive documentation of the topic, much was learned from his lucid demonstration." http://www.designers-books.com/the-visible-word-herbert-spen...

I'm not sure where to find "today's comprehensive documentation" of legibility and I suspect that it is a little tedious and empirical...

But as an example, here's some more up-to-date research+bibliography in this article:

http://gaultney.org/jvgtype/wp-content/uploads/BalanLegEcon....