This is an old article from September, but as I was driving past some old lower case letter signs at exit 14 on I-135 today I thought about this. I don't remember this being posted before and now as I'm noticing more lower case versus upper case signs, I do agree the all caps are much easier to read.
http://cjonline.com/news/local/2010-09-29/city_abiding_by_federal_mandate