How much of this is just a learned behavior?

As an example, it is dead easy to spot common speed limit signs, mainly because there are so few speed limits. Even when my sight was failing, I could make out the speed limits at a distance with little trouble. Not because of some magic of the text, but simply from familiarity.

So, in an age where most of what we read is mixed case, it makes intuitive sense that we would be better at reading it. I fully realize intuitive sense doesn't translate to an explanation and is why I'm asking the question.