The updated Manual on Uniform Traffic Control Devices came with regulations requiring the upgrades of certain traffic signs. Included was the mandate to update street name signs to have upper and lower case letters which are easeir to read. Here's a good article from New York on the topic. Changing the street name signs in New York City alone is estimated to cost $27.5 million (US).
Thanks go to Matt Gauntt of SE3 in Illinois for an email about the topic!