Highway Gothic, the font used on U.S. road signs for half a century, is on the way out. Clearview, created from scratch for readability, is on the way in.
The associated slide show is a lot of fun, as are the research details available on the font's website.
Until the 1920s, when the development of die-cut technology allowed for the shaping and cutting of thin metal alloy, signs were often idiosyncratic, with layouts and typefaces varying by city and region. But as the popularity and accessibility of long-distance road travel increased, so, too, did the need for coherent nationwide standards. Federally approved fonts first appeared in the 1935 edition of the Manual on Uniform Traffic Control Devices, the bible of federal road and highway standards that dictates the size, shape and placement of road signs.
In 1956, President Dwight D. Eisenhower announced his goal of an expanded Interstate System, and highway engineers worked quickly to fashion a rough alphabet by rounding off the square edges of the block lettering created during die-cut sign making. Today, there are six Highway Gothic typefaces in the official Federal Highway Administration series. Most prevalent on the modern highway is the fifth typeface in the family, Series E-Modified, and it is with this that Clearview is most often directly compared.