what is "counter space"?
Counter spaces are the white spaces formed by the black space of a letter. So the loop of the "a" as well as the gap between its stories, the inside of the "o", the hole in the "e" and the space between its tail and the loop, etc. If you compare FHWA fonts to Clearview, you'll notice the holes are quite a lot bigger in Clearview. (Compare the lowercase "e"s in "Street" on my example for instance.) That's because the small counter spaces in FHWA fonts were perceived to be washed out by halation on retroreflective signs.
The larger counter spaces make the font seem more open, which in turn makes me perceive it as a more friendly font than the more down-to-business FHWA Series fonts. AT&T's use of it in their corporate branding certainly doesn't help–while FHWA fonts make me think "Trust me, doing what this sign says is best for your continued well-being", the Clearview fonts make me think "Hey! You know what would be synergistic?! Keeping right except to pass! Oh, and cell phones!"
Of course, the old block lettering makes me think "Keep it under 45 or I will hit you with this stick."
Clearview in general is just not a particularly attractive font. I've seen plenty of custom state fonts ... some ridiculous (Maryland), others quite nice (New York) - why couldn't they have adopted the New York numbers??
You really need to put an index of these together someday.
IIRC, Clearview was not meant to be used in route shields. More often than not, it isn't.
It's more complex than that. The FHWA notice of provisional approval for Clearview only approved it for use as white text on a darker background. Studies had not effectively shown that there was a benefit for dark text on light background at the time (and in fact some studies conducted since then have shown that FHWA Series fonts might actually be more effective than Clearview at these types of applications). Dark-on-light has still not been approved, so really the only states that can properly use Clearview in state route shields are CA and MN, and neither of them seem too terribly interested in doing so. (Interstate shields are fair game.)
Of course Oklahoma in its infinite wisdom totally disregards FHWA best practices and sticks Clearview on whatever it wants.