They could protect their copyright/patents plenty of ways (free or cheap licensing to selective agencies, for example).
The only way I see that concept working is if the Clearview fonts were licensed to state DOTs, say through a sign software vendor like SignCAD or Transoft, with the condition that the DOTs could distribute the fonts free of charge to their approved sign contractors and fabricators.
Remember also that, unlike TrueType or similar software fonts, Clearview is a font intended specifically for use on highway signs on public roads. It disturbs me that the developers of the font, who were obviously paid by somebody to do the inital development and research, still feel the need to collect a fee everytime somebody wants to use Clearview on their signs. If they truly did the research for FHWA, and got paid by government grants (even if those grants were indirect payments through universities), then one could argue that the final work product (Clearview) is actually the property of the Federal Government, and not that of the initial developers.
I design BGS (and LGS) signs, among many other tasks, for a living and, yes, I do get paid for that. But, using this same model the Clearview developers are arguing (pay me for my work again, even though I've already gotten paid for doing that same work), it stands to reason that I should also be able to collect a royality (say a nickel a driver) from every driver who passes under (or by) one of my signs.And don't worry, I value my job too much to even dream of suggesting that last idea to my employer. Still, it's an interesting thought.