It's easy for any agency to go way overboard on mandating specifications for elements in a design or even a typeface. It's also easy for people who aren't actually doing the design work to request things that don't add up mathematically.
If the FHWA was to mandate some specifics for a new type familiy for road signs they should mostly stick to the basics, such as ratio of lowercase characters in proportion to the uppercase characters, approximate stroke thickness (this varies in most typefaces) and not too much else.
am I the only one that finds Helvetica to be positively garish? never mind the tiny-to-moderate differences between Helvetica and Arial and the other ones in that family - I even think Clearview looks a lot better than Helvetica.
A lot of it is a matter of taste -and also which version of Helvetica is being used. The original 1950's version has some odd features. The "Neue" version, created in the 1980's, cleaned up a lot of things and added a bunch of new weights. The latest "Pro" version of Helvetica Neue added some OpenType features and some new compressed weights.
Helvetica can be a hazard to deal with in sign work. The different versions may all look the same, but they do have substantial differences. That can be a nightmare if you're having to replace trim-capped acrylic faces on a channel letter sign. If you don't use the exact same font file your finished sign parts won't fit the existing sign. There's not only varied versions of Helvetica. Clones like Swiss 721 and Nimbus Sans have subtle differences. These days too many people love squeezing and stretching fonts out of their original proportions. In the end it really helps to have the original art files used to make the sign in the first place.
Helvetica is a clean, neutral (if not plain looking) typeface. It looks a lot better than Arial simply because it is a lot more consistent looking than Arial. The problem with Arial is Monotype just couldn't figure out what Arial needed to be visually. They tried to fit all the metrics of Helvetica and mimic a little of its style. But then they tried copying visual cues from Akzidenz Grotesk and Univers. In the end Arial would up being a hodge-podge of ugly crap.
Not everyone likes Clearview, but at least Clearview has a specific visual style -unlike Arial.