Sorry for the delayed response. I've been incredibly busy the past few days. Trying to catch up.

Regarding Zeffy's Illinois example, I don't think it's very fair. The two legends are sized at the same horizontal width rather than the same cap letter height. Normally sign letters are specified according to capital letter height, not overall legend width.

Here is another example of the "Illinois" legend set in Clearview Highway 5W and FHWA Series Gothic E. But in this example both legends are set at the same capital letter height rather than both legends set at identical lengths. Clearview Highway has a significantly larger lowercase letter range. The Clearview lowercase letters are around 7/8 the size of the uppercase letters. Most of the FHWA Series Gothic lowercase letters are just under 3/4 the size of the uppercase letters. And that actually makes their letters insufficiently small in terms of the latest MUTCD requirements of lowercase letters being at least 3/4 the height of the uppercase letters.
At any rate, Clearview Highway done properly is going to be quite a bit more legible than FHWA Series Gothic, but it is going to come at a cost of longer sign panels.
Which glyphs, in particular, do you feel "need to be fixed"? I have heard from several people that the FHWA fonts are "clunky" or "outdated/less modern" or "ugly", but I've never really heard specifics as to why they feel that way. I will agree that on most series, the lowercase "w" is ghastly, but that's the only character in particular I have noticed as being particularly ugly. It should be noted that while the uppercase characters were designed back in the 1940s, for all series other than E(M), the lower-case letters were designed much more recently (I want to say 2000 or so). I understand that "modern" fonts usually incorporate variance in stroke width as Clearview does, but I would guess that a consistent stroke width is more legible.
Glyphs I don't like in FHWA Series Gothic.
In the uppercase range I think the "G" is downright terrible. It stinks in the Series E weight and it only gets worse as the weights get more narrow. The "O" is funky looking. Honestly, the uppercase "S" the only curved letter in FHWA Series Gothic that looks the slightest bit attractive at all. Normally a typeface should retain some harmony in its angled A, K, M, N, V, W, X, Y and Z characters. FHWA Series Gothic has none of this harmony. And in the area where it's most expected, between the V and W characters, the angles have no resemblance to each other. The "v" and "w" relationship is better in the lowercase range, but too many of the lowercase letters have all sorts of clunky, crooked looking issues. Overall, it's just a butt-ugly looking typeface. Familiarity and nostalgia are the only factors helping it.
Some people here like to take pot shots at Font Bureau's Interstate type family, but all of its curved letters are way better drawn than those in FHWA Series Gothic. The V & W relationship still kind of stinks. But even Interstate's numerals look better. Unfortunately Interstate is not a workable substitute for FHWA Series Gothic. It's not so much to do with the typeface having improper spacing as much as it does with it lacking all the appropriate widths from very condensed to semi-extended.
In terms of aesthetics, Clearview Highway easily has FHWA Series Gothic beat hands down in terms of glyph harmony. While there is a lot of nostalgia reserved for the 1950's era typeface, technically and artistically it is inferior to Clearview.
The numerals are the only thing going for FHWA Series Gothic. And even there, not all the numerals are all that great. The "2" is pretty weird on that curve down towards the flat terminal on the bottom. The "6" is pretty odd looking at any weight. The "8" is out of balance with its squished top oval and the round bottom oval.
Sometimes I think I'm the only one who does not become apoplectic when Helvetica or Arial or some other font shows up on a road sign.
I use Helvetica Neue a LOT on various kinds of institutional and way-finding signs. The wealth of weights and widths combined with its very clean and neutral appearance just make it work for so many things. Perhaps that's one reason why some designers hate Helvetica with such a passion. Designers try a lot of different typefaces to see if they'll fit in a project and then after all that experimentation they end up gnashing their teeth and reverting to Helvetica. The work has to get done already and Helvetica works for so many things.
But with that being said, Helvetica doesn't work for traffic signs. The characters are too "closed" and the spacing is just too tight.
On the other hand I just detest Arial. Even though the latest version probably has over 1,000 glyphs I still can't stand how it looks. It's just a much more ugly looking typeface made to fit Helvetica spacing proportions.
IMHO, those states who already spent the money converting to Clearview ought to be able to keep using it if they feel like doing so. Otherwise FHWA Series Gothic should be the default traffic signs typeface.
However, if the powers that be want FHWA Series Gothic to truly perform at Clearview levels they will need to completely redraw that type family and deliver new "cuts" of it. The current cuts kind of suck. And they're missing a lot of features. Add to that the fact the lowercase letters fall just short of that 3/4 cap letter height requirement. If they want the lowercase letters to be something like 7/8 the cap letter height like Clearview they'll really have to redraw the whole thing. If they go to that trouble they might as well extend the character range to include a variety of fraction sets, native small capitals, foreign language support and more like most other contemporary type families are doing now.