If FHWA would just go to Series E instead of Series E(M), Clearview would have no leg to stand on re: stroke width, hole size, and halation.
I don't think that is actually true. I am not aware that Series E has ever proved to have a unit legibility greater than that of Series E Modified. In comparison to Clearview (at either the 5-W or 5-W-R spacings), its lowercase loop height is a smaller percentage of capital letter height.
I am actually not sure that stroke width, hole size, and halation are all that important in determining the relative legibility of Clearview in general, although they are probably the major driver of the enhanced benefits for older drivers. I think the enhanced lowercase loop height is probably a more important factor for the motoring population as a whole. For Clearview this is 84% of capital letter height, versus 75% for the FHWA alphabet series, and if you divide the former by the latter and subtract 1, the size difference you get (12%) is pretty close to the 11% increase in unit legibility that is claimed for Clearview 5-W-R over Series E Modified (in the "equal footprint" scenario).
FHWA's current position with regard to Clearview (as expressed in the Clearview FAQ) is that Clearview produces a 5% legibility increase for older drivers, which is a composite of the legibility effects of using Clearview
and microprismatic sheeting. In contradistinction, FHWA claims a legibility increase of 6.3% for upgrading to microprismatic sheeting alone. This implies some clawback of benefit for using Clearview (in other words, legibility would probably be even better if the microprismatic sheeting upgrade were accompanied by use of Series E Modified). This flatly contradicts the earlier finding (trumpeted off the rooftops by Meeker and Associates) that Clearview produces an 11% legibility increase in the "same footprint" scenario (using Clearview 5-W-R), or 21% with 11% increase in sign panel area when Clearview 5-W is substituted for Series E Modified. However, for FHWA it serves to justify its position that Clearview will not be added to the
MUTCD because it is an "equivalent alternative" rather than a net improvement.
Bottom line: if you are a highway agency, you are better off keeping on trucking with Series E Modified rather than messing around with Series E in an attempt to counter the claimed advantages of Clearview.