Unrelated to everything you've said thus far, J N Winkler, what would you think if the FHWA granted interim approval of the Transport typeface? I see you've spent time in England.
I wouldn't expect FHWA to approve the Transport typefaces for use on US traffic signs. TTI did a comparison of Transport, Series D, and an early version of Clearview about 15 years ago (the study was done long enough ago that only the abstract, not the full report, is available from the usual online free sources). Transport was found, by a small margin, to have the worst legibility performance of the three. Even if we took this result as a "floor" for the performance of Transport and found there were situations where it could outperform the FHWA series, I still don't see FHWA issuing an interim approval for it because it essentially offers "equivalent performance" to something that is already approved rather than being clearly better. This is the hurdle that Clearview failed to cross to progress from interim approval to inclusion in the
MUTCD.
Transport works well in its native (British) context largely due to ways in which British signs differ from the
MUTCD as implemented in most US states. It is not available in multiple levels of condensation and in most situations it is used in sentence case rather than all-uppercase, so in urban settings British drivers tend to be better served by signs in Transport than American drivers are served by signs which use FHWA Series B, C, or even D. On motorways and other high-speed roads, design of direction signs assumes that drivers should be able to pick out the specific destination they need quickly, rather than (as in the US) that they should be able to read the entirety of the sign at least twice in the time that it is visible. Transport thus only has to be able to support visual search of placenames, rather than double-reading of every legend element in full, so some deficit of performance compared to the less-condensed FHWA series (D, E, and E Modified) is tolerable. In situations where legibility performance is especially important, British standards allow Transport to be deployed at
x-heights of up to 400 mm (15.75 inches), which is slightly bigger than the 20" UC/15" LC Series E Modified we use in similar cases.