Well, here’s the relevant quote from the AASHTO (American Assoc. of State Highway and Trans. Officials) Standard Specifications for High Bridges, 17th Ed., 2002:
(any typos are mine)
I had never heard the military angle before (I’m a bridge engineer) but I guess it’s possible. The TexasDOT Bridge Design manual, specifies 14’-6" as the absolute minimum for clearance with 16-6" being preferred if possible. And actually, nowdays, the districts have been wanting 17’-6" if it can be done. (what clearance is possible depends on a lot of things like allowable steepness, length of approach, number of levels to an interchange, superelevation, etc.