According to the FHWA for highways on the Federal Highway System the minimum clearance is 14' with the desirable height of 16'. Originally those heights were lower but were raised and had to be met when there is reconstruction on those routes. Either the roadway has to be lowered under a structure or the structure raised over the Federal Highway. Some of this change had to do with the under-clearances required for the movement of military vehicles. Apparently the highway where this happened was not on the Federal system or there has never been any reconstruction. I remember this information from my DOT designer days.
