Another likely reason why overpasses got ugly, or at least less ornate, is that standards increased. Vertical clearances got higher, medians got wider, outer clearances got wider. The increased costs of building larger structures greatly exceeded the cost of decorative features. It's only been in the last couple of decades that we've adjusted our ideas as to what a highway overpass should cost to allow some aesthetic treatments, and even now they're mostly gewgaws affixed to AASHTO beam bridges.
