Trucks and busses tend to roll so that the upper part of the vehicle is over the barrier. Make sure your ornamental railing is light enough that these vehicles won't snag on it.
You could contact the NY State Bridge Authority and ask for the details they used on the Kingston-Rhinecliff bridge a few years back.
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"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail."
Wm. Phelps Eno, ca. 1928