There is an approach that has been used for decades to keep people from falling down deep window wells next to buildings that could be used for the gap between twin road bridges. Think of it as a horizontal fence.
It consists of galvanized chain link fence fabric that spans the entire width and length of the gap. It is fastened to and rests on a grid of galvanized pipe with sections welded every three or four feet that are horizontally perpendicular to the roadbed.
It is hard to imagine having to pay $50,000, as described in a Dec. 4 article, for a vertical fence on one side of a gap. The approach described above uses half as much material and has the advantage that it catches people jumping from either roadbed or the median.
Since these sections are out of sight, they do not conflict with the aesthetics of a median with planters or grass. For those bridges over a greenway path, they have the additional advantage of preventing objects larger than the openings of the fence fabric from falling through.