Judging by the spray diffusion, it was marked from the top of the beam (left side of the image), which would be easer to access from an overpass rather than climbing up the column, so the writing probably begins with the circle dot and ends with an up-triangle.

The parenthetical numbers below refer to the respective Blissymbolic character (not encoded in the UCS, as a side note).

I'm not seeing an inverted triangle in this symbol set, besides one half of the glyph for "teeth" (17517), but this glyph has to be smaller than the triangle here is in the picture relative to the other glyphs per this symbol set's guidelines, so let's disregard that possibility for a moment and start with the circle dot.

What we get is something like this:

"eye/sight/vision" (14133) "layer(s)/level(s)" (25748) "creation/nature" (13607)

So I read this as an esoteric tag: See (witness/envision) the Levels/Layers of Creation/Nature.

If I'm mistaken about the three lines being equivalent size/length sub-glyphs, it could mean "equal to" (16713) with a "cloud" (13367) modifier above it, rendering it as:

Vision/Sight is a "Clouded-Equivalent" of Creation/Nature or something similar.

And if they couldn't quite remember the more complex arrangement for the term "storm" (23956), perhaps the middle glyph is "cloud" (13367) plus "sky" (17006) plus "earth/ground/land" (13901) on top of one another. This would make sense if you saw the glyph for "cloud" since it's a flat line with a minimal, singular periodic waveform overlapping the line, end to end. It looks like they took a little more time spraying that one, so that would make sense why there's more diffusion there.

With this arrangement, it could be chillingly understood to mean: Witness the Storm of Creation.

That's possibly a powerful enough message to risk one's life while tagging a bridge beam.