People see roads and bridges being blocked as an inconvenience, “snarling traffic” but it is so much more serious than that.
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As I recall, a mass of people on a bridge can be more of a load than vehicles, spaced, passing over.
Didn’t the Golden Gate have a problem with too many people, during a celebration of some sort.
And I have been told that soldiers, martching over a bridge, break step to stop any ‘frequency induced’ forces.
I’ll bet that a bunch of people, gathered on a bridge, and jumping up and down in unison, with the ‘frequency’ of the bridge, could bring it down.
Interesting, I hope some people wade in on that idea.