And yet, in so many movies, bridges in NYC are filled with people and/or military vehicles going one way or another. I wonder if they were tasked with actually making sure the bridges could support a city evacuation or massive protest.
Bridges are designed and built with enough redundancy that if you fit all the people you can find on them, they can comfortably handle the weight. One should be more worried about not how many folks are there but HOW they are moving. Remember if you are in a protest and on a bridge don't "walk in step" like this https://goo.gl/dXIvBN . If you are interested in why this happened watch this https://goo.gl/ZOlzyx (skip to 2:42)
Forget the modern concrete and steel ones, when we built small bridges for villagers in Rwanda and Central America, we would ask all the villagers and their horses to come and fill them (just like in the protest pictures). This is basically done to address the concerns of some folks like you who may have to see it to believe it :-) See picture here: https://goo.gl/RIixT0
Source: Personal experience being a construction engineer who has built cable stayed bridges and the ones in the picture.
“It was probably the biggest load the bridge had ever seen,” said Mark Ketchum, a San Francisco bridge engineer who studied the Golden Gate Bridge from 1989 to 1991. “But it did not exceed the design load capacity of the bridge.”
[1] http://www.mercurynews.com/2012/05/23/the-day-the-golden-gat...
Even the Brooklyn bridge, partially because structural mechanics wasn't quite as well developed at the time, has a factor of safety of 6.