[arjunvpaul]

Short Answer: Don't worry about people's weight. Invite as many people and cars as you like.

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.

[bung]

Very cool. I suppose in hindsight I think the guy was talking about those elevated highway overpasses and/or on/off ramps, not exactly nice new suspension bridges :) Anyway thanks for the details.