>Yes people definitely weigh as much as cars and trains and horses

It's not intuitive, but a shoulder to shoulder crowd of people actually does weigh more than a bumper to bumper traffic jam

Pedestrian bridges are calculated for a loading of around 90 pounds per square foot, basically a person occupying 2 square feet (for comparison elevator capacity is typically 2.3 square feet per person). And having watched from a window high above as people crowded onto a bit of the FDR closed for the fireworks, people really will pack in like that at times.

A typical compact car might be 15 feet long, closest you're going to pack them is 16 feet each. The kicker is that they're still taking up the full lane width of 10 feet.

So basically a car occupies 10*16 = 160 square feet of roadway.

Using the pedestrian loading standard, 160 feet would be a load of 14400 pounds.

But a fully loaded compact car has a gross weight of no more than 4000-5000 pounds - well less than half as much, closer to a third. Some vehicles weigh more, but not as much per area as you might think, and the extreme cases have their own rules with regard to bridges.

Even if you ignore the lanes and pack the cars in side by side, they still have less weight than the load factors used when designing for people.

Trains you say? The trains that ran on the Brooklyn Bridge were lightweights compared to today.

It's sarcasm - Chill