I'm a proponent of public transport and pedestrian transport, but as a question of efficiency, "how many people can fit on a bridge in cars vs. on foot" is not the right way to put it - time has to be a factor, as in, how many can move across it.  More people can fit on a bridge on foot, but can cars move the same number of people across it faster?  I have no idea.  In heavy traffic, pedestrians would win, unless the great number of walkers makes them walk more slowly (pedestrian traffic)...

Anyway, just thought I'd be annoying and point out a detail.

That's a fair point on speed, but I think you will find that our bridges are much less efficient now versus 1900-1920 (before cars) based on the amount of passengers that make trips across them on a daily basis. Mostly because we took off streetcars and replaced them with less efficient automobiles.
Well, see, now, there you go!