It didn't take motor traffic, because when it was built, there was no motor traffic. But the claim I replied to was about all bridges. But it used to take vehicle traffic: the WP article says "In 1934 it was designated a scheduled monument and closed to vehicular traffic."

A tiny fraction reach 500 and so far none that we know of have hit 5,000 though there’s a few possibilities.

For example, if tomorrow we decided to build a new bridge next to every old bridge, average lifespan would half.

> if tomorrow we decided to build a new bridge next to every old bridge, average lifespan would half.

The only way building a bunch of new bridges changes lifespan is if we changed how they where built.

Figuring out lifespan rather than age is at best an approximation after looking at the bridges that didn't survive and the condition of bridges that do, but the uncertainty is low.