Famous example of pedestrian bridge resonances: https://en.m.wikipedia.org/wiki/Millennium_Bridge,_London#Re...
One has to consider what is daily use, not just the exceptional cases.
generally it's the exceptional case that it's designed around though
We have a car bridge designed to withstand exceptional load, and a pedestrian bridge designed to withstand exceptional load. The commenter assumes that on average, the car bridge's load is much closer to it's maximum load than the pedestrian bridge's load, and in consequence, the average wear on the car bridge should be higher than on the pedestrian bridge. As such, the pedestrian bridge should have a much longer lifetime, and the commenter assumes that this is the reason old Roman bridges are still standing.
Their question is: is this assumption correct?
You also have to take failure modes into account, and how degraded the bridge can get before it's "unsafe".