We might be arguing semantics, but safety margins aren't considered 'overbuilding' but part of the bare minimum requirements for a bridge to stand. They aren't there "just in case" they are there because it is known for a fact that bridges in the real world will experience degradation and overloading.

If you build a bridge that is rated to carry 100k lbs of weight, and you build it to hold 100k lbs, you didn't build it to barely meet spec -- you under built it -- because overloading is a known condition that does happen to bridges.