Well you know sometimes you see a lot of small bridges that will have weight limit signs. I believe this is really for safety precautions. Keyword: SMALL bridge -
Due to some bridges being small, the materials and strength of the bridge may not can withstand a certain weight. The contents of the vehicle, axels, and wheels, all add to that weight. So there is a very good reason for that to matter.
I hope my somewhat, tied together answer provides just the information you need!
~Brooke❤️