I wondered about that. Here in Georgia the DOT has load limit signs posted on bridges based on the number of axles. I think the normal indications are for 4 wheel, 6 wheel, and 18 wheel rigs, but not sure.

The load limits you see posted have a large factor a safety built into them. Depends upon the individual agency. Don't go by the load limit on the bridge alone especially if you are close to the posted load limit. Contact the agency in charge of the bridge to see if you can get a waiver.