Where it has been determined that any City bridge has a capacity less than the maximum permitted on the streets of the City, or on the street serving the bridge, the City Council may cause to be posted and maintained signs on said bridge and at suitable distances ahead of the entrances thereof to warn drivers of such maximum load limits, and no person shall drive a vehicle weighing, loaded or unloaded, upon said bridge in excess of such posted limit.
(Code of Iowa, Sec. 321.471)