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 Public Works Director 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. No person shall drive upon said bridge any vehicle weighing, loaded or unloaded, in excess of such posted limit.
(Code of Iowa, Sec. 321.471)