It shall be unlawful for any person to operate a vehicle or any combination of vehicles over any bridge or other elevated structure or through any tunnel or underpass constituting a part of any public highway at a rate of speed or with a gross weight or of a size which is greater at any time than the maximum speed or maximum weight or size which can be maintained or carried with safety over any such bridge or structure or through any such tunnel or underpass when such bridge, structure, tunnel, or underpass is sign posted as hereinafter provided. The secretary of transportation, if it be a bridge, structure, tunnel, or underpass upon a state highway, or the governing body or authorities of any county, city, or town, if it be upon roads or streets under their jurisdiction, may restrict the speed which may be maintained or the gross weight or size which may be operated upon or over any such bridge or elevated structure or through any such tunnel or underpass with safety thereto. The secretary or the governing body or authorities of any county, city, or town having jurisdiction shall determine and declare the maximum speed or maximum gross weight or size which such bridge, elevated structure, tunnel, or underpass can withstand or accommodate and shall cause suitable signs stating such maximum speed or maximum gross weight, or size, or either, to be erected and maintained on the right hand side of such highway, road, or street and at a distance of not less than one hundred feet from each end of such bridge, structure, tunnel, or underpass and on the approach thereto: PROVIDED, That in the event that any such bridge, elevated structure, tunnel, or underpass is upon a city street designated by the department of transportation as forming a part of the route of any state highway through any such incorporated city or town the determination of any maximum speed or maximum gross weight or size which such bridge, elevated structure, tunnel, or underpass can withstand or accommodate shall not be enforceable at any speed, weight, or size less than the maximum allowed by law, unless with the approval in writing of the secretary. Upon the trial of any person charged with a violation of this section, proof of either violation of maximum speed or maximum weight, or size, or either, and the distance and location of such signs as are required, shall constitute conclusive evidence of the maximum speed or maximum weight, or size, or either, which can be maintained or carried with safety over such bridge or elevated structure or through such tunnel or underpass.