No motor truck or trailer with steel tires, or tractors with cleats or lugs on the wheels shall be operated over any public road in the state which has been hard-surfaced with any kind of material, or improved, in any manner by public funds, proceeds of bond issues, or with revenues of the state highway department, or of the United States.  Nothing contained in this Section shall prevent the use of trucks or trailers equipped with steel tires by persons engaged in farming or agricultural pursuits when transporting their products to and from plantations abutting any improved highway of this state, or to persons operating under certificates of permission by the police jury of the parish, or of the municipal authorities of the city or town in which they may be operating.