The several counties of the State are authorized to levy ad valorem taxes upon all property within their respective boundaries for county purposes, except the first Three Thousand Dollars ($3,000) value of residential homesteads of married or unmarried adults, including those living alone, not to exceed thirty cents (30¢) on each One Hundred Dollars ($100) valuation, in addition to all other ad valorem taxes authorized by the Constitution of this State, provided the revenue derived therefrom shall be used for construction and maintenance of Farm to Market Roads or for Flood Control, except as herein otherwise provided.