For the purposes of N.M. Stat. Ann. Chapter 67, necessary bridges, culverts and other appertaining structures on any highway shall be considered a part of such highway. Not more than fifty percent of the cost of all construction or improvement of highways, under the provisions of N.M. Stat. Ann. Chapter 67, shall be paid by the state and not less than fifty percent by the county in which such work is done, except where such work is done by the state transportation commission without the use of county funds.