For all counties where this has not already been done, the state highway department or the state highway engineer shall prepare and file with the clerk of the board of supervisors of the said several counties of the state, plans and specifications for highway sign or guide boards, showing the construction, size, material, color, and how said sign or guide boards are to be erected on the roads or highways within the said several counties. All such specifications are to be uniform, so as to obtain a uniform system of highway sign or guide board markers throughout the state.