Gates shall not be allowed on any county highway except gates allowed by the board of supervisors in accordance with the provisions of section 966. If the expense of the erection and maintenance of such allowed gates is not paid as required by section 966, such gates shall be removed as an obstruction.

Any person who leaves any such gate open, or who wilfully and unnecessarily rides over ground adjoining the highway on which the gate is erected, shall be liable to the injured party for treble damages.