Cities and towns maintaining bridle paths and trails mentioned in sections 3151 and 3152 shall not be under any obligation to keep them in repair or to break them out in winter. If any city or town shall erect a bridge on such bridle path or trail, it shall be under the same obligation to keep such bridge in a safe condition for the use of horses and riders as it is now under to keep highway bridges in repair for the purposes for which they are used. Such city or town shall erect at the entrance of such bridle paths and trails suitable signs, signifying that they are bridle paths or trails only, and not for use of vehicles, and that persons may use them at their own risk.