No ordinance closing a highway under Section 35712 or 35715 is effective until appropriate signs are erected indicating either the highways affected by the ordinance or the highways not affected as the county may determine will best serve to give notice of the ordinance, nor shall any ordinance be effective with respect to any county highway unless the board of supervisors designates in the ordinance an alternate route for the use of the vehicles which shall remain unrestricted by any local regulation as to commercial vehicles so long as the ordinance proposed shall remain in effect.