20B. If the Authority is of opinion that any bridge or toll-road, or any specified part thereof, can not be used without danger to the users or the public, or is no longer in a fit state for the carriage of any specified class of traffic, it may, by notice in writing displayed prominently in or near such bridge or toll-road, order that the bridge or toll-road, or any specified part thereof, shall be closed to the users or the public or to all traffic or to any specified class of traffic