4.  The Secretary of State directs as respects any part of a highway which crosses the route of the new trunk road that—

(a) where the highway is maintainable at the public expense by a local highway authority, the part in question shall be maintained by that authority, and

(b) where the highway is not so maintainable and is not maintainable under a special enactment or by reason of tenure, enclosure or prescription, the Secretary of State shall be under no duty to maintain the part in question,

until in either case, a date to be specified in a notice given by the Secretary of State to the highway authority for that highway. The date specified will not be later than the date on which the new trunk road is opened for traffic.