The director of transportation shall prepare and carry out a plan regarding an implementation schedule for all roads changing jurisdiction as a result of this section. Any road that changes jurisdiction and becomes a municipal road, under the enactment of this section, that such shall be repaired and improved, as may be deemed necessary, by the department of transportation to a safe and usable condition before the road is returned to the jurisdiction of the municipality; provided, however that no road shall be returned to the jurisdiction of a municipality without its consent if the jurisdiction does not have a full-time highway department; and provided further that in the town of Portsmouth to preserve the coastal right-of-way for highway purposes, Park Avenue, Point Road, and Hummock Avenue including the “Cove Bridge” shall not be returned to the jurisdiction of the town of Portsmouth and shall remain the responsibility of the state.