The approval of the Secretary of the department in which the Coast Guard is operating, as required by section 491 of this title, shall be given only subsequent to the President's approval, as provided for in section 535b of this title, and shall be null and void unless the construction of the bridge is commenced within two years and completed within five years from the date of the Secretary's approval: Provided, however, That the Secretary, for good cause shown, may extend for a reasonable time either or both of the time limits herein provided.