No bridge for the use of the traveling public shall be constructed and operated by the parish or by a municipal corporation or other political subdivision of the state, or by any person, and no franchise shall be granted for the operation of a ferry, within fifteen miles of any toll bridge for the acquisition or construction of which bridge revenue bonds have been authorized under this Sub-part, except under a written permit granted by the director of highways.  No such permit shall be granted by the director until he has ascertained by an investigation, including a hearing upon such notice and under such rules he prescribes, that there is an urgent public need for the operation of the bridge or ferry and that its operation will not affect the revenues of the bridge purchased or constructed under the provisions of this Sub-part, so as to impair the security of any bridge revenue bonds issued therefor.