Sections thirteen to twenty-eight, inclusive, of this article shall be deemed to provide an additional and alternative method for the doing of the things authorized hereby and shall be regarded as supplementary and additional to powers conferred by other laws: Provided, however, That no bridge for the use of the traveling public shall hereafter be erected and operated by any public authority authorized by law to construct bridges, or by any person, firm or corporation, across any river or other stream in this state, between any bridge across such river or other stream and any bridge thereover owned by the state and operated as a toll bridge by the state road commissioner, except under a permit of the commissioner in writing, until a certificate of convenience and necessity shall have been granted for the construction and operation thereof by the Public Service Commission. No such certificate of convenience and necessity shall be granted by the Public Service Commission until application in writing shall have been made therefor, and the applicant shall have filed with such application a map showing the location and design of the bridge for the erection and operation of which the certificate of convenience and necessity is sought, and the location of the existing bridges across the river or other stream between which the bridge is proposed to be erected, and the location of all the public roads leading to, and between, the existing bridges for a distance of twenty miles on either side of the river or other stream, and such other information as the Public Service Commission may deem necessary, together with proof that a copy of the application and map was, at least sixty days prior thereto, furnished to the commissioner; nor until the Public Service Commission shall, by investigation and by a hearing had thereon, under such rules as it may prescribe, determine that there is an imperative public need for the erection and operation of such bridge, and that its erection and operation will not materially injure the revenue of any bridge owned by the state and operated by the commissioner as a toll bridge. This proviso shall not apply where any bridge owned by the state and operated by the commissioner as a toll bridge is operated only for the maintenance, repair and operation of such bridge, and not for the purpose of paying any indebtedness against the same.