All work involved in or incidental to the maintenance, repair and improvement of said bridge shall be performed in such manner as is or may be provided by law for the maintenance, repair and improvement of trunk line bridges, except when the manner of performing such work is prescribed by any existing contract and in such cases the work shall be performed in accordance with such contract, it being the intent hereof that said bridge in so far as the public interest therein is concerned, shall be regarded as a trunk line bridge.