§ 17. Advertising for proposals. The board of transportation before awarding any contract except where there is an extraordinary emergency involving danger to life or property or where the estimated expense of a contract does not exceed twenty-five thousand dollars shall advertise for proposals for such contract by a notice to be printed once a week for two successive weeks in no less than two of the daily newspapers published in such city, and in such newspapers published elsewhere than in such city as such board shall determine. Such notice shall set forth and state the points within the city, between which such road is to run, the general method of construction, the route to be followed, the term of years for which it is proposed to make such contract, and such other details and specifications as the board shall deem to be proper. Such notice shall state the time and place at which such proposals will be opened. The board shall attend at the time and place so specified, and shall publicly open all proposals that shall have been received, but the board shall not be bound to accept any proposals so received, but may reject all such proposals and readvertise for proposals in the manner provided in this chapter, or may accept any of such proposals as, in the judgment of such board, will best promote the public interest, and award a contract accordingly. All such contracts must before execution be approved as to form by the corporation counsel and consented to by the board of estimate and the mayor of such city.