The proceeds from the sale of all bonds authorized under the provisions of this chapter shall be paid to the state treasurer for the credit of the department and be deposited as demand deposits forthwith in such depositary or depositaries as may be authorized by law to receive deposits of state funds to the credit of a fund to be designated as the construction fund of the particular toll bridge or toll bridges for which such bonds were issued and sold, which fund shall not be a state fund and shall at all times be kept segregated and set apart from all other funds and in trust for the purposes herein set out. Such proceeds shall be paid out or disbursed solely for the construction of such toll bridge or toll bridges, the acquisition of the necessary lands and easements therefor and the payment of interest on such bonds during the period of actual construction and for a period of six months thereafter, only as the need therefor shall arise. The department may agree with the purchaser of the bonds upon any conditions or limitations restricting the disbursement of such funds that may be deemed advisable, for the purpose of assuring the proper application of such funds. All moneys in such fund and not required to meet current construction costs of the toll bridge or toll bridges for which such bonds were issued and sold, and all funds constituting surplus revenues that are not immediately needed for the particular object or purpose to which they must be applied or are pledged shall be invested in bonds and obligations of the nature eligible for investment of surplus state moneys: PROVIDED, That the department may provide in the proceedings authorizing the issuance of these bonds that the investment of such moneys shall be made only in particular bonds and obligations within the classifications eligible for such investment, and such provisions shall thereupon be binding upon the department and all officials having anything to do with the investment. Any surplus which may exist in the construction fund shall be applied to the retirement of bonds issued for the construction of such toll bridge or toll bridges by purchase or call. If these bonds cannot be purchased at a price satisfactory to the department and are not by their terms callable prior to maturity, the surplus shall be paid into the fund applicable to the payment of principal and interest of the bonds and shall be used for that purpose. The proceedings authorizing the issuance of bonds may provide limitations and conditions upon the time and manner of applying the surplus to the purchase and call of outstanding bonds and the terms upon which they shall be purchased or called. Such limitations and conditions shall be followed and observed in the application and use of the surplus. All bonds so retired by purchase or call shall be immediately canceled.