Whenever any interstate bridge has spans, sections, or parts which are required to be movable by the laws, regulations, or codes of the United States, the department shall have the power and authority to enter into agreements with the adjoining state concerned respecting the operation and maintenance of such spans, sections, or parts. Such agreement may be included as a part of any agreement made for the construction or maintenance of interstate bridges, as provided by section 39-8,107, but the costs for the operation and maintenance of such spans, sections, or parts shall be considered separately from other construction and maintenance costs and shall be shared equally by the department and the adjoining state concerned.