The bonds issued under this article:
1. Are fully negotiable within the meaning and for all purposes provided by title 47.
2. Are payable only according to their terms from bridge construction revenues or highway improvement revenues, as applicable, and are not general, special or other obligations of the issuing county or this state. The bonds do not constitute a legal debt of this state and are not enforceable against this state. Payment of the bonds shall not be made from any monies other than those allowed to be pledged.