Bonds issued pursuant to this article:
1. Are obligations of the board issuing the bonds.
2. Are payable only pursuant to the terms of the bonds.
3. Are not obligations general, special or otherwise of this state.
4. Are not a legal debt of this state or of the respective county imposing the transportation excise tax.
5. Are not enforceable against this state or that county out of any monies other than the income and revenue pledged and assigned to or in trust for the benefit of the holder or holders of the bonds.