Mich. Comp. Laws § 9
All specific taxes, except general sales and use taxes and regulatory fees, imposed directly or indirectly on fuels sold or used to propel motor vehicles upon highways and to propel aircraft and on registered motor vehicles and aircraft shall, after the payment of necessary collection expenses, be used exclusively for transportation purposes as set forth in this section.
Not less than 90 percent of the specific taxes, except general sales and use taxes and regulatory fees, imposed directly or indirectly on fuels sold or used to propel motor vehicles upon highways and on registered motor vehicles shall, after the payment of necessary collection expenses, be used exclusively for the transportation purposes of planning, administering, constructing, reconstructing, financing, and maintaining state, county, city, and village roads, streets, and bridges designed primarily for the use of motor vehicles using tires, and reasonable appurtenances to those state, county, city, and village roads, streets, and bridges.
The balance, if any, of the specific taxes, except general sales and use taxes and regulatory fees, imposed directly or indirectly on fuels sold or used to propel motor vehicles upon highways and on registered motor vehicles, after the payment of necessary collection expenses; 100 percent of the specific taxes, except general sales and use taxes and regulatory fees, imposed directly or indirectly on fuels sold or used to propel aircraft and on registered aircraft, after the payment of necessary collection expenses; and not more than 25 percent of the general sales taxes, imposed directly or indirectly on fuels sold to propel motor vehicles upon highways, on the sale of motor vehicles, and on the sale of the parts and accessories of motor vehicles, after the payment of necessary collection expenses; shall be used exclusively for the transportation purposes of comprehensive transportation purposes as defined by law.
The legislature may authorize the incurrence of indebtedness and the issuance of obligations pledging the taxes allocated or authorized to be allocated by this section, which obligations shall not be construed to be evidences of state indebtedness under this constitution.
MCL 9