Mich. Comp. Laws § 252.58

Sec. 8. For the purpose of carrying out the provisions of this act, and to enable limited access highways to be constructed, the state transportation commission is authorized to make annual contributions to the cost of construction of limited access highways as provided in this act, and to make an irrevocable pledge of funds of the state transportation department derived from taxes imposed upon gasoline or other motor fuels, and on motor vehicles registered in this state for the purpose of meeting its annual obligations under the contract or contracts. The annual contributions of the state transportation department for any such project shall be payable in manner designated by the contract or contracts over a fixed period of not exceeding 30 years.