Mich. Comp. Laws § 252.59
For the purpose of carrying out the provisions of this act, each county, city or village which is a party to a contract or contracts as herein authorized, for the construction of such limited access highways is authorized to make annual contributions to the cost thereof as hereinbefore provided, and to make an irrevocable pledge of funds received, and to be received, by each of said units from the state of Michigan derived from taxes imposed upon gasoline or other motor fuels, and on motor vehicles registered in the state for the purpose of meeting its annual obligations pursuant to said contract or contracts. The annual contributions of each county, city or village for any such project shall be payable in manner designated by the contract or contracts over a fixed period of not exceeding 30 years.
MCL 252.59