State fuel tax and vehicle registration fee revenues are collected in a single "pot" known as the Michigan Transportation Fund. After money is taken off the top for a number of items including the Bridge Fund and Mass Transit, the remaining money is divided between the Michigan Department of Transportation, county road commissions and cities and villages according to a formula established by the state Legislature.
The formula calls for 39.1% of the money to go to MDOT (which has jurisdiction over 8% of Michigan's roads), 39.1% to go to county road commissions (which have jurisdiction over 75% of Michigan's roads) and 21.8% to go to cities and villages (which have jurisdiction over 17% of Michigan's roads).
Roads in Oakland County brochure.