Construction, maintenance, or improvement of county unit roads. Any county board in a county with an established county unit district highway system may, with the approval of the Department, use a maximum of 30% of the motor fuel tax funds, provided for in Section 8 of the Motor Fuel Tax Law, allotted to it to pay for the cost of construction, maintenance, or improvements of roads in the county unit system. The Department shall not approve use of the funds unless the Department determines that the county highway system is being maintained in an acceptable condition and that the use of the motor fuel tax funds would not jeopardize adequate maintenance of existing county highways.