605 ILCS 5/5-204
Whenever the office of county superintendent of highways is vacant, the county board may with the consent in writing of the Department appoint any competent person as acting county superintendent of highways until the vacancy is filled in the manner provided in Section 5--201.
The office shall not be deemed vacant except at the end of the incumbent's 6 year term or in case of his death, his removal from office in accordance with the provisions of section 5--203, or his resignation submitted in writing to the county board. However, if the incumbent enters the military service of the United States, the county board may, with the approval of the Department, appoint any competent person as acting county superintendent of highways to perform the duties of the office until the end of the incumbent's 6 year term or the discharge of the incumbent from such service, whichever shall first occur.
605 ILCS 5/5-204