Tenn. Code § 54-7-116
There is created a presumption of law that the chief administrative officer of each county highway department is authorized to sign agreements with the department of transportation on behalf of the county. Once an agreement is executed by the chief administrative officer of the county highway department for any county of the state, the agreement shall be fully binding upon the applicable county. The presumption of law may only be overcome by the provision of notice by the county legislative body that the chief administrative officer of the county highway department does not have the authority to execute these agreements on behalf of the county. The receipt of the notice shall be acknowledged by the department in order to overcome the presumption of law set forth in this section.
T.C.A. § 54-7-116