In those counties under the County Uniform Highway Law, a very strict conflict of interest statute applies.  The statute, T.C.A. § 54‑7‑203(a), provides:

Neither the chief administrative officer, county highway commissioner, member of the county governing body nor any employee of the county road department shall be financially interested in or have any personal interest, either directly or indirectly, in the purchase of any supplies, machinery, materials, equipment or contractual services for the department or system of roads for the county, nor in any firm, corporation, partnership, association or individual selling or furnishing such machinery, equipment, supplies and materials. 

Note that this prohibition is so broad as to preclude all employees of the highway department, whether or not they have any discretion or control over the purchase, from having a direct or indirect interest in these purchases. A violation of this statute constitutes official misconduct and is a Class C misdemeanor and is grounds for removal from office.  T.C.A. § 54‑7‑203(b).