Tennessee Code Title 55. Motor and Other Vehicles § 55-8-153

(2) The legislative bodies of municipalities shall also possess the power to prescribe lower speed limits within certain areas or zones, or on designated highways, avenues or streets that are not designated as state highways in their respective jurisdictions, and to erect appropriate signs and traffic signals.

(d)(1) The legislative body of any county, including a county having a metropolitan form of government, has the power to prescribe such lower speed limits as it may deem appropriate on any road being maintained by the county and shall erect appropriate signs and traffic signals.

(2) A county legislative body may delegate, by resolution of the body, the authority to lower speed limits on roads described in this subsection (d) to the chief administrative officer of the county highway department if the department employs an engineer capable of completing engineering and traffic investigations and ensures the completion of the same.  For purposes of this subdivision (d)(2), “chief administrative officer” has the same meaning as defined in § 54-7-103.

(3) Engineering and traffic investigations used to establish speed limits by counties on roads maintained by the county must be made in accordance with established traffic engineering practices and in a manner that conforms to the guidelines in the manual on uniform traffic control devices (MUTCD), and within state supplemental guidelines.  The investigations must be documented, and documentation must be maintained by the jurisdiction performing or sponsoring the investigation.

(4) Notwithstanding subdivision (d)(1), the board of commissioners has the power prescribed in this section in a county that has a commission form of government.