Speed Zones in General
Municipalities are authorized to establish upper speed limits not exceeding 55 mph on the streets within their jurisdictions that are not part of an interstate or controlled-access highway in the state road and highway system. T.C.A. § 55-8-152(c) sets the speed limit on state controlled-access highways and interstates at 70 mph. The minimum speed in a left lane is 55 mph. T.C.A. § 55-8-152(h) provides that only the state Department of Transportation may set speed limits on access-controlled state highways and interstates. It also is authorized to set minimum speed limits "below which no person shall drive a vehicle except when necessary for safe operation or in compliance with the law." The minimum speed limit must be set on the basis of an engineering and traffic investigation showing that "slow speeds on any part of a highway consistently impede the normal and reasonable movement of traffic ... " Presumably, that authority would not extend to interstate or controlled-access highways in the state road and highway system. A municipal governing body must have an engineering and traffic investigation done that shows the public safety requires a lower speed limit on a state highway in the municipality before the governing body may lower the speed limit below the state-set speed limit. These investigations must be done in accordance with engineering practices and with the Manual on Uniform Traffic Control Devices (MUTCD). Signs and markings also must conform to the MUTCD. Municipalities have the authority to erect the appropriate speed limit signs and traffic signals. T.C.A. §§ 55-8-152---154.

School Zones
With the exception of municipalities in Hawkins and Sullivan counties, municipalities are authorized to set special speed limits on streets adjacent to school grounds (except at school entrances and exits from controlled-access highways in the state highway system where the Department of Transportation has the authority to set speed limits). The special speed limits must be:

• based on an engineering investigation;
• not less than 15 mph; and
• in effect only when the proper signs are posted with a warning flasher; or flashers in operation and only while children are actually present.

Where the municipal legislative body has not set a special speed limit, a person is prima facie guilty of reckless driving if exceeding 15 mph:

• When passing a school during recess while a warning flasher or flashers are in operation; or
• Ninety minutes before or after the opening or closing of a school when children are actually going to or leaving the school.

The same rules apply to municipalities in Hawkins and Sullivan counties except that the distance from schools at which the municipality may set speed limits is one-quarter mile, and the period before the opening and closing hours of school is 40 minutes. T.C.A. § 55-8-152.

Tennessee Department of Transportation
The Tennessee Department of Transportation (TDOT) has the authority to lower the speed limit:

• on the state system of roads and highways as it deems appropriate due to conditions affecting the roadway or traffic; and
• "in business, urban, or residential districts or at any congested area, dangerous intersection, or whenever and wherever the department shall determine, on the basis of an engineering and traffic investigation, that the public safety requires a lower speed limit."

Apparently, this is limited to the state system of roads and highways. The violation of TDOT-set speed limits where construction workers are present is a Class B misdemeanor, carrying a fine of from $250 to $500. T.C.A. §§ 55-8-152–53.