In Tennessee, a fog detection and warning system implemented in 1994 significantly improved safety as no fog-related accidents have occurred since implementation.

In December 1990, a chain-reaction collision involving 99 vehicles prompted the design and implementation of a fog detection and warning system on Interstate 75 in southeastern Tennessee. The system covers 19 miles including a 3-mile, fog-prone section above the Hiwassee River and 8-mile sections on each side.

Center managers with Tennessee DOT access a central computer system that collects data from 2 ESS, 8 fog detectors, and 44 vehicle speed detectors. By continually monitoring fog and speed sensor data, the computer system predicts and detects conditions conducive to fog formation, and alerts managers when established threshold criteria are met. Highway Patrol personnel visually verify onsite conditions. The computer system correlates field sensor data with pre-determined response scenarios, which include advising motorists of prevailing conditions via flashing beacons atop 6 static signs, 2 HAR transmitters, and 10 DMS; reducing speed limits using 10 VSL signs; and restricting access to the affected highway section with ramp gates.

Center managers select pre-programmed DMS messages (see table below), pre-recorded HAR messages, and appropriate speed limits (i.e., 50 mi/h or 35 mi/h) based upon response scenarios proposed by the system. Under the worst-case scenario (i.e., visibility less than 240 feet), the Highway Patrol activates eight automatic ramp gates to close the interstate and detour traffic to US Route 11.


TDOT Fog Detection/Warning System DMS Messages

Conditions	Displayed Messages
Reduced Speed Detected	Flashing "CAUTION" with "SLOW TRAFFIC AHEAD"
Fog Detected	Flashing "CAUTION" with "FOG AHEAD TURN ON LOW BEAMS"
Speed Limit Reduced	Flashing "FOG AHEAD" with "ADVISORY RADIO TUNE TO XXXX AM"
	Flashing "FOG AHEAD" with "REDUCE SPEED TURN ON LOW BEAMS"
	Flashing "FOG" with "SPEED LIMIT YY MPH"
Roadway Closed	Flashing "DETOUR AHEAD" with "REDUCE SPEED MERGE RIGHT"
	Flashing "I-75 CLOSED" with "DETOUR w/ arrow""
	Flashing "FOG AHEAD" with "ADVISORY RADIO TUNE TO XXXX AM"

There have been over 200 crashes, 130 injuries and 18 fatalities on this highway section since the interstate opened in 1973. Since the fog detection and warning system began operating in 1994, safety has been significantly improved as no fog-related accidents have occurred.