Weather-Related Signal Timing, Charlotte, NC

In North Carolina, the Charlotte Department of Transportation (DOT) manages 615 traffic signals with a computerized control system. In the central business district, the city make use of weather-related signal timing plans at 149 signals to reduce traffic speeds during severe weather. Signal timing also can be employed at more than 350 intersections regulated by closed-loop systems.

System Components: The traffic signal control system comprises signal controllers located at city intersections, a closed-circuit television (CCTV) surveillance system, twisted-pair cable and fiber-optic cable communication systems, and a signal system control computer in the traffic operations center. Images from more than 25 CCTV cameras on major arterial routes are passed to the operations center and shown on video monitors. Traffic managers can select and download various timing plan patterns (stored in the computer) to field controllers with the aid of the communication systems.

System Operations: System operators assess traffic and weather conditions by observing CCTV video images and reviewing weather forecasts. Forecast data are available through radio and television broadcasts, the Website for the National Oceanic and Atmospheric Administration's National Weather Service, and a private weather service vendor. When operators notice heavy rain, snow, or icy conditions, they access the signal computer and manually implement weather-related timing plans. To slow the speed of traffic, these signal timing plans increase the cycle length—which is typically 1minute and 30 seconds—while offsets and splits remain the same. During offpeak periods operators also may select peak-period timing patterns, which are designed for lower traffic speeds.

After establishing the weather-related signal timing plans, operators monitor traffic flow on the roadways. If warranted by field conditions, operators can increase cycle lengths to further reduce traffic speeds. When weather conditions return to normal, operators access the central computer to restore normal time-of-day and day-of-week timing plans.

Transportation Outcome: When weather-related signal timing is engaged, travel speeds decrease by 8 to 16 kilometers per hour (5 to 10 miles per hour). By selecting signal timing plans based on prevailing weather conditions, traffic managers improve roadway safety by minimizing speeds and reducing the probability and severity of crashes.