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Dynamic message signs that warn drivers of high winds and harsh driving conditions can reduce traffic speeds by 16% and improve uniformity of traffic flow.

A section of I-84 near Rupert, Idaho was equipped with a road weather information system (RWIS) to provide low visibility warnings to drivers. Roadside Environmental Sensor Stations (ESS) were installed to measure visibility, wind, precipitation, air temperature, relative humidity, and pavement conditions. Vehicle detectors recorded the lane number, time, speed, vehicle type (passenger car or truck), and length of each vehicle passing the sensor site. A closed circuit television (CCTV) camera system was installed to confirm sensor visibility readings. The data generated by these systems were transmitted to a central computer system and appropriate driver warnings were posted on dynamic message signs (DMS) along the corridor.

To assess the degree to which driver behavior was modified through display of road weather information on the DMS system, an analysis of vehicle speeds was conducted with and without DMS during 19 weather events. The evaluation was carried out utilizing weather, pavement and traffic sensor data collected from 1997 to 2000.

RESULTS

Drivers were influenced by the system more extensively during windy conditions:

• Heavy winds (greater than 30 mph)
• Heavy winds and moderate to heavy precipitation
• Heavy winds when roads were snow covered

During these conditions, the average vehicle speed decreased 23% (12.5 mph) when road weather information was displayed compared to a 7% (3.7 mph) decrease when advisory messages were not displayed. In addition, when messages were posted on DMS, the standard deviation of vehicle speeds (speed variability) decreased 14% compared to virtually no change when no DMS messages were shown. Speed variability can be an important factor in vehicular crashes.

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Last Modified: 01/03/2006