From the Rural ITS Toolbox report: Subsection 7.1 Speed Warning Systems (Travel Aid)
The Washington State Department of Transportation (WSDOT) deployed a variable speed limit (VSL) system to improve safety and reduce winter-weather related accidents on a 40-mile stretch of I-90 over the Snoqualmie Pass. The system consists of radar detection, six weather stations, nine variable message signs (VMS), and radio and microwave transmission systems. The weather stations monitor temperature, wind, humidity, precipitation and road surface conditions. Current traffic speed and weather condition data are collected and transmitted by packet radio and microwave communication to the Travel Aid control center. The system calculates safe speeds which are confirmed by WSDOT staff and transmitted to the dynamic message signs deployed along Snoqualmie Pass. The variable message signs were installed on the mainline downstream of each on-ramp, and the signs displayed either the maximum legal speed limit, or an enforceable reduced speed limit when conditions warranted. Pass conditions are also posted to the WSDOT website. Travel Aid cost $5 million to design and implement. The system has been in operation beginning winter 1997-98.
Each of the nine variable message signs in the test area was formatted to display the speed limit on the left hand side of the sign and a three-line message (ten characters per line) on the right. Four additional variable message signs were installed outside the test area; however, these signs did not post speed limits and were formatted to use all 15 characters per line for traveler information on roadway conditions, tire chain requirements, and road closures.
Speed limits were changed in 10 mph increments. If traction tires were advised, the speed limit was reduced to 55 mph; if traction tires were required, the speed limit was reduced to 45 mph; and if chains were required, the speed limit was reduced to 35 mph.
Robinson. "Examples of Variable Speed Limit Applications." Presentation given at the Speed Management Workshop held in conjunction with the 79th Annual Meeting of the Transportation Research Board. January 2000.
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Design and implementation cost: $5 million (1997).