System Components:
The speed management system is comprised of six Environmental Sensor Stations (ESS), 22 radar vehicle detectors, Remote Processing Units (RPUs) housed in roadside cabinets, 13 Dynamic Message Signs (DMS), Variable Speed Limit (VSL) signs, a central control system, as well as digital radio and microwave communication systems. The ESS are installed along the interstate to detect air temperature and humidity, precipitation, wind speed, pavement temperature and condition (e.g., dry, wet, icy), and pavement chemical concentration. ESS data and vehicle speed data are collected by RPUs and transmitted to a control computer in the maintenance office located in Hyak. Advisory messages and reduced speed limits are posted on the DMS and VSL signs, as shown in the figure.

System Operations:
The central control computer provides decision support by utilizing software algorithms to process field data, calculate safe speeds, and suggest speed limit reductions during adverse conditions. If system operators agree with the recommendations DMS and VSL signs are activated to display road weather advisories, reduced speed limits, and the reasons for lower speeds. The control computer allows system operators to modify speed limits by direction and by road section. DMS may also be used to alert drivers of roadway closures necessitated by winter maintenance and avalanche control activities. When warranted, the speed limit is reduced in 10-mph (16-kph) increments from 65 mph (104.5 kph) to 35 mph (56.3 kph) based upon prevailing road, weather, and traffic conditions. Vehicle equipment (e.g., tire chains) may be regulated to improve vehicle traction.