Weather-controlled variable speed limit signs deployed on I-84 in Oregon


The Oregon Department of Transportation has deployed variable speed limit (VSL) signs along a section of Interstate 84 that is prone to changeable weather conditions, as part of an intelligent transportation system (ITS) project to improve road safety in the region.

The new digital speed limit signs have been mounted over a 30-mile (48km) stretch of I-84 between Baker City and La Grande, replacing standard speed signs in the area. The new signs will use traffic, road, weather and visibility sensors to lower the legal speed limit when ice, snow, fog or an incident ahead requires everyone to slow down. Along with identifying the current legal speed limit, the digital displays can also show the reason for a reduced speed, such ‘ice’ or ‘low visibility’. The new VSL signs were installed this summer and displayed the state freeway speed limits for about two months. The signs have now been connected to a variety of road sensors and are fully functional, showing the current legal speed limit for the area, depending on conditions.

Ice, high winds with blowing snow, and limited visibility are some of the challenging conditions that catch motorists off-guard on this section of I-84, resulting in a higher number of crashes than similar freeway sections. Road weather information systems (RWIS) that measure temperature, visibility, road surface grip (traction), vehicle speeds, moisture levels, wind speed, and other data along the highway, feed that real-time information into the VSL sign system. If a majority of cars and trucks are slowing down due to weather, poor road conditions, or a crash ahead, the sensors can send a message to the signs to reduce the speed for all drivers. Likewise, when sensors detect poor traction, low visibility or other hazardous conditions, speed limits will be lowered, and an appropriate warning message can be displayed. ODOT staff will also be able to manually adjust the speed limits, such as when a crash occurs or when construction activities require a slower speed.

“The section of freeway between Baker City and La Grande is positioned between the Eagle Cap and Elkhorn mountain ranges,” explained ODOT area manager Ken Patterson. “This often creates microclimates with more severe weather patterns than adjacent sections of freeway. The idea behind these new signs is to warn motorists ahead of hazardous road conditions and get them to slow down before they enter a potentially dangerous situation.

“This is the first time in Oregon a speed limit sign system has been installed that can adjust the legal speed. Similar projects have been constructed in other states with positive results. Over the winter we will be monitoring our new system and making adjustments as needed. The goal of these projects is to reduce the number and severity of crashes in known trouble spots. It will save lives and limit the amount of time the freeway is closed due to a crash or severe weather conditions.”

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Tom has edited Traffic Technology International (TTi) magazine and its Traffic Technology Today website since May 2014. During his time at the title, he has interviewed some of the top transportation chiefs at public agencies around the world as well as CEOs of leading multinationals and ground-breaking start-ups. Tom's earlier career saw him working on some the UK's leading consumer magazine titles. He has a law degree from the London School of Economics (LSE).