Variable Speed Limit (VSL)

The Variable Speed Limit Program is an innovative traffic operations program developed by UDOT to allow the Traffic Operations Center to control speed limits in canyon roads where speed limit differentials can create both congestion and safety issues.

So you are wondering why this is a thing. You are not alone! The Variable Speed Limit Program is an innovative traffic management strategy that draws its roots deep into traffic flow theory. The short answer is that VSL helps protect the capacity of the freeway, it helps prevent back of queue crashes and it helps set appropriate speed limits when weather conditions demand it by encouraging uniform traffic speeds. Uniform traffic speeds optimize capacity which means that we can get more vehicles through to their destination. Uniform traffic speeds, also ideally, prevent groups of high speed vehicles from approaching low speed vehicles which reduces the risk of back-of-queue crashes. So the VSL makes our roadways safer too. 

Utah hosts a lot of out-of-state travelers and our mountains make us unique. Many drivers are not familiar with our steep grades and heavy snow. When snow storms hit, the VSL reacts to reflect lower, safer speeds. This helps regulate higher speed drivers to bring them in-line with lower speed drivers. These higher speed drivers represent outliers, meaning that the majority of traffic is already driving at the slower speeds that the VSL adjusts to reflect. The safe speed is actually partially calculated based on the speeds collected from all of the drivers passing through the VSL corridor using radar detection. In even more severe conditions, like when blizzards hit, inexperienced drivers are often not aware that chains are sometimes required and driving with those chains is only safe at slow speeds. The VSL reacts to reflect safe speeds specifically when we post the "chains required" messages to the VMS.

The reason why VSL works to protect freeway capacity is a little bit more complicated. Consider your garden hose. Every time you turn the water on, that hose generally produces the same amount of water. Yes, for those hydraulic engineers out there, we know about Manning's n and the hydraulic grade line... but stick with us here. This maximum capacity is the ideal situation. But traffic, unfortunately, doesn't work the same way. Vehicles speed up and slow down, every driver has a different level of comfort, weather conditions change, every vehicle has a different weight to power ratio, the road curves and the grades change, even the pavement condition can change. So all of these changes (and there are more out there!) mean that the flow of traffic is constantly changing too. The VSL targets one of our biggest changing elements: speed. Freeways operate at their very best capacity when vehicles are able to travel close to the same optimal speed limit. Large variations in speed creates a ripple effect. As vehicles speed up and slow down, a shock wave travels backwards through traffic. And the strength of the shock wave is amplified by the severity of congestion. This is why, sometimes, after a crash or a delay, traffic will still be experiencing that 'speed up and slow down' behavior an hour after that crash has been completely cleared. All of that 'speed up and slow down' behavior robs the freeway of capacity that it would otherwise have with uniform flows. So the VSL helps vehicles move along the corridor in a more uniform way.

UDOT currently has VSL running on I-80 through Parleys Canyon and on I-15 through Baker Canyon and both of these systems are automated.

The speed limits displayed on the VSL are calculated based on the speed of each vehicle measured through radar detection along the sides of the road. Speeds are collected in 20 second intervals across all lanes. The live 5 minute rolling average is used to determine the appropriate speed limit for the corridor. Weather data, including roadway conditions and visibility, will sometimes influence these speed limits if they indicate unsafe conditions. When our snow plows cannot keep up with the snowfall, the accumulating snow may lead to conditions that require chains. When chains are required for just heavy trucks, the VSL system will automatically reduce speed limits by 20 mph.  When chains are required for all vehicles, the VSL system will automatically reduce speeds by 30 mph. The speed limit posted between the two sections are tempered to prevent faster vehicles from approaching slower vehicles at high differentials. The speeds of the VSL are also bounded so the VSL will never drop below 25 mph nor will it exceed the normal speed limit of the freeway. And finally, the VSL speeds are coded to prevent large changes over short amounts of time and they are only capable of changing in set intervals. This means that traffic conditions have to be consistently sustained at a new level before the VSL will react.

Yes! But it didn't start out that way. We first developed VSL to be controlled manually. Live traffic data, in 20 second intervals, was collected from our radar and aggregated into reports. Those reports were pulled and evaluated by traffic operation engineers at 5 minute intervals. Each day of the week was assigned to a traffic operation engineer who responded on-call to the Traffic Operations Center to turn on and run the VSL system. A few of us ran this system manually for a few years until we felt comfortable developing the algorithms to automate it. The automation was built to reflect those same decisions we were making, and learning, during the manual pilot program.

VSL signs are connected to power and fiber that run through the canyon, ultimately connected back to the Traffic Operations Center. A program, developed by UDOT, DTS and The Narwhal Group, sends commands to all of the signs at once to post new speed limits.

VSL signs are coded to remain at the same speed until new traffic conditions are sustained such that the new rolling average of 3 consistent data intervals produces a different speed outside of a 5 mph range.  The speed limits are also bounded by the adjacent static sections of the freeway.

Yes. In order for VSL to be effective, the speed limits are enforced by Utah Highway Patrol. Note that these are Regulatory signs with matching black and white colors indicating that these are enforceable based on the actual posted speed.